WO1998050438A1

WO1998050438A1 - Chloride linker for solid phase organic synthesis

Info

Publication number: WO1998050438A1
Application number: PCT/US1998/009485
Authority: WO
Inventors: Ravi S. Garigipati
Original assignee: Smithkline Beecham Corporation
Priority date: 1997-05-08
Filing date: 1998-05-08
Publication date: 1998-11-12
Also published as: EP0981554A4; EP0981554A1; JP2001525866A; CA2289719A1

Abstract

This invention relates to a linker for use in solid phase synthesis, its preparation and methods of use. The linker consists essentially of an alkoxy substituted phenyl ring bearing an optionally substituted benzylic chloride. Synthetic components bearing a suitable nucleophile can displace the chlorine atom and form a covalent linkage to the support.

Description

CHLORIDE LINKER FOR SOLID PHASE ORGANIC SYNTHESIS

FIELD OF THE INVENTION

This invention relates to a novel linker for use in solid phase chemistry, its preparation and methods of use of the linker.

BACKGROUND OF THE INVENTION

In the continuing search for new chemical moieties that can effectively modulate a variety of biological processes, the standard method for conducting a search is to screen a variety of pre-existing chemical moieties, for example, naturally occurring compounds or compounds which exist in synthetic libraries or databanks. The biological activity of the pre-existing chemical moieties is determined by applying the moieties to an assay which has been designed to test a particular property of the chemical moiety being screened, for example, a receptor binding assay which tests the ability of the moiety to bind to a particular receptor site. In an effort to reduce the time and expense involved in screening a large number of randomly chosen compounds for biological activity, several developments have been made to provide libraries of compounds for the discovery of lead compounds. The chemical generation of molecular diversity has become a major tool in the search for novel lead structures. Currently, the known methods for chemically generating large numbers of molecularly diverse compounds generally involve the use of solid phase synthesis, in particular to synthesize and identify peptides and peptide libraries. See, for example, Lebl et al., Int. J. Pept. Prot. Res., 41, p. 201 (1993) which discloses methodologies providing selectively cleavable linkers between peptide and resin such that a certain amount of peptide can be liberated from the resin and assayed in soluble form while some of the peptide still remains attached to the resin, where it can be sequenced; Lam et al., Nature, 354, p. 82 (1991) and (WO 92/00091) which disclose a method of synthesis of linear peptides on a solid support such as polystyrene or polyacrylamide resin; Geysen et al., J. Immunol. Meth., 102, p. 259 (1987) which discloses the synthesis of peptides on derivatized polystyrene pins which are arranged on a block in such a way that they correspond to the arrangement of wells in a 96-well microtiter plate; and Houghten et al., Nature, 354, p. 84 (1991) and WO 92/09300 which disclose an approach to de novo determination of antibody or receptor binding sequences involving soluble peptide pools.

The major drawback, aside from technical considerations, with all of these methods for lead generation is the quality of the lead. Linear peptides historically have represented relatively poor leads for pharmaceutical design. In particular, there is no rational strategy for conversion of a linear peptide into a non-peptide lead. As noted above, one must resort to screening large databanks of compounds, with each compound being tested individually, in order to determine non-peptide leads for peptide receptors. In this respect, there has been increasing interest in the application of solid phase synthesis to the preparation of organic compounds, especially in the context of combinatorial chemistry and multiple simultaneous synthesis, or parallel synthesis. One of the limitations in the solid phase approach in general, involves the linker by which the organic molecule is attached to the solid support. The Rink linker (Rink, H., Tetrahedron Lett., 1987, 28, 3787-3790) has been effectively applied to the synthesis of some chemical libraries (Gordeev, M. F.; Patel, D. V.; Gordon, E. M. J. Org. Chem. 1996, 61, 924-928; Norman, T. C; Gray, N. S.; Koh, J. T.; Schultz, P. G. J. Am. Chem. Soc. 1996, p. 118, 7430-7431; Ward, Y. D.; Farina, V. Tetrahedron Lett., 1996, 37, 6993-6996), because of the ease of use and mild conditions for release of the library component. However the Rink linker is currently limited to use in the preparation of amides and carboxylic acids. Therefore, a need exists for a linker useful for a broader range of solid phase chemistry. Herein, we describe the preparation of a chloride linker, specifically a Rink-chloride linker, which allows a very general and practical method for the attachment of, ter alia, amines, alcohols and thiols to a solid support.

