The cleavage of the spiroketal ring has been widely investigated under a great variety of reaction conditions providing different kinds of skeletons, sometimes under similar reaction con
Trang 1Contents lists available atScienceDirect
Steroids
j o u r n a l h o m e p a g e :w w w e l s e v i e r c o m / l o c a t e / s t e r o i d s
Novel steroidal penta- and hexacyclic compounds derived from
12-oxospirostan sapogenins
José Oscar H Pérez-Díaza, José Luis Vega-Baezb, Jesús Sandoval-Ramírezb, Socorro Meza-Reyesb,∗, Sara Montiel-Smithb, Norberto Farfánc, Rosa Santillana,∗∗
a Departamento de Química, Centro de Investigación y de Estudios Avanzados del IPN, Apdo Postal 14-740, 07000 México D.F., Mexico
b Facultad de Ciencias Químicas, Benemérita Universidad Autónoma de Puebla, Ciudad Universitaria, Col San Manuel, C.P 72570, Puebla, Pue., Mexico
c Facultad de Química, Departamento de Química Orgánica, Universidad Nacional Autónoma de México, Ciudad Universitaria 04510, Mexico D.F., Mexico
a r t i c l e i n f o
Article history:
Received 12 May 2010
Received in revised form 12 July 2010
Accepted 14 July 2010
Available online 22 July 2010
Keywords:
12-Oxosapogenins
Polycyclic steroids
Intramolecular cyclocondensation
a b s t r a c t
The E ring regioselective acid-catalyzed opening of spirostanic sapogenins possessing a carbonyl group at C-12, such as botogenin and hecogenin, provided the new 12,23-cyclo-22,26-epoxycholesta-11,22-diene skeleton, in addition to new compounds of the already known 12,23-cyclocholest-12(23)-en-22-one frameworks This transformation proceeds in a single step, under slightly acidic conditions Both, penta-and hexacyclic steroids were obtained with retention of configuration of all asymmetric centers
© 2010 Elsevier Inc All rights reserved
1 Introduction
Steroidal derivatives have been a rich source of agents with
potential pharmaceutical applications that have inspired the
syn-thesis of new analogs with increased pharmacological activity By
1990 about 80% of the drugs used were either natural products or
synthetic analogs; nowadays almost 50% of the approved drugs are
still based on natural products[1]
The spirostan sapogenins are a particular type of steroidal
derivatives, widely spread in plants and some marine organisms,
which exhibit a broad range of biological activities[2–5] These
natural products are found as steroidal glycosides (saponins) The
sugar moiety is greatly diversified and the carbohydrate units can
be obtained through acidic or enzymatic hydrolysis The steroidal
sapogenins have served for many years as cheap raw
mate-rial for the pharmaceutical industry in the synthesis of sex and
adrenocortical hormones, analogs of vitamin D, anabolics and
anti-inflammatory drugs[6–8] Steroidal sapogenins can be divided into
cholestanic, furostanic and spirostanic derivatives; the latter ones
contain a particular spiroketal system (E/F rings) at the side chain,
which is stabilized by anomeric effects[9]
∗ Corresponding author Tel.: +52 22 2229 5500x7382; fax: +52 22 2229 5584.
∗∗ Corresponding author Tel.: +52 55 5747 3725; fax: +52 55 5747 3389.
E-mail addresses: msmeza@siu.buap.mx (S Meza-Reyes), rsantill@cinvestav.mx
(R Santillan).
The variants of 12-oxosapogenins, botogenin (1a) and heco-genin (2a) (Fig 1) are suitable starting materials for the synthesis of 9,11 and 11,12 steroidal epoxides[10], cephalostatines[2,11]and ritterazines[4,12], compounds with potential application for can-cer chemoprevention; also they have been used in the synthesis
of drugs such as cortisone, betamethasone[13,14], as well as the preparation of compounds which are potent cholesterol absorption inhibitors[15]
The C-12 ketone group shows remarkable low reactivity mainly attributed to the steric hindrance caused by the angular 18 and
C-19 methyl groups in the face This hypothesis was corroborated
by the reduction of the C-12 carbonyl group with NaBH4 and by hydrogenation[16–18]
The cleavage of the spiroketal ring has been widely investigated under a great variety of reaction conditions providing different kinds of skeletons, sometimes under similar reaction conditions [19–22] Suárez and coworkers described for the first time the cleavage of the E ring of spirostanic sapogenins catalyzed by BF3 obtaining the 22,26-epoxy-5␣-cholest-22-ene 3 with an excellent
yield, from a non-functionalized (25R)-spirostan compound[21] Singh and Dhar however, obtained the
22,26-epoxycholesta-3,5-dien-16-one skeleton 4, treating diosgenin with BF3·OEt2at high temperature[23] Similar results to those of Suárez were obtained
by our work group, thus the 22,26-epoxy-5
␣-cholest-22-en-12-one 5 was prepared by regioselective cleavage of the E ring of
(25R)-sapogenins such as hecogenin (Fig 2)[24,25] Further stud-ies showed that when the same reaction conditions are applied
to sarsasapogenin, a (25S)-sapogenin with a pronounced steric 0039-128X/$ – see front matter © 2010 Elsevier Inc All rights reserved.
Trang 2Fig 1 Spirostan sapogenins with a carbonyl group at C-12.
hindrance due to the axial C-27 methyl group, the
22,26-epoxy-5-cholest-22-ene derivative is obtained in a lower yield together
with the acetyl pseudosapogenins 6 and 7[26] Moreover,
modify-ing reaction conditions, the treatment with BF3·OEt2on sapogenins
could direct to the 22-oxocholestanic frameworks 8 and 9[27,28]
More recently, LaCour et al [29] reported that cleavage of
hecogenin (2a) with Ph3P·I2 at high temperatures gives an
aro-matic hexacyclic product derived from an intramolecular aldol
condensation In this transformation, aromaticity is achieved by
the elimination of four asymmetric centers
In continuation with our studies on the synthesis of new
steroidal derivatives, we report hereupon the synthesis of new
polycyclic frameworks from botogenin (1b) and hecogenin (2b)
acetates
2 Experimental
2.1 General methods
IR spectra were acquired on a FT-IR Perkin-Elmer Spectrum
GX spectrophotometer using KBr pellets, or a FT-IR Varian 640
spectrophotometer using ATR ( cm−1) Optical rotations [˛]25
