Total synthesis of (-)-tantazole B

Recent advances in the synthesis and utility of thiazoline and its derivatives

Thiazolines and their derivatives hold significant importance in the field of medicinal chemistry due to their promising potential as pharmaceutical agents. These molecular entities serve as critical scaffolds within numerous natural products, including curacin A, thiangazole, and mirabazole, and play a vital role in a wide array of physiological reactions. Their pharmacological versatility encompasses anti-HIV, neurological, anti-cancer, and antibiotic activities. Over the course of recent decades, researchers have extensively explored and developed analogs of these compounds, uncovering compelling therapeutic properties such as antioxidant, anti-tumor, anti-microbial, and anti-inflammatory effects. Consequently, thiazoline-based compounds have emerged as noteworthy targets for synthetic endeavors. In this review, we provide a comprehensive summary of recent advancements in the synthesis of thiazolines and thiazoline-based derivatives, along with an exploration of their diverse potential applications across various scientific domains.

Synergistic Cu/Pd-catalyzed asymmetric allylation: a facile access to α-quaternary cysteine derivatives

An efficient synthetic methodology to access biologically important and synthetically useful α-quaternary cysteine derivatives via asymmetric catalytic α-allylation of readily available 2-thiazoline-4-carboxylates was successfully developed through a synergistic Cu/Pd catalytic system. A wide array of α-quaternary cysteine derivatives were obtained in moderate to high yields with good to excellent enantioselectivities (45-98% yields and 69->99% ee). Gram-scale asymmetric allylation was performed to obtain high yields maintaining the enantioselectivity. Moreover, some synthetic transformations to access chiral spirocyclic compounds proceeded smoothly, which exhibited the important utility of this methodology.

A comprehensive review of chemistry and pharmacological aspects of natural cyanobacterial azoline-based circular and linear oligopeptides

The cyanobacterial oligopeptides are recognized for being highly selective, efficacious and relatively safer compounds with diverse bioactivities. Azoline-based natural compounds consist of heterocycles which are reduced analogues of five-membered heterocyclic azoles. Among other varieties of azoline-based natural compounds, the heteropeptides bearing oxazoline or thiazoline heterocycles possess intrinsic structural properties with captivating pharmacological profiles, representing excellent templates for the design of novel therapeutics. The specificity of heteropeptides has been translated into prominent safety, tolerability, and efficacy profiles in humans. These peptidic congeners serve as ideal intermediary between small molecules and biopharmaceuticals based on their typically low production complexity compared to the protein-based biopharmaceuticals. The distinct bioproperties and unique structures render these heteropeptides one of the most promising lead compounds for drug discovery. The high degree of chemical diversity in cyanobacterial secondary metabolites may constitute a prolific source of new entities leading to the development of new pharmaceuticals. This review focuses on the azoline-based natural oligopeptides with emphasis on distinctive structural features, stereochemical aspects, biological activities, structure activity relationship, synthetic and biosynthetic aspects as well as mode of action of cyanobacteria-derived peptides.

Palladium-Catalyzed Insertion of Isocyanides into the C-S Bonds of Heteroaryl Sulfides

Insertion of tert-butyl isocyanide into the C(sp² )-S bonds of heteroaryl sulfides is catalyzed by a palladium diphosphine complex. Thioimidates generated through this reaction could be readily hydrolyzed under acidic conditions to yield the corresponding thioesters, which are of synthetic use. This insertion is useful because starting heteroaryl sulfides were readily prepared by either conventional ways or through sulfur-specific extended Pummerer reactions.

Total synthesis of the azolemycins

The first total syntheses of newly isolated polyazole natural products azolemycins A–D, along with the synthesis of the tetra-oxazole non-natural analogue, are described.

