TỔNG HỢP PHA RẮN (SOLID PHASE SYNTHESIS)

Synthesis of Functionalized Polystyrene Resin Polymerization of styrene can be conducted with functionalized monomers: X Ph Ph 2 Radical Initiator Styrene monomer Divinylbenzene Crossli

TỔNG HỢP PHA RẮN

(SOLID PHASE SYNTHESIS)

TỔNG HỢP PHA RẮN

Là kỹ thuật tổng hợp theo đó chất cần tổng hợp được tổng hợp trên một chất mang rắn (polymeric resin).

S S

S S

S S

S S

S S S S

TỔNG HỢP PHA RẮN

Ưu điểm của Tổng hợp Pha rắn?

 Không cần cô lập chất trung gian.

 Có thể dùng dư tác nhân phản ứng → thúc đẩy

phản ứng xảy ra hoàn toàn.

 Sau phản ứng, lượng dư chất phản ứng có thể loại

bỏ bằng phương pháp rửa đơn giản.

 Có thể tự động hóa quá trình tổng hợp.

TỔNG HỢP PHA RẮN

TỔNG HỢP PHA RẮN

Tổng hợp pha rắn: Chất mang và Cầu nối

Resin

Chất mang (Resin)

 Một loại polymer.

 Mức độ trương nở trong dung môi khác nhau:

Polystyrene trương nở tốt trong dung môi không phân cực; Polyethylene glycol (PEG): trương nở tốt trong cả dung môi phân cực và không phân cực.

Cầu nối (Linker)

 Cấu trúc trung gian nối chất mang và tác chất phản ứng.

 Nhiều loại cầu nối khác nhau.

inert-• Need to be chemically functionalized.

• Sufficient permeability or swelling capacity.

• Mostly are polystyrene and copolymers of styrene with various cross-linking agents.

• Cross-linking imparts mechanical stability and improved diffusion and swelling properties to the resin.

Effects of Crosslinking

Without cross-linking, each polymer chain can dissolve under thermodynamically favored conditions.

Cross-linking can induce some

sites of “permanent

entanglement” maintaining

structural integrity.

‘Swollen’ state : Permeable to

solvent and reagent

‘Shrunken’ stateSwelling of Polymer by Solvent

Classification of Solid Support

1.1 Polystyrene (PS) Resins

Essentially insoluble if cross-linking exceeds 0.2%, but can swell to a variable extent in organic solvents

Synthesis of Functionalized Polystyrene Resin

Polymerization of styrene can be conducted with

functionalized monomers:

X

Ph Ph

2

Radical Initiator Styrene

monomer

Divinylbenzene Crosslinking element

Functionalized monomer

Polystyrene chain Crosslink

Functionalized aryl group for attachment

of linkers and substrates

Ph Ph

Ph Ph Ph

Tl(OAc)3

Br2, CH2Cl2

Ph Ph

Ph Ph Ph

Br

Br

96%

Synthesis of Functionalized Polystyrene Resin

Synthesis of Functionalized Polystyrene Resin

Synthesis of Functionalized Polystyrene Resin

Synthesis of Functionalized Polystyrene Resin

Synthesis of Functionalized Polystyrene Resin

Synthesis of Functionalized Polystyrene Resin

Synthesis of Functionalized Polystyrene Resin

Synthesis of Functionalized Polystyrene Resin

1.2 Polyacrylamide Resins

1.2 Polyacrylamide Resins

• Swells in a wider variety of solvents (e.g water,

1.3 Polystyrene-Poly(ethylene glycol) graft (TentaGel)

Preparation of PEG-polystyrene graft supports

Functionalized PEG-PS Resins

90 µ m (TentaGel) 0.75 mmol/ g

350 pmol/ bead

200 µ m (PS) 1.05 mmol/ g

4 nmol/ bead

500 µ m (PS) 1.05 mmol/ g

60 nmol/ bead

Considerations in Choosing a Solid Support

• Mode of attachment and cleavage of materials from the resin

(linker)

• Compatibility of the chemistry planned for the library synthesis.

• The amount of material desired (loading level).

• Size - affects efficiency of diffusion within the polymer (reaction rates!).

