Synthesis of diverse tetrahydro-β-carboline-3-carboxamides and -2,3-bis-lactams on a versatile 4-hydroxythiophenol-linked solid support

Safety-Catch Linkers for Solid-Phase Peptide Synthesis

Solid-phase peptide synthesis (SPPS) is the preferred strategy for synthesizing most peptides for research purposes and on a multi-kilogram scale. One key to the success of SPPS is the continual evolution and improvement of the original method proposed by Merrifield. Over the years, this approach has been enhanced with the introduction of new solid supports, protecting groups for amino acids, coupling reagents, and other tools. One of these improvements is the use of the so-called "safety-catch" linkers/resins. The linker is understood as the moiety that links the peptide to the solid support and protects the C-terminal carboxylic group. The "safety-catch" concept relies on linkers that are totally stable under the conditions needed for both α-amino and side-chain deprotection that, at the end of synthesis, can be made labile to one of those conditions by a simple chemical reaction (e.g., an alkylation). This unique characteristic enables the simultaneous use of two primary protecting strategies: tert-butoxycarbonyl (Boc) and fluorenylmethoxycarbonyl (Fmoc). Ultimately, at the end of synthesis, either acids (which are incompatible with Boc) or bases (which are incompatible with Fmoc) can be employed to cleave the peptide from the resin. This review focuses on the most significant "safety-catch" linkers.

Application of Pictet-Spengler Reaction to Indole-Based Alkaloids Containing Tetrahydro-β-carboline Scaffold in Combinatorial Chemistry

Indole-based alkaloids are well-known in the literature for their diverse biological properties. Polysubstituted optically active tetrahydro-β-carboline derivatives functionalized on C-1 position are the common structural motif in most of the indole-based alkaloids, as well as highly marketed drugs. The stereoselective Pictet-Spengler reaction is one of the currently most important synthetic techniques used for the preparation of these privileged tetrahydro-β-carboline scaffolds. To date, there are numerous research reports that have been published on the synthesis of the tetrahydro-β-carboline scaffold both on solid phase, as well as in solution phase. Moreover rapid growth has been observed for the enantioselective synthesis of tetrahydro-β-carboline scaffold using chiral organocatalysts. In this Review, efforts have been taken to shed light on the latest information available on different strategies to synthesize tetrahydro-β-carboline both on solid phase and in solution phase during the last 20 years. Furthermore, we believe that the present synthetic methodologies covered in this Review will help to improve the status of this privileged tetrahydro-β-carboline scaffold in its use for drug discovery.

Immobilized coupling reagents: synthesis of amides/peptides

The primary idea of using immobilized reagents in organic synthetic chemistry is to simplify the downstream process, product workup and isolation, and therefore avoiding time-consuming and expensive chromatographic separations, which are intrinsic to every synthetic process. Numerous polymer-bounded reagents are commercially available and applicable to almost all kinds of synthetic chemistry conversions. Herein, we have covered all known supported-coupling reagents and bases which have had a great impact in amide/peptide bond formation. These coupling reagents have been used for the activation of a carboxyl moiety; thus generating an active acylating species that is ready to couple with an amine nucleophile liberating the amide/peptide and polymeric support which can be regenerated for reuse. This also addresses a large variety of anchored coupling reagents, additives, and bases that have only been employed in amide/peptide syntheses during the last six decades.

Convenient Synthesis of a Library of Discrete Hydroxamic Acids Using the Hydroxythiophenol (Marshall) Resin

Several resins have reportedly been used to synthesize hydroxamic acids except for the hydroxythiophenol (Marshall) resin. Herein we report the use of the Marshall resin to synthesize hydroxamic acids from carboxylic acids and its application to convert a library of fourteen discrete aliphatic and aromatic carboxylic acids including N-protected amino acids to their corresponding hydroxamic acids in good yields.

Stereochemical and Skeletal Diversity Employing Pipecolate Ester Scaffolds

[structure: see text] The stereocontrolled synthesis of pyridooxazinones by Mg(OTf)2-promoted epoxide ring-opening with use of chiral pipecolates as nucleophiles is described. Pyridooxazinone products derived from azido-epoxides can be further rearranged to seven-membered pyridodiazepinones by azide reduction. The sequence of functional group interconversions generates diversity through topological and stereochemical variation.

Efficient solid-phase synthesis of a library of imidazo[1,2-a]pyridine-8-carboxamides

A versatile method for the solid-phase synthesis of imidazo[1,2-a]pyridine-based derivatives, imidazo[1,2-a]pyridine-8-carboxamides, has been developed. They were obtained by treatment of the amino group of the polymer-bound 2-aminonicotinate with different alpha-haloketones, followed by halogenation at the 3-position of the polymer-bound imidazo[1,2-a]pyridine. The derived polymer-bound imidazo[1,2-a]pyridines 5, 6, and 7 were finally cleaved from the solid-support with an excess of primary or secondary amines. The final crude products were purified from excess amines by solid-supported liquid-liquid extraction (SLE).