SUMMARY OF THE INVENTION

This invention relates to a novel resin-bound solid phase linker of formula (I)

Formula (I) wherein

X is a bond or HNC(O)(CH₂)_n; R is hydrogen or an optionally substituted phenyl ring; Z is hydrogen or alkoxy or, when R is an optionally substituted phenyl ring it can optionally, together with Z, form a fused ring, wherein Z is oxygen or (CH2)_m; Y is hydrogen or C1 -.4 alkoxy; n is 1 to 6; and m is 1 or 2.

The resin-bound solid phase linker of formula (I) is hereinafter referred to as a resin-bound chloride linker or a chloride linker. This represents a significant improvement over the current use of the Rink-acid linker. At present the use of the known Rink-acid linker is limited to preparing amides and carboxylic acids. The use of the instant improved chloride linker of formula (I) makes the technology available to a broad number of functional group attachments. The use of the instant chloride linker allows a very general and practical method for the attachment of amines, alcohols and thiols, including phenols and thiophenols to a solid support. Therefore, another aspect of the instant invention is in a method for making compounds by resin-bound synthesis using the chloride linker of formula (I) in solid phase synthesis. This method is applicable to making combinatorial libraries of compounds designed around a core molecular structure using known methods of solid phase combinatorial chemistry or multiple simultaneous synthesis ("parallel synthesis"). The compounds or libraries of compounds made using this linker may be tested in biologically assays designed to test for a particular physical characteristic potentially useful in drug therapy.

DETAILED DESCRIPTION OF THE INVENTION The terms "resin," "solid support," "inert resin," polymeric resin" or

"polymeric resin support" are used herein at all occurrences to mean a bead or other solid support such as beads, pellets, disks, capillaries, hollow fibers, needles, solid fibers, cellulose beads, pore-glass beads, silica gels, grafted co-poly beads, poly- acrylamide beads, latex beads, dimethylacrylamide beads optionally cross-linked with N,N'-bis-acryloyl ethylene diamine, glass particles coated with a hydrophobic polymer, etc., i.e., a material having a rigid or semi-rigid surface. The solid support is suitably made of, for example, cross linked polystyrene resin, polyethylene glycol- polystyrene resin, and any other substance which may be used as such and which would be known or obvious to one of ordinary skill in the art. In the schemes herein the resin is represented in part by the shaded circle.

The term "substituted resin-bound chloride intermediate" is used herein at all occurrences to mean the intermediate produced by coupling a resin-bound chloride linker with a suitable nucleophile (with displacement of the chloride of the linker) such that to the nucleophile is linked to the resin through the chloride linker. The term "additional synthetic chemistry" is used herein at all occurrences to mean chemical reactions which are performed on the substituted resin-bound chloride intermediate prior to cleavage of the nucleophile from the polymeric resin, wherein said chemical reactions are compatible with and non-reactive with the chloride linker and may be used to prepare derivatives of the nucleophile. It will be understood by the skilled artisan that the additional synthetic chemistry performed on the substituted resin-bound chloride intermediate, is done so prior to cleavage of the derivatized nucleophile. Chemical reactions which are incompatible with the nucleophile/chloride linkage, i.e., they cause cleavage of the nucleophile from the chloride linker prior to complete derivatization, are not among the additional synthetic chemistry that may be used in the methods of this invention.