D were obtained using dichloromethane or chloroform solutions on a
Perkin-Elmer 241 polarimeter NMR spectra (1H,13C, DEPT, HSQC,
HETCOR and COSY) were determined with a JEOL eclipse +400, and
a VARIAN Mercury 400 spectrometer, chemical shifts are stated
in ppm (ı), and are referred to the residual 1H signal (ı = 7.27)
or to the central13C triplet signal (ı = 77.0) for CDCl3, compound
12 was referred to the residual1H signals (ı = 8.74) or to the13C
pyridine-d5(ı = 150.35) HMBC and ROESY experiments were
per-formed on a JEOL ECA 500 Mass spectra were obtained at 70 eV
with a Hewlett Packard 5989A and a Hewlett Packard 6890A mass spectrometer High resolution mass spectra (HRMS) were deter-mined on an Agilent Technologies, model 1100 coupled MSD-TOF spectrophotometer with APCI as ionization source Thin-layer chro-matograms were developed on aluminum TLC pre-coated sheets with silica gel 60 with fluorescent indicator F254, visualized by UV and by calcinations with 50% sulfuric acid
2.2 Crystal structure determination
Crystals of 16 suitable for X-ray were obtained from hexane:
ethyl acetate, by slow evaporation of the solvent at room tem-perature Data collection was performed at 293 K on a Kappa CCD diffractometer with Mo K␣-radiation, = 0.7107 ˚A The structure was solved by direct methods SHELXS-86[30]and refined using CRYSTALS[31] All non-hydrogen atoms were refined anisotrop-ically The H atoms attached to O atoms were refined freely and the remaining H atoms were refined using a riding model Crystallographic data have been deposited at the Cambridge Crys-tallographic Center No 757895 Copy of the data can be obtained free of charge from CCDC, 12 Union Road, Cambridge CB2 1EZ, UK E-mail:deposit@ccdc.cam.ac.uk
2.2.1 Crystal data for compound16
C33H46O7, colorless prisms, formula weight M = 554.72, orthorhombic, P 212121, a = 11.7915(2) ˚A, b = 12.2100(2) ˚A,
c = 25.5888(6) ˚A, ˛ = ˇ = = 90◦, V = 3684.13(12) ˚A3 Z = 4,
Dx = 1.00 Mgm−3, = 0.07 mm−1, F(0 0 0) = 1200 Collected reflec-tions: 8564 within a theta range of 2.9–27.9◦ (−14 ≤ h ≥ 14,
−16 ≤ k ≥ 16, −33 ≤ l ≥ 33) Refinement: final R = 0.075, goodness-of-fit = 1.083; 8564 reflections, 362 parameters, maximum and
·OEt
Trang 3minimum difference electron densities were −0.24 e ˚A−3 and
0.14 e ˚A−3respectively
2.3 Synthesis of polycyclic steroidal derivatives from
12-oxosapogenin acetate with BF3·OEt2
In a 25 mL round bottom flask were dissolved 0.50 g (1.06 mmol)
of sapogenin acetate (1b or 2b) in 5.0 mL of dichloromethane;
0.70 mL (6.36 mmol) of acetic anhydride and 0.72 mL (4.24 mmol)
of BF3·OEt2were added slowly The mixture was stirred at 25◦C
and monitored by TLC until complete disappearance of starting
material (45 min) The reaction mixture was poured over ground
ice (20 g) and vigorously shaken The organic phase was extracted
with AcOEt, washed with brine (2× 20 mL), followed by
neutral-ization with saturated solution of NaHCO3, dried over anhydrous
Na2SO4and concentrated to dryness under vacuum
The crude product of the reaction with botogenin acetate (1b)
(0.60 g) was chromatographed over silica gel (230–400 mesh) using
hexane/AcOEt (90:10) to give 0.16 g of 10 (27%), 0.13 g of 12 (22%),
0.03 g of 14 (5%), 0.04 g of 16 (7%), and 0.17 g of 17 (29%).
The purification of the crude reaction product using hecogenin
acetate (2b) (0.54 g) afforded 0.06 g of 11 (12%), 0.31 g of 13 (57%),
0.02 g of 15 (4%), 6 mg of 5 (1%), and 16 mg of 18 (3%).
2.4 Acetolysis of 12-oxosapogenin acetate catalyzed by ZnCl2
To a solution of 0.50 g (1.06 mmol) of the 12-oxosapogenin
acetate 1b or 2b in 11.60 mL (105 mmol) of acetic anhydride, were
added 0.29 g (2.12 mmol) of anhydrous ZnCl2, then the mixture
was stirred at room temperature for 40 h Once finished, the
reac-tion was poured over ground ice (20 g) and vigorously shaken
The organic phase was extracted with AcOEt, washed with brine
(2× 20 mL), followed by neutralization with saturated solution of
NaHCO3, dried over anhydrous Na2SO4 then concentrated until
dryness under vacuum
The products from the reaction with botogenin acetate (1b)
(0.68 g) were isolated by column chromatography over silica gel
(230–400 mesh) using hexane/AcOEt (85:15) to give 0.55 g of 16
(81%), 0.10 g of 17 (10%) and 0.03 g of 19 (5%).
The crude reaction product of hecogenin acetate (2b) (0.54 g),
was separated by column chromatography over silica gel (230–400
mesh) affording 0.36 g of 5 (65%), 0.14 g of 18 (25%), and 0.05 g of
20 (10%).
2.5 Acid-catalyzed cyclization of 26-hydroxypentacyclic
derivatives (12–13) into hexacyclic frameworks (10–11)
In a 5 mL round bottom flask were dissolved 0.10 g (0.19 mmol)
of the corresponding 26-hydroxypentacyclic derivative (12 or 13)
in 3 mL of dichloromethane and 5.0L of glacial AcOH were added
This mixture was magnetically stirred at 25◦C and monitored by
TLC for 1 h The reaction mixture was poured over ground ice (3 g)
and vigorously shaken The organic phase was extracted with AcOEt
and washed with brine (2× 5 mL), followed by neutralization with
saturated NaHCO3solution, dried over Na2SO4and concentrated to
dryness under vacuum
The reaction products were isolated by column chromatography
over silica gel (230–400 mesh) using hexane/AcOEt (9:1) to give
0.093 g of 10 (97%) and 0.082 g of 11 (87%), respectively.
2.6 Base-catalyzed cyclization of 26-hydroxypentacyclic
derivative (13) into hexacyclic framework (11)
In a 10 mL round bottom flask were dissolved 0.10 g (0.19 mmol)
of 13 in 5.0 mL of dichloromethane and 8.5 mg de DMAP were
added The mixture was magnetically stirred at 25◦C and mon-itored by TLC for 1 h The reaction mixture was poured over ground ice (5 g) and vigorously shaken The organic phase was extracted with AcOEt, washed with 5% HCl solution (1× 10 mL), brine (2× 10 mL), followed by neutralization with saturated solu-tion of NaHCO3, dried over anhydrous Na2SO4and concentrated to dryness under vacuum
The reaction product was isolated by column chromatography performed over silica gel (230–400 mesh) using hexane/AcOEt (9:1)
to give 0.087 g of 11 (92%).