Analytical Methods for Support-Bound Intermediates

• Combustion Analysis: determine the amount of

halogens, nitrogen, or sulfur present in samples of cross- linked polystyrene → the loading

• Many linkers are adapted from protecting group chemistry

• Are organized according to the functional group that results upon cleavage from the support

• Multifunctional linkers

Linkers: Introduction

Ester-Type Linkers

Cleavage methods are similar for both types of linkage but the resulting molecules end up with different functional groups.

Esters Type A

Cleavage Possibilities of Ester and Thioester Linkers

Cleavage Possibilities of Ester and Thioester Linkers

(Ley et al (1995) Synlett p 1017)

• Cleavage Yielding Carboxylic Acids

Cleavage Possibilities of Ester and Thioester Linkers

Cleavage of esters (a–g) acidic cleavage, (h–k) basic cleavage, (l) fluorous cleavage, (m) hydrogenation,

(n) Pd-catalyzed cleavage.

• Cleavage Yielding Carboxylic Acids

Cleavage Possibilities of Ester and Thioester Linkers

Photolabile carboxylic acid linkers

Photo-labile linker

• Photolytic conditions can be very mild and selective

• Dimerization of the support-bound nitroso by-product sometimes hampers further cleavage

• Aryl nitro group is incompatible with some

O R

O

h ν , 350 nm

+

• Cleavage Yielding Ketones, Aldehydes and Alcohols

Cleavage Possibilities of Ester and Thioester Linkers

• Cleavage Yielding Ketones, Aldehydes and Alcohols

• Traceless Cleavage: is a possibility to completely remove ester linkages from the substrate.

Cleavage Possibilities of Ester and Thioester Linkers

• Cleavage Yielding Primary and Secondary Amides

Cleavage Possibilities of Ester and Thioester Linkers

• Cleavage Yielding Alkyl-O- and S-Esters

Cleavage Possibilities of Ester and Thioester Linkers

Methods for transesterification of ester linkers

• Cleavage Yielding Hydroxamates

Cleavage Possibilities of Ester and Thioester Linkers

Synthesis of hydroxamates via ester linker

• Ring-Forming Strategies

Cleavage Possibilities of Ester and Thioester Linkers

Synthesis of isoindoles and phthalides

Esters Type B

Amide-Type Linkers

Amide formation is one of the most important reactions

on solid supports

Amide-Type Linkers

Linkers for Carboxylic Acids

• Carboxylic acids are generally attached to polymeric supports as esters or amides

• Depending on the type of linker and on the cleavage conditions used, cleavage can lead either to the regeneration of a carboxylic acid,

or to the formation of a new product, such as

an ester, amide, ketone, or alcohol

X= H, Wang linker:

X= OMe, Sasrin linker:

Acid Labile Linkers for Carboxylic Acid

• Many historically important resins (Merrifield, Wang, Sasrin, Sieber, Rink resins) have linkers that are cleaved under acidic conditions.

• Acidic conditions were intended to prevent racemization of amino acids during solid phase peptide synthesis.

Linkers for Carboxylic Acids

Acid Labile Linkers for Carboxylic

Acid:

Benzyl Alcohol Linkers

• Most are based on the acidolysis of benzylic C-O

bonds

Immobilization of carboxylic acids as benzyl esters

and acidolytic cleavage M + : Cs + , NMe4+ , Na + , Zn 2+

Y: leaving group; HX: HBF4, HBr, HF, TFA, TfOH

Acid Labile Linkers for Carboxylic

Acid:

Benzyl Alcohol Linkers

Acid Labile Linkers for Carboxylic Acid

Diarylmethanol (Benzhydrol) Linkers

• “Rink acid resin”

• Esters of this linker can be cleaved with weak acids

• Lesser useful compared to benzyl alcohol or trityl linkers.