The terms "resin-bound synthesis" and "solid phase synthesis" are used herein interchangeably to mean a series of chemical reactions used to prepare either a single molecule/compound or a library of molecularly diverse compounds, wherein the chemical reactions are performed on a compound which is bound to a polymer resin through a linkage, in particular, a chloride linkage such as in formula (I). The term "optionally substituted phenyl ring" is used herein at all occurrences to mean a phenyl ring substituted with zero to two electron donating groups in the ortho or para position. It will be understood by the skilled artisan what is meant by an electron donating group, for example ,an alkoxy group of 1-6 carbon atoms, preferably methoxy.

For the compounds of formula (I) various embodiments are as follows. X is suitably a bond or HNC(O)(CH2)_n, wherein n is 1 to 6, preferably 1 to

4.

R is suitably hydrogen, an optionally substituted phenyl ring. R is preferably hydrogen or dimethoxyphenyl.

Z is suitably hydrogen or C 1.4 alkoxy, preferably methoxy, or when R is an optionally substituted phenyl ring it can optionally, together with Z, form a fused ring, wherein Z is oxygen or (CH2)_m, wherein m is 1 or 2.

Y is suitably hydrogen or C 1.4 alkoxy, preferably hydrogen or methoxy. Chemical synthesis on solid supports has become a cornerstone in the generation of small organic molecule combinatorial libraries. Paramount to the success of any solid- phase synthetic strategy is a reliable and general method for coupling the initial starting materials onto the solid support, namely through linker technology. Such linker technology should also be amenable to ready cleavage of the reaction products under relatively mild conditions, and without compromising the structure of the reaction products. The instant resin-bound chloride linker of formula (I), particularly the Rink linker, has been effectively applied to the synthesis of some chemical libraries because of its ease of use and the mild conditions for release of library components from the solid support. However the Rink technology is currently limited to the preparation of amides and carboxylic acids. Herein, we describe the preparation and utility of a resin-bound chloride linker, which allows a very general and practical method for the attachment of amines, alcohols and thiols to a solid support, derivatization of these resin-bound compounds, and their eventual release from the resin with trifluoroacetic acid. Generally, resin-bound chloride linkers of formula (I) can be made according to

Scheme 1. The commercially available ketone starting material is coupled with a resin by known methods to obtain 1-Scheme 1. Reduction of the resin-bound ketone 1-Scheme 1 is carried out with a suitable commercially available metal hydride, such as NaBH4 (sodium borohydride), LAH (lithium aluminum hydride) or L1BH4 (lithium borohydride), preferably LiBH4, in THF (tetrahydrofuran).

Scheme 1

1-Scheme 1 2-Scheme 1

After reduction of the ketone to an alcohol according to Scheme 1 , the resin-bound alcohol linker is converted to a resin-bound chloride linker according to Scheme 2.

The conversion in Scheme 2 is carried out with triphenylphosphine and a suitable chlorinating agent, preferably hexachloroethane, in THF. Schemes 1 and 2 are applicable for making resin-bound chloride linkers of formula

(I) from known solid phase linkers such as the Sieber linker (P. Sieber, Tetrahedron Lett, 1988, 28, p. 2107); the Wang linker (S. S. Wang, J. Am. Chem. Soc. 1973, p. 1328); and the HAL linker (G. Breipohl, J. Knolle, R. Geiger, Tetrahedron Lett. 1987, 28, p. 5647), shown below. Sieber Linker:

conversion to chloride

linker of formula (I)

Compound of formula (I) wherein X is

Sieber linker a bond and R is phenyl which together with Z forms a fused ring, wherein 2 is oxygen

Sieber linker Compound of formula (I) wherein X is HNC(θ)(CH₂)₄, and R is phenyl which together with Z forms a fused ring wherein Z is oxygen

Wang Linker:

conversion to chloride linker of formula (I)

Wang linker Compound of formula (I) wherein X is a bond and R is hydrogen

HAL Linker:

conversion to chloride linker ^{of for ula} (

Compound of formula (I) wherein X is HNC(0)(CH2)4 and R is hydrogen

Ramage suberone Linker: conversion to chloride linker of formula (I)

Compound of formula (I) wherein X is a bond,

Ramage suberone linker and R is phenyl which together with Z forms a fused ring, wherein Z is (CH₂)₂

A specific example using the Rink-chloride linker is described below in Scheme 3. The resin-bound Rink-acid linker 1-Scheme 3 was converted to the resin-bound Rink- chloride resin of this invention, 2-Scheme 3, by treatment of the resin-bound Rink-acid linker with triphenyl phosphine and hexachloroethane. 2-Scheme 3 so obtained was stable at room temperature for several days and can be used without any loss of activity.