2.6.1 (20S, 25R)-12,23-cyclo-22,26-epoxycholesta-5, 11,22-triene-3ˇ,16ˇ-diyl diacetate (10)
Oil, [˛]25
D −81.5◦ (c 1.6, CH2Cl2); IR ¯vmax: 2928, 1732, 1682,
1634, 1244 cm−1; MS, m/z (%): 494 (M+ 10), 419 (6), 314 (100),
254 (6), 239 (97), 184 (4) HRMS calcd m/z for C31H42O5 (M+ +1): 495.3105; found: 495.3112 1H NMR, ı: 5.38 (1H, d,
J = 4.7 Hz, H-6), 5.28 (1H, ddd, J16␣-17␣= 8.0 Hz, J16␣-15␣= 7.0 Hz,
J16␣-15= 4.0 Hz, H-16␣), 4.98 (1H, s, H-11), 4.57 (1H, m, H-3␣ 3.94 (1H, dd, Jgem= 10.2 Hz, J26eq-25ax= 3.2 Hz, H-26eq), 3.29 (1H, dd,
Jgem= J26ax-25ax= 10.2 Hz, H-26ax), 2.46 (1H, m, H-20), 2.38 (1H, ddd,
Jgem= 13.0 Hz, J15 ␣-14␣= 7.0 Hz, J15 ␣-16␣= 7.0 Hz, H-15␣), 2.31 (1H,
dd, Jgem= 13.3 Hz, J4eq-3ax= 4.6 Hz, H-4eq), 1.97 and 1.98 (3H each,
s, 3-, 16-OCOCH3), 1.40 (1H, ddd, Jgem= 13.0 Hz, J15-14␣= 13.0 Hz,
J15-16␣= 4.0 Hz, H-15), 1.02 (3H, d, J21-20= 6.6 Hz, CH3-21), 0.97 (3H, s, CH3-19), 0.92 (3H, d, J27-25= 6.6 Hz, CH3-27), 0.86 (3H, s,
CH3-18).13C NMRı: 37.0 (C-1), 27.7 (C-2), 74.1 (C-3), 38.3 (C-4), 140.9 5), 122.8 6), 29.0 7), 31.1 8), 55.7 9), 38.3 (C-10), 110.0 (C-11), 144.9 (C-12), 41.4 (C-13), 50.5 (C-14), 35.6 (C-15), 73.9 (C-16), 55.1 (C-17), 17.5 (C-18), 19.6 (C-19), 31.1(C-20), 15.6 21), 154.6 22), 104.6 23), 31.4 24), 27.4 25), 71.6 (C-26), 17.2 (C-27), 170.6 and 170.5 (3-, 16-OCOCH3), 21.4 and 21.1 (3-,16-OCOCH3)
2.6.2 (20S,25R)-12,23-cyclo-22,26-epoxy-5 ˛-cholesta-11,22-diene-3ˇ,16ˇ-diyl diacetate (11)
Oil, [˛]25
D −27.1◦ (c 0.2, CHCl3); IR ¯vmax: 2958, 1735,
1244 cm−1; MS, m/z (%): 496 (M+, 100), 437 (37), 377 (8),
327 (55), 57 (12), 43 (16); HRMS calcd m/z for C31H44O5 (M++ 1): 496.3189; found 496.3194 1H NMR ı: 5.31 (1H, ddd, J16 ␣-17␣= 7.0 Hz, J16 ␣-15␣= 7.0 Hz, J16 ␣-15= 3.3 Hz,
H-16␣), 5.05 (1H, d, J = 2.0 Hz, H-11), 4.71 (1H, m, H-3␣), 3.99 (1H, dd Jgem= 10.4 Hz, J26eq-25ax= 3.2 Hz, H-26eq), 3.34 (1H, dd,
Jgem= J26ax-25ax= 10.4 Hz, H-26ax), 2.53 (1H, m, H-20), 2.42 (1H, ddd, Jgem= 13.8 Hz, J15 ␣-14␣= 7.0 Hz, J15 ␣-16␣= 7.0 Hz, H-15␣), 2.29 (1H, dd, Jgem= 16.0 Hz, J4eq-3ax= 5.6 Hz, H-4eq), 2.02 and 2.01 (3H each, s, 3-, 16-OCOCH3), 1.06 (3H, d, J21-20= 6.4 Hz, CH3-21), 0.96 (3H, d, J27-25= 6.8 Hz, CH3-27), 0.90 (3H, s, CH3-18), 0.83 (3H, s,
CH3-19);13C NMRı: 36.4 (C-1), 27.5 (C-2), 73.6 (C-3), 34.1 (C-4), 44.8 (C-5), 31.2 (C-6), 29.4 (C-7), 34.0 (C-8), 58.9 (C-9), 36.8 (C-10), 110.0 (C-11), 144.4 (C-12), 41.8 (C-13), 50.7 (C-14), 35.5 (C-15), 73.9 (C-16), 55.1 (C-17), 18.1 (C-18), 13.2 (C-19), 31.2 (C-20), 15.8 (C-21), 154.1 (C-22), 104.6 (C-23), 29.0 (C-24), 27.4 (C-25), 71.5 (C-26), 17.3 (C-27), 170.4 and 170.3 (3-, 16-OCOCH3), 21.5 and 21.2 (3-, 16-OCOCH3)
2.6.3 (20S,25R)-12,23-cyclo-26-hydroxy-22-oxocholesta-5,12(23)-diene-3ˇ,16ˇ-diyl diacetate (12)
Oil [˛]25
D +19.4◦(c 0.93, CH2Cl2); IR ¯vmax: 3426, 2898, 1734, 1654,
1616, 1246 cm−1 MS, m/z (%): 512 (M+, 10), 494 (59), 434 (66), 419 (78), 359 (94), 332 (49), 314 (100), 239 (83), 197 (13), 145 (8), 43 (5) HRMS calcd for C31H45O6(M++ 1): 513.3208; found: 513.3211.1H NMR (Py-d5)ı: 5.36 (2H, m, H-6 and H-16), 4.75 (1H, m, H-3␣ 3.83 (1H, dd, Jgem= 10.4 Hz, J26a-25= 5.6 Hz, H-26a), 3.76 (1H, dd,
Jgem= 10.4 Hz, J26b-25= 5.6 Hz, H-26b), 2.93 (1H, dd, Jgem= 13.0 Hz,
J = 5.0 Hz, H-7 ), 2.83 (1H, dd, J = 16.5 Hz, J = 5.2 Hz,
Trang 4H-11eq), 2.67 (1H, dq, J20-21= J20-17␣= 7.0 Hz, H-20), 2.44 (1H, ddd,
Jgem= 13.5 Hz, J15␣-14␣= 7.0 Hz, J15␣-16␣= 7.0 Hz, H-15␣), 2.29 (1H,
dd, Jgem= 16.5 Hz, J11ax-9ax= 12.6 Hz, H-11ax), 2.07 and 2.04 (3H
each, s, 3-, 16-OCOCH3), 1.29 (3H, d, J21-20= 7.0 Hz, CH3-21), 1.13
(3H, d, J27-25= 6.8 Hz, CH3-27), 1.06 (3H, s, CH3-19),1.05 (3H, s,
CH3-18) 1H NMR (CDCl3) ı: 5.40 (1H, d, J = 5.0 Hz, H-6), 5.25
(1H, ddd, J16␣-17␣= J16␣-15␣= 7.0 Hz, J16␣-15= 3.2 Hz, H-16␣), 4.60
(1H, m, H-3␣), 3.26 (1H, dd, Jgem= 10.4 Hz, J26a-25= 5.6 Hz, H-26a),
3.18 (1H, dd, Jgem= 10.4, J26b-25= 5.6 Hz, H-26b), 2.68 (1H, dd,
Jgem= 16.5 Hz, J11eq-9ax= 5.2 Hz, H-11eq), 2.60 (1H, q, J20-21= 6.6 Hz,
H-20), 2.54 (1H, ddd, Jgem= 13.5 Hz, J15␣-16␣= J15␣-14␣= 7.0 Hz,
H-15␣), 2.36 (1H, dd, Jgem= 13.5 Hz, J4eq-3ax= 4.7 Hz, 4eq), 1.46 (1H,
ddd, Jgem= J15-14␣= 13.5 Hz, J15-16␣= 3.2 Hz, H-15), 2.04 and 2.03
(3H each, s, 3-, 16-OCOCH3), 1.14 (3H, d, J21-20= 6.6 Hz, CH3-21),
1.12 (3H, s, CH3-18), 1.07 (3H, s, CH3-19), 0.88 (3H, d, J27-25= 6.8 Hz,
CH3-27).13C NMR (Py-d5)ı: 36.8 (C-1), 27.9 (C-2), 73.4 (C-3), 38.4