Discovery of protein phosphatase inhibitor classes by biology-oriented synthesis

Protein phosphatases have very recently emerged as important targets for chemical biology and medicinal chemistry research, and new phosphatase inhibitor classes are in high demand. The underlying frameworks of natural products represent the evolutionarily selected fractions of chemical space explored by nature so far and meet the criteria of relevance to nature and biological prevalidation most crucial to inhibitor development. We refer to synthesis efforts and compound collection development based on these criteria as biology-oriented synthesis. For the discovery of phosphatase inhibitor classes by means of this approach, four natural product-derived or -inspired medium-sized compound collections were synthesized and investigated for inhibition of the tyrosine phosphatases VE-PTP, Shp-2, PTP1B, MptpA, and MptpB and the dual-specificity phosphatases Cdc25A and VHR. The screen yielded four unprecedented and selective phosphatase inhibitor classes for four phosphatases with high hit rates. For VE-PTP and MptpB the first inhibitors were discovered. These results demonstrate that biology-oriented synthesis is an efficient approach to the discovery of new compound classes for medicinal chemistry and chemical biology research that opens up new opportunities for the study of phosphatases, which may lead to the development of new drug candidates.

Total asymmetric synthesis of (?)-Phenylhistine, (?)-Aurantiamine and related compounds. Part I

A new general, short, and efficient strategy for the construction of dehydro-diketopiperazines was developed. Horner-Emmons type coupling between a phosphinyl glycine ester and a formyl heterocycle is the key coupling reaction, which proceeds in good-to-excellent yields on several sterically-hindered substrates. Moreover, racemization of the parent L-amino acids is avoided as a result of the mild basic conditions used. The selection of the NH protective group of the formyl heterocycle was crucial. N-tosylated heterocycles proved ideal for this reaction sequence. Thus, the title compounds, (-)-Phenylhistine and (-)-Aurantiamine, were prepared in high yield (four steps, 47% overall) and optical purity. Furthermore, the synthesis of unnatural derivatives including an indole analogue was successfully completed.

Solid-phase intramolecular N-acyliminium Pictet-Spengler reactions as crossroads to scaffold diversity

A novel solid-phase intramolecular Pictet-Spengler reaction is presented. The approach utilizes masked aldehyde building blocks protected as their N-Boc-1,3-oxazinanes for the clean generation of solid-supported aldehydes. When exposed to simple acidic treatment, the aldehyde functionality is rapidly released and becomes susceptible to nucleophilic attack from an amide nitrogen of the peptide backbone, which results in the formation of a highly reactive cyclic N-acyliminium ion. Subsequently, a quantitative and highly stereoselective Pictet-Spengler reaction takes place by attack of the indole from a neighboring tryptophan, thus appending two new N-fused rings to the indole moiety. Extension of this intramolecular reaction to substituted indoles, and other reactive heterocycles, such as furane and thiophenes, provides a convenient and rapid access to a range of pharmacologically interesting tri- and tetracyclic scaffolds. Finally, the reaction products may conveniently be released from the solid support (PEGA) by cleavage of the base-labile linker (HMBA).

Solid-Phase Synthesis of 1,2,3,4-Tetrahydroisoquinoline Derivatives Employing Support-Bound Tyrosine Esters in the Pictet−Spengler Reaction

The solid-phase synthesis of 1,2,3,4-tetrahydroisoquinoline-3-carboxamides employing carboxyl-supported, o-alkylated tyrosine esters in a Pictet-Spengler reaction is described. Esterification of [4-(hydroxyphenyl)thiomethyl]polystyrene (Marshall resin) with ethers of N-BOC-L-tyrosine using diisopropylcarbodiimide (DIC) and 4-dimethylaminopyridine (4-DMAP) afforded the solid-supported ester derivatives. Removal of the BOC group with trifluoroacetic acid (TFA) afforded the carboxyl-supported tyrosine ester, which was then treated with paraformaldehyde and TFA to afford the desired solid-supported counterpart. Acylation of the secondary amine with arylsulfonyl chlorides followed by reaction with amines resulted in the formation of the desired 2-arylsulfonyl-7-alkoxy-1,2,3,4-tetrahydroisoquinoline-3-carboxamides. Alternatively, the support-bound tetrahydroisoquinoline-3-carboxylate derivatives could be treated with an aldehyde and a reducing agent to give the corresponding support-bound tertiary amine. Exposure of these resin-bound products to amines afforded the corresponding 2-alkyl-7-alkoxy-1,2,3,4-tetrahydroisoquinoline-3-carboxamides after cleavage from the resin. Alternative routes to the desired chemotypes, as well optimization of the conditions for the Pictet-Spengler reaction and the conditions for the acylation and reductive amination of the support-bound secondary amines, are also described.