Scheme 3

2-Scheme 3 was reacted cleanly with a variety of nucleophiles ("Nu") under mild reaction conditions (see, Scheme 4 and Table I below). The nucleophiles depicted herein are either commercially available or can be made using known procedures.

Scheme 4

Table I

Rink-chloride efficiently reacts with primary and secondary amines, anilines, alcohols, phenols, thiols, thiophenols, and carboxylic acids. The coupling is usually carried out in dichloroethane in the presence of Hϋnig's base, under inert atmosphere for 18-26 hours at room temperature. The extent of coupling efficiency is monitored by MASNMR and then by cleaving the product from the resin with about 3-5% TFA in CH2CI2. Release of the ligands from the resin is complete within 30 minutes as evidenced by MASNMR of the residual resin. As is apparent from Table I, the coupling is general and highly efficient. While cleavage from the resin is facile, it is sufficiently stable enough to carry out a wide range of chemistry commonly used in small molecule library construction. Some illustrative examples are shown in Scheme 3.

Scheme 5

β-naphthyl amme

Cl

Resin-bound

Rink-Chloride

Linker

It will be understood that after making the substituted resin-bound chloride intermediate, and prior to cleavage with TFA, additional synthetic chemistry may be performed on the intermediate in order to derivatize the nucleophile core. Therefore, another aspect of this invention is in a method for synthesizing a compound by resin- bound synthesis comprising the steps of: (a) converting a resin-bound linker into a resin- bound chloride linker of formula (I) :

Formula (I) wherein

X is a bond or HNC(O)(CH₂)_n; R is hydrogen or an optionally substituted phenyl ring;

Z is hydrogen or alkoxy or, when R is an optionally substituted phenyl ring it can optionally, together with Z, form a fused ring, wherein Z is oxygen or (CH2)_m; Y is hydrogen or C1-4 alkoxy; n is 1 to 6; and m is 1 or 2;

(b) coupling the resin-bound chloride linker with a suitable nucleophile under appropriate conditions to provide a substituted resin-bound chloride intermediate; and (c) performing additional synthetic chemistry on the substituted resin-bound chloride intermediate to provide a resin-bound derivatized nucleophile. The resin-bound derivatized nucleophile can remain bound to the resin for storage and/or further derivatization, or it may be cleaved from the resin with between about 3 and 5% TFA.

More specifically the invention is in a method for synthesizing a compound by resin-bound synthesis comprising the steps of: (a) converting a resin-bound Rink-acid linker into a resin-bound Rink-chloride linker of formula (I)

(b) coupling the resin-bound Rink-chloride linker with a suitable nucleophile under appropriate conditions to provide a substituted resin-bound Rink-chloride intermediate; and (c) performing additional synthetic chemistry on the substituted resin-bound Rink- chloride intermediate to provide a resin-bound derivatized nucleophile. Again, the resin- bound derivatized nucleophile can remain bound to the resin for storage and/or further derivatization, or it may be cleaved from the resin with between about 3 and 5% TFA. In yet another aspect, this invention is in a method for synthesizing a library of molecularly diverse compounds by resin-bound synthesis, comprising the steps of: (a) converting a resin-bound linker into a resin-bound chloride linker of formula

(I)

Formula (I) wherein X is a bond or HNC(O)(CH₂)_n; R is hydrogen or an optionally substituted phenyl ring;

Z is hydrogen or alkoxy or, when R is an optionally substituted phenyl ring it can optionally, together with Z, form a fused ring, wherein Z is oxygen or (CH2)_m; Y is hydrogen or C1 -.4 alkoxy; n is 1 to 6; and m is 1 or 2;

(b) coupling the resin-bound chloride linker with a suitable nucleophile under appropriate conditions to provide a substituted resin-bound chloride intermediate; (c) optionally dividing said substituted resin-bound chloride intermediate into a plurality of portions; (d) performing additional synthetic chemistry on the substituted resin-bound chloride intermediate to provide a resin-bound derivatized nucleophile; and (e) optionally recombining the portions.