Comparison of the reactivity of β-thiolactones and β-lactones toward ring-opening by thiols and amines

An investigation into the comparative reactivity of simple β-lactones and β-thiolactones toward a thiol and a primary amine is reported. A simple 3-mercaptomethyl-2-oxetanone is found to undergo rearrangement in the presence of aqueous base to give the corresponding thietane-3-carboxylic acid rather than the 3-hydroxymethyl-2-thietanone. Implications for the use of β-thiolactones in bioorganic and medicinal chemistry are discussed.

A convenient route to optically pure α-alkyl-β-(sec-amino)alanines

The cyclization of N-Boc-α-alkylserines to corresponding β-lactones under Mitsunobu reaction conditions and the ring opening with heterocyclic amines (pyrrolidine, piperidine, morpholine and thiomorpholine) produced N-Boc-α-alkyl-β-(sec-amino)alanines. The removal of the Boc group gives di-hydrochlorides of non-protein amino acids.

Total syntheses of the histone deacetylase inhibitors largazole and 2-epi-largazole: application of N-heterocyclic carbene mediated acylations in complex molecule synthesis

Details of the evolution of strategies toward convergent assembly of the histone deacetylase inhibiting natural product largazole exploiting γ,δ-unsaturated-α,β-epoxy-aldehydes and a thiazole-thiazoline containing ω-amino-acid are described. The initial N-heterocyclic carbene mediated redox amidation exploying these two types of building blocks representing largazole's structural domains of distinct biosynthetic origin directly afforded the seco-acid of largazole. This was accomplished without any protecting groups resident upon either thioester bearing epoxy-aldehyde or the tetrapeptide. However, the ineffective production of largazole via the final macrolactonization led to an alternative intramolecular esterification/macrolactamization strategy employing the established two building blocks. This provided largazole along with its C2-epimer via an unexpected inversion of the α-stereocenter at the valine residue. The biological evaluation demonstrated that both largazole and 2-epi-largazole led to dose-dependent increases of acetylation of histone H3, indicating their potencies as class I histone deacetylase selective inhibitiors. Enhanced p21 expression was also induced by largazole and its C2 epimer. In addition, 2-epi-largazole displayed more potent activity than largazole in cell viability assays against PC-3 and LNCaP prostate cancer cell lines.

Novel Structural Motifs Consisting of Chiral Thiazolines: Synthesis, Molecular Recognition, and Anticancer Activity

The facile synthesis of linear and cyclic chiral oligo(4-alpha/beta-methyl)thiazolines is described. Linear oligothiazolines have been efficiently synthesized by the iterative formation of thiazoline rings and two-directional block condensation. The construction of 24- to 36-membered cyclic oligothiazolines was achieved through the head-to-tail cyclo-oligomerization of doubly deprotected linear fragments. Studies of the interactions of both the linear and cyclic oligomers with chiral compounds revealed that cyclic oligomers displayed a strong binding affinity towards mandelic acid, whereas linear oligomers showed a poor affinity. Linear oligomers have been proven to inhibit the cell growth of the cancer cell lines HPAC, PC-3, and HCT-116. Studies of the structure-activity relationships showed that the IC50 values are clearly dependent on both the length and the terminal functionalities of the linear oligomers. Longer derivatives showed more potent activity (e.g., hexi- and octithiazolines exhibit IC50<1 microM) against all three cancer cell lines. In sharp contrast, cyclic oligomers were inactive to all three cell lines.

Enantioselective Synthesis of (R)- and (S)-α-Alkylcysteines via Phase-Transfer Catalytic Alkylation

We reported efficient enantioselective synthetic methodologies for (R)-alpha-alkylcysteines and (S)-alpha-alkylcysteines. The phase-transfer catalytic alkylation of 2-phenyl-2-thiazoline-4-carboxylic acid tert-butyl ester and 2-o-biphenyl-2-thiazoline-4-carboxylic acid tert-butyl ester, in the presence of chiral catalysts (1 or 2), gave the corresponding alkylated products, which could be hydrolyzed to provide (R)-alpha-alkylcysteines (67->99% ee) and (S)-alpha-alkylcysteines (66-88% ee), respectively.