(C-4), 139.9 (C-5), 122.2 (C-6), 28.5 (C-7), 29.9 (C-8), 51.6 (C-9), 37.2
10), 25.2 11), 164.0 12), 43.1 13), 54.1 14), 35.0
(C-15), 73.6 (C-16), 55.5 (C-17), 15.3 (C-18), 19.1 (C-19), 39.1 (C-20),
13.5 (C-21), 201.3 (C-22), 131.3 (C-23), 31.3 (C-24), 37.0 (C-25),
67.4 (C-26), 17.1 (C-27), 170.1, 170.0 (3-, 16-OCOCH3), 21.1, 20.8
(3-, 16-OCOCH3)
2.6.4 (20S,25R)-12,23-cyclo-26-hydroxy-22-oxo-5
˛-cholesta-12(23)-ene-3ˇ,16ˇ-diyl diacetate
(13)
Oil, [˛]25
D +59.4◦ (c 0.11, CHCl3); IR ¯vmax: 3475, 1732,
1660, 1244 cm−1; MS, m/z (%): 514 (M+, 5), 496 (100), 453
(10), 437 (11), 55 (30), 43 (65); HRMS calcd for C31H47O6
(M++ 1) 515.3367, found 515.3382 1H NMR ı: 5.23 (1H, ddd,
J16 ␣-17␣= 7.0 Hz, J16 ␣-15␣= 7.0 Hz, J16 ␣-15= 3.2 Hz, H-16␣), 4.67
(1H, m, H-3␣), 3.36 (1H, dd, Jgem= 10.5 Hz, J26a-25= 5.7 Hz,
H-26a), 3.29 (1H, dd, Jgem= 10.5 Hz, J26b-25= 5.7 Hz, H-26b), 2.64
(1H, dd, Jgem= 16.0 Hz, J11eq-9ax= 4.8 Hz, H-11eq), 2.60 (1H, dq,
J20-17␣= 14.0 Hz, J20-21= 6.9 Hz, H-20), 2.51 (1H, ddd, Jgem= 14.2 Hz,
J15␣-16␣= J15␣-14␣= 7.0 Hz, H-15␣), 2.10 (1H, dd, Jgem= 16.0 Hz,
J11ax-9ax= 13.0 Hz, H-11ax), 1.96 and 1.95 (3H each, s, 3-,
16-OCOCH3), 1.06 (3H, d, J21-20= 6.9 Hz, CH3-21), 0.99 (3H, s, CH3-18),
0.86 (3H, s, CH3-19), 0.81 (3H, d, J27-25= 6.6 Hz, CH3-27);13C NMR
ı: 36.1 (C-1), 27.3 (C-2), 73.1 (C-3), 33.9 (C-4), 44.7 (C-5), 31.4
(C-6), 28.0 (C-7), 33.7 (C-8), 53.9 (C-9), 36.7 (C-10), 25.4 (C-11),
165.0 (C-12), 43.4 (C-13), 55.3 (C-14), 34.8 (C-15), 73.3 (C-16),
55.4 (C-17), 15.8 (C-18), 12.2 (C-19), 39.0 (C-20), 13.3 (C-21),
202.6 (C-22), 130.8 (C-23), 27.4 (C-24), 36.4 (C-25), 66.4 (C-26),
17.1 (C-27), 170.3 and 170.1 (3-, 16-OCOCH3), 21.4 and 21.1
(3-, 16-OCOCH3)
2.6.5 (20S,
25R)-12,23-cyclo-22-oxocholesta-5,12(23)-diene-3ˇ,16ˇ,
26-triyl triacetate (14)
Oil, [˛]25
D +54.6◦ (c 0.2, CHCl3); IR max: 2927, 2858,
1733, 1657, 1615, 1235 cm−1; MS, m/z (%): 554 (M+, 1), 494
(90), 434 (100), 419 (46), 359 (57), 314 (66); HRMS calcd
For C31H47O6 (M+-60) 494.3032, found 494.3025 1H NMR ı:
5.41 (1H, d, J = 5.0 Hz, H-6), 5.25 (1H, ddd, J16␣-15␣= 10.6 Hz,
J16␣-17␣= 7.0 Hz, J16␣-15= 3.7 Hz, H-16␣), 4.61 (1H, m, H-3␣
3.93 (1H, dd, Jgem= 10.5 Hz, J26a-25= 5.9 Hz, H-26a), 3.90 (1H, dd,
Jgem= 10.5 Hz, J26b-25= 5.9 Hz, H-26b), 2.62 (1H, dd, Jgem= 13.5 Hz,
J11eq-9ax= 6.0 Hz, H-11eq), 2.53 (1H, q, J20-21= 7.0 Hz, H-20), 2.47
(1H, dd, Jgem= 14.0 Hz, J15 ␣-14␣= 7.0 Hz, H-15␣), 2.39 (1H, dd,
Jgem= 13.5 Hz, J4eq-3ax= 6.0 Hz, H-4eq), 2.24 (1H, dd, Jgem= 13.5 Hz,
J11ax-9ax= 7.9 Hz, H-11ax), 2.06, 2.05 and 2.04 (3H each, s, 3-,
16-, 26-OCOCH3), 1.46 (1H, ddd, Jgem= 14.0 Hz, J15-14␣= 11.6 Hz,
J15-16␣= 3.7 Hz, H-15), 1.13 (3H, d, J21-20= 7.0 Hz, CH3-21), 1.11
(3H, s, CH3-19), 1.08 (3H, s, CH3-18), 0.83 (3H, d, J27-25= 6.9 Hz,
CH3-27).13C NMRı: 36.9 1), 27.7 2), 73.5 3), 38.1 (C-4), 139.6 (C-5), 122.1 (C-6), 28.4 (C-7), 31.2 (C-8), 51.5 (C-9), 37.1 (C-10), 25.0 (C-11), 164.5 (C-12), 43.2 (C-13), 54.2 (C-14), 35.0 (C-15), 73.5 (C-16), 55.6 (C-17), 15.6 (C-18), 19.3 (C-19), 39.0 (C-20), 13.2 (C-21), 201.6 (C-22), 130.6 (C-23), 30.0 (C-24), 32.8 (C-25), 69.0 (C-26), 16.7 (C-27), 171.3 and 170.5 (3-, 16-, 26-OCOCH3two signals are overlapped), 21.4, 21.2 and 21.0 (3-, 16-, 26- OCOCH3)
2.6.6
(20S,25R)-12,23-cyclo-22-oxo-5˛-cholesta-12(23)-ene-3ˇ,16ˇ, 26-triyl triacetate (15)
Oil, [˛]25
D +54.2◦(c 0.2, CHCl3); IR ¯vmax: 1737, 1660, 1244 cm−1;
MS, m/z (%): 556 (M+, 1), 496 (100), 438 (27), 421 (51), 56 (39), 43 (63);1H NMRı: 5.25 (1H, ddd, J16 ␣-15␣= J16 ␣-17␣= 7.0 Hz,
J16␣-15= 3.0 Hz, H-16␣), 4.66 (1H, m, H-3␣), 3.86 (1H, dd,
Jgem= 10.4 Hz, J26a-25= 6.0 Hz, H-26a), 3.84 (1H, dd, Jgem= 10.4,
J26b-25= 6.0, H-26b), 2.62 (1H, dd, Jgem= 16.0 Hz, J11eq-9ax= 5.0 Hz, H-11eq), 2.54 (1H, dq, J20-17␣= 12.2 Hz, J20-21= 6.8 Hz, H-20), 2.51 (1H, ddd, Jgem= 14.1 Hz, J15␣-16␣= J15␣-14␣= 7.0 Hz H-15␣), 2.03, 2.01 and 2.00 (3H each, s, 3-, 16-, 26-OCOCH3), 1.04 (3H, s, CH3-18), 1.10 (3H, d, J21−20= 6.8 Hz, CH3-21), 0.91 (3H, s, CH3-19), 0.81 (3H,
d, J27-25= 7.2 Hz, CH3-27);13C NMRı: 36.1 (C-1), 27.3 (C-2), 73.1 (C-3), 33.9 (C-4), 44.7 (C-5), 31.4 (C-6), 28.0 (C-7), 33.7 (C-8), 54.0 9), 36.5 10), 25.3 11), 164.4 12), 43.4 13), 55.2 (C-14), 34.9 (C-15), 73.4 (C-16), 55.4 (C-17), 15.8 (C-18), 12.2 (C-19), 39.0 (C-20), 13.2 (C-21), 201.7 (C-22), 130.5 (C-23), 28.2 (C-24), 32.7 (C-25), 69.0 (C-26), 16.6 (C-27), 171.2, 170.6 and 170.5 (3-, 16-, 26- OCOCH3), 21.4, 21.1 and 20.9 (3-, 16-, 26-OCOCH3); Anal calcd for C33H48O7: C 71.19, H 8.69, O 20.12 Found: C 71.19, H 8.98