More specifically this invention is in a method for synthesizing a library of molecularly diverse compounds by resin-bound synthesis, comprising the steps of: (a) converting a resin-bound Rink-acid linker into a resin-bound Rink-chloride linker of formula (I)

(b) coupling the resin-bound Rink-chloride linker with a suitable nucleophile under appropriate conditions to provide a substituted resin-bound Rink-chloride intermediate; (c) optionally dividing said substituted resin-bound Rink-chloride intermediate into a plurality of portions; (d) performing additional synthetic chemistry on the substituted resin-bound Rink-chloride intermediate to provide a resin-bound derivatized nucleophile; and (e) optionally recombining the portions.

Based upon the disclosure herein, it will be clear to one of ordinary skill in the art that there are many possible synthetic approaches to creating the libraries of this invention. For example the libraries may be prepared using the split and mix technique or parallel synthesis techniques. The libraries generated from either of the synthetic methods are molecularly diverse and are prepared simultaneously. The libraries are prepared on the polymer resins using the chloride linker described herein. For example, the compound to be derivatized (suitable nucleophile), is attached to the polymer resin through the chloride linker to give a substituted resin- bound chloride intermediate. In one embodiment, the substituent(s) are modified by reacting the resin-bound chloride intermediate, with a mixture of reagents. In an alternative embodiment, aliquots of the resin-bound chloride intermediate are reacted with individual reagents each one of which will modify a position on the core of the resin-bound nucleophile, and then the resultant products are mixed together to form the library of derivatized resin-bound intermediates. This library may then be further derivatized by repeating the process of dividing and recombining the intermediates formed by the additional synthetic chemistry. It will be clear to one of ordinary skill in the art that when the libraries of the invention are prepared according to the instant disclosure, each polymer support bears a single species created by the additional synthetic chemistry performed on the substituted resin-bound chloride intermediate.

It will be obvious to one of skill in the art that the steps of optionally dividing and recombining the resin-bound chloride intermediate into portions are for purposes of varying the derivatization on the resin-bound nucleophiles which are generated by the combinatorial synthesis. Of course, it will be obvious to the skilled artisan that the resin-bound nucleophile intermediates may be divided into portions at any point during the synthetic sequence. The portions may be recombined at any point during the sequence or, further iterations may be applied if more derivatization is required. Therefore, it will be obvious to the skilled artisan that the steps of dividing the portions, performing additional synthetic chemistry and recombining the portions, may each be carried out more than once, depending upon the type of diversity required for the library of end-product compounds being prepared. According to this invention, after the additional synthetic chemistry has been performed on the resin-bound chloride intermediate so that a library of molecularly diverse compounds has been prepared, the compounds can be separated and characterized by conventional analytical techniques known to the skilled artisan, for example infrared spectrometry or mass spectrometry. The compounds may be characterized while remaining resin-bound or they can be cleaved from the resin using the conditions described above, and then analyzed. In addition, a partial array of compound members of the library may be cleaved from the resin, characterized and analyzed, while leaving a partial array of the compound members of the library bound to the resin.

EXAMPLES Preparation of Rink-chloride (2-Scheme 3)

To a suspension of resin-bound Rink-acid, 1-Scheme 3, (l.Og, l.όmmol.) in THF (25mL) was added triphenylphosphine (2.32g, 8.8mmol.) and hexachloroethane (2.13g, 8.8mmol.). The reaction mixture was agitated with a constant flow of argon for 6h. The resin-bound Rink-chloride, 2-Scheme 3, was filtered and washed with THF and acetone. Completion of the reaction was affirmed by chlorine analysis and MASNMR (the signal for CH(OH)at 5 5.85 disappears completely). Chlorine analysis indicated a stoichiometric amount of chlorine.