Acid Labile Linkersfor Carboxylic Acid

Trityl Alcohol Linkers

• Esters of unsubstituted, polystyrene-bound trityl alcohol are too acid-sensitive to be useful for solid phase synthesis

• More stable towards solvolysis is the 2-chlorotrityl alcohol linker High stability towards nucleophiles and bases

• Cleavage can be effected by treatment with dilute TFA, acetic acid, or hexafluoro- isopropanol

Acid Labile Linkers for Carboxylic Acid

Acid Labile Linkers for Carboxylic Acid:

Non-Benzylic Alcohol Linkers

Tertiary aliphatic alcohol linkers

Acid Labile Linkers for Carboxylic Acid:

Non-Benzylic Alcohol Linkers

(4-Acyloxy-2-buten-l-yl)silanes

2-acyloxyethylsilanes

Base Labile Linkers for Carboxylic Acid:

• Mainly developed for the preparation of peptide amides or cyclic peptides, or non-peptides

• Several different carboxylic acid derivatives are available by nucleophilic cleavage, the most common being carboxylates, esters, and amides

• The type of support chosen can have an impact on the facility with which nucleophilic cleavage takes place

Base Labile Linkers for Carboxylic Acid:

Benzyl Alcohol Linkers

• Can be cleaved by treatment with various types of nucleophile

• Benzyl esters become more sensitive towards nucleophiles with decreasing electron density of the benzene ring.

• On the other hand, acid-sensitive benzyl alcohol linkers, in particular those with electron-donating alkoxy groups (e.g the Wang linker), are rather resistant towards nucleophilic attack

Base Labile Linkers for Carboxylic Acid:

Non-Benzyl Alcohol Linkers

• The advantage of this type of linker is its stability towards electrophiles

• Attachment of carboxylic acids is usually realized by acylation of the resin-bound alcohol with a reactive acid derivative

Linkers for Amides

• Yield amides upon cleavage

• These linkers can often also be used to prepare sulfonamides, carbamates, or ureas

• Three different strategies for the release of amides from insoluble supports:

(a) Cleavage of the benzylic C-N bond of resin-bound N-alkyl-N-benzylamides

(b) Nucleophilic cleavage of resin-bound acylating agents with amines

(c) Acylation/debenzylation of resin- bound N,N-dialkylamines

N-benzyl-Benzylamine Linkers

Based on the scission of benzylic C-N bonds:

Because of the lower polarization of C-N bonds compared with C-O bonds, benzylic amines or amides are generally more difficult to cleave with electrophiles than benzylic ethers or esters

Benzylamine Linkers

Based on the scission of benzylic C-N bonds:

The reactivity of the N-benzylamides towards electrophiles depends to a great extent on the electronic properties of all the substituents, and is not always easy to predict

R = Ar < R = CHR1-CONHR2) < R = Et

Benzylamine Linkers

Linkers for Amines

cleavage by TFA cleavage by strong acid

A tertiary amine

Alkylsilyl Linker - Fluoride Labile

*

*

Si

Me Me Me

Me

OMe *

Si

Me Me Me

Me

OMe

Me Me Me

• Mild cleavage conditions compatible with various functional groups.

• Designed for attachment through an alcohol

• Compatibile with strong anionic, cationic, oxidative, and reductive conditions

Nucleophile Labile Linkers

Kaiser Oxime linker

• Advantage: Introduction of diversity in cleavage step

• Difficulty: Often too reactive for common

nucleophilic reaction conditions

NO2

R O

Safety-catch linker

Kenner’s sulfonamide linker

• A “safety-catch” linker can solve the reactivity problem with a two step cleavage

• 1) An activation step that is orthogonal to common functional groups

• 2) Cleavage of the activated linker under mild conditions

S N

i

dilute BnNH2

Bn

Safety-catch linker

• This type of linker creates a C-C or a C-H bond at the site of cleavage

• C-H bond generation : Si-Ge linker (protonolysis or radical reduction)

Cl

S cat.

Traceless Linkers

PEPTIDE SYNTHESIS

COUPLING METHOD: SOLID PHASE FORMATION OF AMIDE AND ESTER

• The formation of amides and esters are key steps in the solid phase organic synthesis

• Requires activation of the carboxylic group prior to reaction with the amino or alcohol component

RACEMIZATION

Coupling Reagents

• Carbodiimides:

TỔNG HỢP PEPTIDE TRÊN PHA RẮN

Fmoc Protecting Group

• Is cleaved under very mild basic conditions (e.g piperidine)

• Stable under acidic conditions

Boc Protecting Group