2.6.7 (25R)-23-acetyl-22,26-epoxy-12-oxocholesta-5,22-diene-3ˇ,16ˇ-diyl diacetate (16)
Colorless crystals, m.p 111–113◦C (hexane/ethyl acetate) [˛]25
D +30.9◦ (c 1.0, CH2Cl2); IR ¯vmax: 1733, 1720, 1664, 1570, 1372,
1246 cm−1 MS, m/z (%): 554 (M+, 7), 479 (32), 451 (17), 206 (100),
205 (54), 179 (53), 163 (45), 116 (19), 43 (8) HRMS calcd m/z for C33H47O7 (M+ +1): 555.3320; found: 555.3316 1H NMR, ı: 5.39 (1H, d, J = 5.8 Hz, H-6), 5.16 (1H, ddd, J16 ␣-17␣= J16 ␣-15␣= 7.8 Hz,
J16␣-15= 3.5 Hz, H-16␣), 4.58 (1H, m, H-3␣), 4.04 (1H, dd,
Jgem= 10.3 Hz, J26eq-25ax= 3.2 Hz, H-26eq), 3.96 (1H, dq, J20-17␣= 11.2,
J20-21= 7.1 Hz, H-20), 3.45 (1H, dd, Jgem= J26ax-25ax= 10.3 Hz,
H-26ax), 2.77 (1H, dd, J17␣-16␣= 7.8 Hz, J17␣-20= 11.2 Hz, H-17␣ 2.71 (1H, dd, Jgem= J11ax-9ax= 12.8 Hz, H-11ax), 2.21 (1H, dd,
Jgem= 12.8 Hz, J11eq-9ax= 8.2 Hz, H-11eq), 2.17 (3H, s, 23-COCH3), 2.03 (3H, s, 3-OCOCH3), 1.85 (3H, s, 16-OCOCH3), 1.71 (1H, ddd, Jgem= 13.5 Hz, J1eq-2eq= J1eq-2ax= 3.5 Hz, H-1eq), 1.49 (1H, ddd,
J9ax-11ax= 12.8 Hz, J9ax-11eq= 8.2 Hz, J9ax-8ax= 11.3 Hz, H-9ax) 1.26 (3H, s, CH3-19), 1.17 (1H, dd, Jgem= 13.5 Hz, J1ax-2eq= 3.5 Hz, H-1ax), 1.14 (3H, s, CH3-18), 1.09 (3H, d, J21-20= 7.1 Hz, CH3-21), 0.97 (3H,
d, J27-25= 6.2 Hz, CH3-27).13C NMR,ı: 36.7 (C-1), 27.6 (C-2), 73.5 (C-3), 37.9 (C-4), 139.7 (C-5), 122.1 (C-6), 31.3 (C-7), 31.7 (C-8), 53.9 (C-9), 37.6 (C-10), 38.0 (C-11), 213.4 (C-12), 56.8 (C-13), 55.9 (C-14), 34.8 (C-15), 73.9 (C-16), 46.9 (C-17), 12.6 (C-18), 19.1 (C-19), 33.0 (C-20), 19.4 (C-21), 171.2 (C-22), 106.9 (C-23), 31.9 (C-24), 26.6 (C-25), 71.7 (C-26), 16.9 (C-27), 198.0 (23-COCH3), 170.4 (3-, 16-OCOCH3, two signals overlapped), 29.6 (23-COCH3), 21.4 and 21.1 (3-, 16-OCOCH3) Anal calcd for C33H46O7: C 71.45, H 8.36, O 20.19 Found: 71.49, H 8.60
2.6.8 (25R)-23-acetyl-22,26-epoxy-12-oxo-5
˛-cholesta-22-ene-3ˇ,16ˇ-diyl diacetate (5)
Colorless crystals m.p 195◦C (MeOH), [˛]25
D + 37.6◦ (c 0.62, CHCl3); IR¯max: 1732, 1708, 1665 cm−1 MS, m/z (%): 556 (M+).1H NMRı: 5.13 (1H, ddd, J16␣-17␣= J16␣-15␣= 8.2 Hz, J16 ␣-15= 4.1 Hz,
Trang 5H-16␣), 4.66 (1H, m, H-3␣), 4.03 (1H, dd, Jgem= 10.4 Hz,
J26eq-25ax= 3.6 Hz, H-26eq), 3.98 (1H, m, H-20), 3.45 (1H, dd,
Jgem= J26ax-25ax= 10.4 Hz, H-26ax), 2.75 (1H, dd, J17 ␣-16␣= 8.2 Hz,
J17␣-20= 11.2, H-17␣), 2.59 (1H, dd, Jgem= J11ax-9ax= 12.8 Hz,
H-11ax), 2.17 (3H, s, 23-COCH3), 2.01 (3H, s, 3-OCOCH3), 1.84 (3H,
s, 16-OCOCH3), 1.23 (3H, s, CH3-18), 1.08 (3H, d, J20-21= 7.0 Hz,
CH3-21), 0.97 (3H, d, J27-25= 6.8 Hz, CH3-27), 0.93 (3H, s, CH3-19)
13C NMRı: 36.3 1), 27.2 2), 73.1 3), 33.7 4), 44.4
(C-5), 28.3 (C-6), 31.8 (C-7), 35.0 (C-8), 57.1 (C-9), 36.3 (C-10), 38.3
11), 213.2 12), 57.0 13), 55.5 14), 34.7 15), 73.8
(C-16), 46.8 (C-17), 12.7 (C-18), 11.9 (C-19), 33.0 (C-20), 19.4 (C-21),
171.0 (C-22), 106.6 (C-23), 31.1 (C-24), 26.5 (C-25), 71.5 (C-26),
16.9 (C-27), 197.7 (23-COCH3), 170.3 and 170.1 (3-, 16-OCOCH3),
29.6 (23-COCH3), 21.4 and 21.1 (3-, 16-OCOCH3) Anal calcd for
C33H48O7: C 71.19, H 8.69, O 20-12 Found C 71.03, H 8.69
2.6.9 (E)-(20S,
25R)-20,23-diacetyl-12-oxofurosta-5,22-diene-3ˇ,26-diyl diacetate (17)
Oil, [˛]25
D −5.1◦(c 2.7, CH2Cl2); IR ¯vmax: 2928, 1732, 1708, 1667,
1374, 1243, 1034 cm−1 MS, m/z (%): 596 (M+, 7), 554 (24), 494,
(19), 451 (28), 391 (27), 266 (42), 223 (100), 205 (84), 163 (44), 121
(16), 43 (23) HRMS calcd for C35H48O8(M++ 1): 597.3421; found:
597.3437.1H NMRı: 5.42 (1H, d, J = 5.0 Hz, H-6), 4.70 (1H, ddd,
J16 ␣-15␣= J16 ␣-17␣= 6.9 Hz, J16␣-15= 4.0 Hz, H-16␣), 4.53 (1H, m,
H-3 ), 3.95 (2H, d, Jgem= 6.1 Hz, H-26), 2.76 (1H, d, J17␣-16␣= 6.9 Hz,
H-17␣), 2.55 (1H, dd, Jgem= J11ax-9ax= 13.3 Hz, H-11ax), 2.45 (3H, s,
20-COCH3), 2.13 (3H, s, 23-COCH3), 2.01 and 2.00 (3H each, s, 3-,
26-OCOCH3), 1.72 (1H, dd, Jgem= 13.2 Hz, J1eq-2eq= 3.4 Hz, H-1eq),
1.56 (3H, s, CH3-21), 1.18 (3H, s, CH3-18), 1.10 (3H, s, CH3-19),
0.92 (3H, d, J27-25= 6.8 Hz, CH3-27).13C NMR ı: 36.6 (C-1), 27.5
2), 73.4 3), 37.9 4), 139.6 5), 121.8 6), 31.1
(C-7), 30.8 (C-8), 52.6 (C-9), 37.4 (C-10), 37.2 (C-11), 212.6 (C-12),
56.2 (C-13), 57.4 (C-14), 31.7 (C-15), 83.4 (C-16), 56.4 (C-17),
15.1 (C-18), 19.0 (C-19), 61.8 (C-20), 17.0 (C-21), 174.6 (C-22),
109.6 (C-23), 31.9 (C-24), 33.3 (C-25), 68.8 (C-26), 17.3 (C-27),