Reaction of rink-chloride with various nucleophiles

To a suspension of 2-Scheme 3 (0.96g, l.όmMol.) in dichloroethane (25mL) was added Hϋnig's base (lmL) and the requisite nucleophile (lOmMol.). The reaction mixture was agitated with a constant flow of argon for 6-12h. The resin was filtered and washed with dichloromethane, MeOH, H2O, EtOH, CH2CI2 and MeOH.

Completion of the reaction was confirmed by MASNMR, IR or elemental analysis. Elemental analysis indicates no chlorine and a stoichiometric amount of nitrogen or sulfur for the appropriate compounds.

Cleavage from Rink-linked ligands

3% TFA/CH2CI2 was added to the substituted resin-bound Rink chloride moieties, either as individual beads or in bulk quantity. The cleavage was carried out for 30 min., and the product was isolated by extraction with 1 : 1/ MeOH:MeCN. All the cleaved products from Table I were identified by comparing them with an authentic sample of the starting material (nucleophile). The above description fully discloses the invention including preferred embodiments thereof. Modifications and improvements of the embodiments specifically disclosed herein are within the scope of the following claims. Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. Therefore any examples are to be construed as merely illustrative and not a limitation on the scope of the present invention in any way. The embodiments of the invention in which an exclusive property or privilege is claimed, are defined as follows.

Claims

What is claimed is:

1. A resin-bound compound of formula (I)

Formula (I) wherein

Z is hydrogen or alkoxy or, when R is an optionally substituted phenyl ring it can optionally, together with Z, form a fused ring, wherein Z is oxygen or (CH2)_m, Y is hydrogen or C 1-4 alkoxy; n is 1 to 6; and m is 1 or 2.

2. A method for synthesizing a compound by resin-bound synthesis comprising the steps of:

(a) converting a resin-bound linker into a resin-bound chloride linker of formula (I)

Formula (I) wherein

(b) coupling the resin-bound chloride linker with a suitable nucleophile under appropriate conditions to provide a substituted resin-bound chloride intermediate; and

(c) performing additional synthetic chemistry on the substituted resin-bound chloride intermediate to provide a resin-bound derivatized nucleophile.

3. The method as claimed in claim 2, further comprising the step of cleaving the derivatized nucleophile from the substituted resin-bound chloride intermediate.

4. A method for synthesizing a compound by resin-bound synthesis comprising the steps of:

Formula (I) wherein

Z is hydrogen or alkoxy or, when R is an optionally substituted phenyl ring it can optionally, together with Z, form a fused ring, wherein Z is oxygen or (CH2)m^ Y is hydrogen or C1 -.4 alkoxy; n is 1 to 6; and m is 1 or 2;

(b) coupling the resin-bound chloride linker with a suitable nucleophile under appropriate conditions to provide a substituted resin-bound chloride intermediate;

(c) performing additional synthetic chemistry on the substituted resin-bound chloride intermediate to provide a resin-bound derivatized nucleophile; and

(d) cleaving the resin-bound derivatized nucleophile.

5. A method for preparing a library of diverse compounds by resin- bound synthesis, comprising the steps of:

Formula (I) wherein

(b) coupling the resin-bound chloride linker with a suitable nucleophile under appropriate conditions to provide a substituted resin-bound Rink-chloride intermediate;

(c) optionally dividing said substituted resin-bound chloride intermediate into a plurality of portions;

(d) performing additional synthetic chemistry on the substituted resin-bound chloride intermediate to provide a resin-bound derivatized nucleophile; and

(e) optionally recombining the portions.

6. The method of claim 5 wherein the steps of (c) dividing the portions, (d) performing additional synthetic chemistry, and (e) recombining the portions, are carried out more than once.

7. The method of claim 5 wherein the derivatized nucleophile is cleaved from the resin-bound Rink-chloride intermediate by reaction with between about 3 to 5% TFA.