The development and preparation of the 2,4-dimethoxybenzyl arylhydrazine (DMBAH) "latent" safety-catch linker: solid phase synthesis of ketopiperazines

The development and preparation of the 2,4-dimethoxybenzyl arylhydrazine (DMBAH) linker 3, a new class of "latent" safety-catch linker which is stable under Mitsunobu alkylation conditions and in the presence of amines and hydrazine, is reported. The utility of the new linker is exemplified by the synthesis of ketopiperazines (MKPs) 24 bearing up to four points of diversity using a cyclitive cleavage approach.

FluoMar, a fluorous version of the Marshall resin for solution-phase library synthesis

[structure: see text] The fluorous counterpart of the Marshall resin, 4-(1H,1H,2H,2H-perfluorodecylsulfanyl)phenol (FluoMar), is prepared by S-alkylation of 4-mercaptophenol with C(8)F(17)CH(2)CH(2)I and employed in the synthesis of amide and diamide analogues. The final products are purified by solid-phase extraction (SPE) over FluoroFlash silica cartridges.

Kinetics study of amine cleavage reactions of various resin-bound thiophenol esters from marshall linker

The kinetics of cleavage reactions of seven resin-bound thiophenol esters with three amines has been studied by single-bead FTIR. The reactivity of these seven thiophenol esters was dependent on their structures and could be summarized as follows: 5-benzimidazolecarboxylic thiophenol ester > alkyl thiophenol ester > aromatic thiophenol ester. The reactivity of three amines was summarized as follows: n-butylamine > 3,4-dimethoxyphenethylamine > 1-piperonylpiperazine. The rate of the cleavage reaction increased 2-fold per 10 degrees C rise in reaction temperature. Oxidation of the thiophenol linker increased the rate of the cleavage reaction by 580-fold.

Solution-phase library synthesis of furanoses

The solution-phase synthesis of amido-, urea-, and aminofuranoses was achieved. Alkylated furanose aldehydes were treated with primary amines in the presence of sodium triacetoxyborohydride to give secondary amines. Subsequent acylation with acid chlorides and isocyanates afforded amidofuranoses and ureafuranoses, respectively. Second, reductive amination of furanose aldehydes with secondary amines yielded tertiary amines. The resulting acetonides were treated with alcohols in the presence of acid to yield mixed acetals. In the library syntheses, functionalized scavenger resins were used in the purification of intermediates and products.

A novel method for the generation of nitrile oxides on solid phase: application to the synthesis of substituted benzopyranoisoxazoles

A solid-phase synthesis of substituted benzopyranoisoxazoles is described. The six-step synthesis features a novel method of generating nitrile oxides on a polymer support followed by an intramolecular 1,3-dipolar cycloaddition with a tethered alkyne for assembly of the benzopyranoisoxazole scaffold. Furthermore, the utilization of single-bead attenuated total reflectance Fourier transform infrared (ATR-IR) microspectroscopy as an essential analytical tool for reaction optimization is highlighted.

Solid-phase library synthesis of alkoxyprolines

The library synthesis of alkoxyprolines was achieved using an acid-stable, nucleophile-cleavable solid support. A hydroxythiophenol linker derived from Merrifield resin was esterified with the corresponding ethers of BOC-hydroxyproline. Removal of the BOC protecting group with trifluoroacetic acid followed by acylation gave solid-supported hydroxyproline derivatives. Cleavage from the solid support with excess primary amines or excess secondary amines followed by purification of the crude products from the excess amine by supported liquid-liquid extraction gave the alkoxyproline library in high purity.

Tracelessness Unmasked: A General Linker Nomenclature

Nowadays it is rare to find an issue of a major chemistry journal without at least one article on solid-phase synthesis. This is hardly surprising: the technique promises an end to arduous work-up procedures and the ability to facilitate the creation of vast libraries of compounds using combinatorial techniques. No longer is the technique only of interest to those involved in peptide synthesis: an enormous variety of product classes have now been prepared on and isolated from the solid phase. It is the "linker" which is the focus of this article. The linker's ultimate function is to release a product from the support into solution: it does this, without exception, with a chemical change to the product at the former linkage site. Some linkers, apparently, are "traceless". But what, in fact, is "tracelessness"? Twenty years ago, in a climate where cleavage of a linker resulted in formation of a polar carboxylic acid as the vestige of the support, the concept was attractive. Today the chemist is faced with a myriad of novel linkers which have the ability to release products bearing most major functionalities at the former linkage site and we will argue here that the term "traceless", although currently in widespread use, is meaningless. Instead, we propose a new categorization of linkers based on the functionality they release upon cleavage, and suggest a nomenclature to underpin this categorization. We anticipate that the article will also serve to highlight areas of linker technology in need of further research.