206.9 (20-COCH3), 199.0 (23-COCH3), 170.5 and 170.3 (3-,
26-OCOCH3), 28.7 (23-COCH3), 26.1 (20-COCH3), 21.4 and 21.0 (3-,
26-OCOCH3)
2.7 (E)-(20S, 25R)-20,23-diacetyl-12-oxo-5
˛-furosta-22-ene-3ˇ, 26-diyl diacetate (18)
Colorless crystals m.p 172–174◦C, [˛]25
D −10.7◦(c 1.6, CHCl3);
IR ¯vmax: 1735, 1707, 1667, 1362 cm−1 MS, m/z (%): 598(M+, 23), 556
(52), 496 (33), 453 (71), 43 (100).1H NMRı: 4.68 (2H, m, 3,
H-16␣), 3.97 (2H, m, H-26), 2.74 (1H, d, J17␣-16␣= 7.0 Hz, H-17␣), 2.46
(3H, s, 20-COCH3), 2.17 (3H, s, 23-COCH3), 2.05 and 2.03 (3H each,
s, 3-, 26-OCOCH3), 1.57 (3H, s, CH3-21), 1.17 (3H, s, CH3-18), 0.94
(3H, d, J27-25= 7.0 Hz, CH3-27), 0.92 (3H, s, CH3-19).13C NMRı: 36.2
1), 27.0 2), 72.9 3), 33.5 4), 44.2 5), 27.9 6), 30.8
(C-7), 33.0 (C-8), 57.0 (C-9), 36.0 (C-10), 37.4 (C-11), 212.4 (C-12), 56.3
13), 55.7 14), 31.3 15), 83.2 16), 56.3 17), 14.9
(C-18), 11.7 (C-19), 61.6 (C-20), 16.8 (C-21), 174.5 (C-22), 109.3 (C-23),
31.7 (C-24), 34.1 (C-25), 68.6 (C-26), 17.1 (C-27), 206.7 (20-COCH3),
198.7 (C, 23-COCH3), 171.0 and 170.5 (3-, 26- OCOCH3); 28.5 (CH3,
23-COCH3), 25.9 (20-COCH3), 21.2 and 20.8 (3-, 26-OCOCH3) Anal
calcd for C35H50O8, C 69.55, H 8.58, O 21.61 Found C 70.07, H 8.16
2.8 (E)-(25R)-23-acetyl-12-oxofurosta-5,22-diene-3ˇ, 26-diyl
diacetate (19)
Oil, [˛]25
D +36.7◦(c 1.2, CH2Cl2) IR ¯vmax: 2856, 1732, 1706, 1656,
1240, 1030 cm−1 MS, m/z (%): 554 (M+, 16), 494 (25), 451 (27),
391 (28), 223 (58), 205 (100), 206 (47), 163 (52), 97 (24), 71(25),
60 (26), 43 (34) HRMS calcd for C H O (M+ +1): 555.3316;
found: 555.3341 1H NMR ı: 5.40 (1H, d, J = 5.0 Hz, H-6), 4.91 (1H, ddd, J16 ␣-15␣= J16 ␣−17␣= 7.2 Hz, J16␣-15= 4.2 Hz, H-16␣), 4.57 (1H, m, H-3␣), 3.92 (1H, dd, Jgem= 11.5 Hz, J26a-25= 5.0 Hz, H-26a), 3.86 (1H, dd, Jgem= 11.5 Hz, J26b-25= 5.0 Hz, H-26b), 3.74 (1H, q,
J20-21= 7.1 Hz, H-20), 2.63 (1H, d, J17 ␣-16␣= 7.2, H-17␣), 2.55 (1H, dd,
Jgem= 13.0 Hz, J11ax-9ax= 8.0 Hz, H-11ax), 2.24 (1H, dd, Jgem= 13.0 Hz,
J11eq-9ax= 6.3 Hz, H-11eq), 2.19 (3H, s, 23-COCH3), 2.04 and 2.02 (3H each, s, 3-, 26-OCOCH3), 1.24 (3H, d, J21-20= 7.1 Hz, CH3-21), 1.10 (3H, s, CH3-19), 0.94 (3H, s, CH3-18), 0.93 (3H, d, J27-25= 7.1 Hz, CH3 -27).13C NMRı: 36.7 (C-1), 27.6 (C-2), 73.4 (C-3), 38.0 (C-4), 139.8 (C-5), 122.1 (C-6), 31.3 (C-7), 30.8 (C-8), 52.2 (C-9), 37.4 (C-10), 36.9 11), 212.0 12), 55.6 13), 54.9 14), 33.2 15), 84.3 (C-16), 53.9 (C-17), 13.4 (C-18), 19.0 (C-19), 39.0 (C-20), 19.4 (C-21), 178.0 (C-22), 108.1 (C-23), 31.6 (C-24), 33.3 (C-25), 69.0 (C-26), 17.3 (C-27), 198.0 (23-COCH3), 171.3 and 170.6 (3-, 26-OCOCH3), 29.3 (23-COCH3), 21.5 and 21.1 (3-, 26-OCOCH3)
2.9 (E)-(25R)-23-acetyl-12-oxo-5˛-furosat-22-ene-3ˇ, 26-diyl diacetate (20)
Colorless crystals m.p 144–145◦C, [˛]25
D +83.56◦(c 1.0, CHCl3);
IR ¯vmax: 1732, 1707, 1665, 1243 cm−1 MS, m/z (%): 556(M+, 24), 496 (30), 453 (52), 205 (48), 163 (33), 43 (100).1H NMRı: 4.84 (1H, m, H-16␣), 4.61 (1H, m, H-3), 3.84 (2H, m, H-26), 3.69 (1H, m, H-20), 2.54 (1H, d, J17 ␣-16␣= 7.3 Hz, H-17␣), 2.14 (3H, s, 23-COCH3), 1.99 and 1.96 (3H each, s, 3-, 26-OCOCH3), 1.18 (3H, d, J21-20= 7.0 Hz,
CH3-21), 0.88 (3H, d, J27-25= 7.0 Hz, CH3-27), 0.87 (3H, s, CH3-19), 0.85 (3H, s, CH3-18),13C NMRı: 36.3 (C-1), 27.2 (C-2), 73.1 (C-3), 33.8 4), 44.5 5), 28.1 6), 31.3 7), 33.3 8), 55.6 (C-9), 36.3 (C-10), 37.3 (C-11), 212.5 (C-12), 55.8 (C-13), 54.7 (C-14), 33.0 (C-15), 84.2 (C-16), 54.0 (C-17), 11.9 (C-18), 13.5 (C-19), 38.9 20), 19.3 21), 177.8 22), 108.1 23), 31.4 24), 34.2 (C-25), 68.9 (C-26), 17.3 (C-27), 198.4 (23-COCH3), 171.2 and 170.6 (3-, 26-OCOCH3); 29.2 (23-COCH3), 21.4 and 21.1 (3-, 26-OCOCH3) Anal calcd for C33H48O7, C 71.05, H 8.67, O 20.32 Found C 71.22, H 8.63
3 Results and discussion
The one pot BF3·OEt2catalyzed reaction of botogenin (1b) and hecogenin (2b) acetates in acetic anhydride using dichloromethane
as solvent proceeded under mild conditions to give hexa- and
pen-tacyclic compounds (10–15) with retention of configuration of all
asymmetric centers (Scheme 1) The effect of the concentration of acetic anhydride and the use of CH2Cl2on the regioselectivity of the reaction was evaluated The use of strong mineral acids and high temperatures was avoided to retain the stereochemistry of all chi-ral centers, since previous reports describe that these conditions promote inversion of configuration and epimerization[29] Tables 1 and 2summarize the product distribution and the
reac-tion condireac-tions for the formareac-tion of derivatives 5, 10–20 The results
indicate that formation of penta- and hexacyclic compounds is favored under low acetic anhydride concentrations, while condi-tions similar to those described in the literature direct mainly to
the 22,26-epoxy (16 and 5) and furostene (17–20) derivatives, as
in entries 2, 3, 5, and 6[24] The selectivity to obtain the hexacyclic derivatives was not increased even at low temperatures (−30 or 0◦C).
The preference for the E ring opening has been documented and attributed to: (a) the higher stability of the tetrahydropyran ring under several acidic reaction conditions[32–34], (b) the use of hard Lewis acids (as compared to the use of softer BBr3or Ph3P·I2 where the cleavage is directed toward the F ring)[21,29]and (c) the higher reactivity based on the larger basicity of the tetrahydrofuran oxygen[21,35]
Trang 6Scheme 1 Acetolysis of botogenin (1b) and of hecogenin (2b) acetate.
Table 1
Product distribution in the spiroketal cleavage of botogenin acetate (1b) at 25◦ C.
a Reaction in 5 mL of CH 2 Cl 2
To explain the formation of compounds 10–15, we propose a
reaction mechanism (Scheme 2) in which the ring E opening is
acti-vated by the Lewis acid The first step involves the E ring opening
promoted by the BF3·OEt2producing the oxocarbenium ion A The
elimination of a proton from position 23, leads to the
dihydropy-ranic intermediate B Rotation of the bond between C-17 and
C-20 allows to attain an adequate disposition of the enol ether to
attack the system of the carbonyl group at C-12, directing to the
tertiary alcoholate C The loss of the proton in 23 allows the
tetrahy-dropyranic oxygen atom to recover its electronic pair, to afford D.
Elimination of the fluoroborate from D provides the oxocarbenium
ion E, which is the common intermediate; when the H-11 is
elimi-nated compound 10 or 11 are obtained, while attack by the acetate
nucleophile yields 14 or 15 During the work-up, the intermediate
E can be attacked by H2O directing to 12 or 13.
The pentacyclic compounds (12 and 13) can undergo acid
or base-catalyzed cyclization (Scheme 3), a process which is
extremely fast and nearly quantitative When compound 13 is
allowed to stand in AcOH/CH2Cl2 or is treated with catalytic
amounts of 4-dimethylaminopyridine, compound 11 is obtained
in almost quantitative yields
The formation of the epoxycholestanes (5 and 16) has
been visualized from the intermediate B, under a mechanism
already proposed [24] On the other hand, the generation of
furostenes 17–20 implies the regioselective cleavage of ring F
[26]
3.1 Spectroscopic analysis The structures of the compounds were unambiguously estab-lished using two-dimensional NMR experiments The assignments were accomplished by combined utilization of DEPT, COSY, HETCOR experiments at 400 MHz, as well as comparison with previously reported data on steroidal sapogenins[24–26,36,37] ROESY and long-range connectivity from HMBC experiments were performed
at 500 MHz (see Supplementary information) Additional infor-mation was obtained from IR, and mass spectroscopy Some characteristic1H and13C NMR signals are shown inTables 3 and 4
In1H NMR, compounds 10 and 11 show a characteristic signal
aroundı = 4.99 (ı 0.1 ppm) for the vinylic proton (H-11), that is confirmed by the signal at 110.0 ppm in the13C NMR spectra In the HMBC spectrum H-11 shows a three bond correlation with
C-13 (41.4 ppm) and C-10 (38.3 ppm) which are quaternary carbons
In addition, the HMBC experiment allows the assignment of C-22 from the three bonds correlation with Me-21; C-12 correlates with Me-18 and C-5 with Me-19 The AMX system gives signals at 3.94 (ı 0.01 ppm) and 3.29 (ı 0.04 ppm) which correspond to the diastereotopic protons of H-26eqand H-26axrespectively coupled with H-25, with aı = 0.65 ppm indicative of a cyclic system In
13C NMR there are 4 carbon signals for C-22, C-23, C-12 and C-11 at (ı 154, 104, 144 and 110 respectively), these chemical shifts agree with the conjugated diene system present in the molecule, as is
Table 2
Product distribution in the spiroketal cleavage of hecogenin acetate (2b) at 25◦ C.
a Reaction in 5 mL of CH Cl
Trang 7Scheme 2 Reaction pathway for the formation of compounds 10–15.
corroborated by the IR spectra that shows two bands in 1682 and
1634 cm−1 Additionally, in1H NMR spectra the singlet signal for
Me-18 is slightly shielded (ı 0.14 ppm) due to the proximity of
the system
Derivatives 12 and 13 show a distinctive carbonyl signal at 202.2
(ı 1.0 ppm) in13C NMR which corresponds to C-22, also the
sig-nals atı = 164.7 ± 0.7 ppm (C-12) and 131.2 ± 0.1 ppm (C-23) for
the␣,-unsaturated system On the other hand the1H NMR
spec-tra show a distinctive ABX system for the diastereotopic protons of
C-26 around 3.6 ppm, this is characteristic for open chain steroids
systems The ROESY experiment allowed establishing the
config-uration at C-20 as (S), furthermore the scalar coupling between
H-20 and H-17 is 7.0 Hz evidences the same orientation for both
protons IR spectroscopy proved the presence of the 26-hydroxyl group from the broad band around 3500 cm−1, two strong bands at
1730 and 1655 cm−1are distinctive frequencies for the acetate and conjugated carbonyls groups, respectively
The NMR spectra of steroidal derivatives 14 and 15 are very similar to analogs 12 and 13, except for the 3 signals for
CH3COO around 2.03, the signals in the 13C NMR at 201, 130 and 164 are assigned to the ketone and␣,-unsaturated system, respectively
Derivatives 5 and 16–20 show distinctive signals for the
diastereotopic protons at C-11, which in these compounds are eas-ily observed, due to the deshielding effect on the H-11ax with respect to the H-11eq This difference in chemical shifts can be
Trang 8Table 3
Selected 1 H chemical shiftsı (ppm) of 5, 10–20 in CDCl3
12 a 5.25 (m) 3.83 (dd) 26a 3.76 (dd) 26b 2.83 (dd) 11 eq 2.29 (dd) 11 ax 2.67 (dq) 1.29 (d) 1.13 (d) 1.05 (s) 1.06 (s)
13 5.23 (m) 3.36 (dd) 26a 3.29 (dd) 26b 2.64 (dd) 11 eq 2.10 (dd) 11 ax 2.60 (dq) 1.06 (d) 0.81 (d) 0.99 (s) 0.86 (s)
14 5.25 (m) 3.93 (dd) 26a 3.90 (dd) 26b 2.62 (dd) 11 eq 2.24 (dd) 11 ax 2.53 (dq) 1.13 (d) 0.83 (d) 1.08 (s) 1.11 (s)
15 5.25 (ddd) 3.86 (dd) 26a 3.84 (dd) 26b 2.62 (dd) 11 eq 2.54 (dq) 1.10 (d) 0.81 (d) 1.04 (s) 0.91 (s)
16 5.16 (ddd) 4.04 (dd) 3.45 (dd) 2.71 (dd) 11 ax 2.21 (dd) 11 eq 3.96 (dq) 1.09 (d) 0.97 (d) 1.14 (s) 1.26 (s)
19 4.91 (ddd) 3.92 (dd) 26a 3.86 (dd) 26b 2.55 (dd) 11 ax 2.24 (dd) 11 eq 3.74 (q) 1.24 (d) 0.93 (d) 0.94 (s) 1.10 (s)
a Determined in Py-d 5
Fig 3 Some ROESY interactions observed in 10, 12, 14 and 17.
Table 4
13 C chemical shiftsı (ppm) for representative signals of 5, 10–20.
Carbon Compound
a Determined in Py-d
Trang 9Fig 4 X-ray of (25R)-23-acetyl-22,26-epoxy-12-oxocholesta-5,22-diene-3,16-diyl diacetate (16) Ellipsoids are drawn at the 35% probability level.
explained by: (a) the carbonyl effect, which deshields the alpha
axial protons 0.3 ppm more than the equatorial ones[38–40]and
(b) through steric compression (also called van der Waals
deshield-ing) [40–43] Besides, there is a considerable deshielding (ı
1.2 ppm) of H-17␣due to the␥ effect of the ␣,-conjugated system
[44]
In the1H NMR spectra of 5 and 16 the distinctive signal of the
AMX system for the diastereotopic protons H-26 at 3.75± 0.3 ppm
is characteristic of a cyclic system in the F ring The13C NMR spectra
of 5 and 16 confirmed the presence of a carbonyl from the signal at
213 ppm (C-12), the␣,-unsaturated system in the F ring gives
sig-nals at 198, 106, 171 ppm, this is corroborated in the IR spectra from
the stretching bands for C O in 1733 (acetate) and 1664 cm−1
(con-jugated carbonyl) The signals1H and13C NMR are in accordance
with those analogs previously reported[24–26]
As expected, the1H NMR spectra of furostenes 17–20 show
signals at 4.80± 0.1 ppm (H-16) where the E ring remains
unaf-fected, as compared to those of compounds 5 and 10–16, this proton
appears deshielded (5.22 ppm) due to the acetate group from the
E ring cleavage In13C NMR the signal at 84 ppm (C-16) provides
evidence that the cleavage did not occurred at the E ring, this is in
contrast with compounds 5 and 10–16, where the E ring is cleaved
and C-16 appears at 74 ppm Particularly in compounds 17 and 18,
the H-26 protons give a doublet signal in the region of 3.95 ppm
(J = 6.1 Hz), which is a distinctive signal for open side chain steroid
systems The signal at 1.56 ppm corresponds to the-Me-21 which
shows a singlet signal due the presence of the␣-acyl substituent in
C-20 The1H and13C NMR spectra of derivatives 17–20 perfectly
fit the expected chemical shifts previously reported[24–26]
The configuration at C-20 in representative compounds (10, 12,
14 and 17) was established by ROESY experiments which allowed to
detect the vicinal interaction between CH3-18 and CH3-21 (Fig 3);
additionally, in compound 12 a correlation between H-20 and H-17,
both placed in the␣ face was observed, confirming the
config-uration at C-17 as S, being the same of the starting material An
interaction in compound 17 between H-17 and H-16, both with an
alpha orientation confirms this configuration at C-17
ROESY experiments also allowed confirming the configuration
at C-16 in compounds 10, 12–15 from the correlation between
H-16␣ and H-15␣; in addition the large J values of H-16␣,
evi-dences the pseudo-axial orientation as reported previously[45]
The J values of the H-16␣ in compound 12 were obtained from
the experiment1H determined in CDCl3, because in Py-d5and in
Py-d5+ D2O the signal is overlapped with H-6
Suitable crystals for X-ray analysis of the
(25R)-23-acetyl-22,26-epoxy-12-oxocholesta-5,22-diene-3,16-diyl diacetate
(16) were obtained by slow evaporation of a mixture of
hexane-ethyl acetate (9:1), crystallizing in the orthorhombic system, space group P 212121 The X-ray crystal structure analysis established that the stereochemistry at C-20 and C-25 is S, and R respectively (Fig 4).The steroidal nucleus in 16 shows that rings A and C adopt a
chair conformation The C-5–C-6 (Csp2-Csp2) distance of 1.307(7) ˚A confirms [46] a double bond, therefore ring B has a half-chair conformation because of the rigidity of the olefinic bond at5 The D ring displays an envelope conformation as well, and the F pyran ring shows a twisted conformation because of the double bond between C-22 and C-23, as can be corroborated with the C-22–C-23 (Csp2-Csp2) distance of 1.359(6) ˚A, besides the Me-27 shows an equatorial orientation[24–26,47]
In conclusion, we have developed a new, efficient and accessi-ble route for the preparation of polycyclic steroidal skeletons via regioselective cleavage of the E ring of 12-oxosteroidal sapogenins under acid conditions in dichloromethane, catalyzed by BF3 ether-ate at room temperature High concentration of Ac2O directs the
reaction to the epoxycholestene (5,16) and furostene (17–20)
derivatives, while the use of low Ac2O concentration, favors the intramolecular reaction and the new hexa- and pentacyclic
steroidal (10–15) derivatives are obtained as major products This
reaction is appealing because it involves the regioselective open-ing of ropen-ing E, followed by aldol condensation with the carbonyl
at C-12 without the loss of asymmetric centers The polycyclic steroidal derivatives were synthesized in one pot under mild con-ditions Further investigation is ongoing regarding the application
of this method to the synthesis of versatile frameworks, as brassi-nosteroids analogs
Acknowledgments
The authors thank CONACYT, VIEP and SEP-PADES (project No 2009-01-09-006-220) for financial support and scholarships to J.O.H.P.D and J.L.V Also we thank V González for the NMR spec-tra, G Cuéllar for the mass spectra and M.A Leyva for X-ray data collection
Appendix A Supplementary data
Supplementary data associated with this article can be found, in the online version, atdoi:10.1016/j.steroids.2010.07.008
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