Synthesis of chiral cyclic oligothiazolines: a novel structural motif for a macrocyclic molecule

The synthesis of chiral cyclic oligo(4-beta-methyl)thiazolines is described; linear oligothiazolines were efficiently prepared by the iterative formation of a thiazoline ring and a two-directional block condensation, and construction of 24- to 36-membered cyclic oligothiazoline systems could be achieved by the head-to-tail cyclooligomerization of doubly deprotected linear fragments and subsequent thiazoline formation.

Total synthesis of the marine cyanobacterial cyclodepsipeptide apratoxin A

A total synthesis of apratoxin A was developed. Apratoxin A, isolated from Lyngbya spp. cyanobacteria, is representative of a growing class of marine cyanobacterial cyclodepsipeptides wherein discrete polypeptide and polyketide domains are merged by ester and amide or amide-derived linkages. In the apratoxins, the N terminus of the peptide domain [(Pro)-(N-Me-Ile)-(N-Me-ala)-(O-Me-Tyr)-(moCys)] is a modified vinylogous cysteine that is joined to a novel ketide [3,7-dihydroxy-2,5,8,8-tetramethylnonanoic acid (Dtna)] by an acid-sensitive thiazoline. The C-terminal proline is esterified to a hindered hydroxyl vicinal to the ketide's tert-butyl terminus. Major synthetic challenges included assembly and maintenance the thiazoline-containing moiety and macrolide formation involving acylation of the C39 hydroxyl. The Dtna domain was assembled in the biogenetic direction beginning with a Brown allylation of trimethylacetaldehyde to establish the C39 alcohol configuration. Diastereofacial selective addition of a higher-order dimethylcuprate upon a ring-closing metathesis-derived alpha,beta-unsaturated valerolactone installed the C37 methyl-bearing center. A Paterson anti-aldol process was used to incorporate the remaining two ketide stereogenic centers at C34 and C35. Although attempts to incorporate the thiazoline moiety by condensations of thiol esters bearing alpha-amino carbamate derivatives failed, an intramolecular Staudinger reduction-aza-Wittig process using alpha-azido thiol esters was uniquely successful. Late-stage macrocycle closure proceeded well by lactam formation between Pro and N-Me-Ile residues, but attempted lactonizations of the Pro carboxylate with the C39 hydroxyl failed. Optimization of C35 hydroxyl group protection-deprotection completed the effort, which culminated in the first total synthesis of apratoxin A and will enable analog generation toward improving differential cytotoxicity.

Efficient Asymmetric Synthesis of (S)- and (R)-N-Fmoc-S-Trityl-α-methylcysteine Using Camphorsultam as a Chiral Auxiliary

(1R)-(+)-2,10- and (1S)-(-)-2,10-camphorsultam were acylated with ethyl 2-phenylthiazoline 4-carboxylate to afford (+)- and (-)-2-phenylthiazolinylcamphorsultam, which were stereoselectively alkylated with MeI in the presence of n-BuLi. Alkylation of these phenylthiazolinylcamphorsultams occurred from the beta-face rather than alpha-face, resulting in the formation of (S)-alpha-methylcysteine from (1R)-(+)-2,10-camphorsultam and (R)-alpha-methylcysteine from (1S)-(-)-2,10-camphorsultam after acidic hydrolysis. Subsequent protection of the side chain thiol group with trityl alcohol and alpha-amine function with Fmoc-OSu furnished fully protected (S)- and (R)-N-Fmoc-S-trityl-alpha-methylcysteine in overall 20% yield.

A New Method for the Stereoselective Synthesis of α-Substituted Serine Amino Acid Analogues

[reaction: see text]. A new method was developed for the stereoselective synthesis of alpha-substituted serine amino acids. The strategy utilizes a common enantiomerically enriched intermediate obtained through an enzymatic desymmetrization. A variety of amino acids were synthesized in good ee's through nucleophilic acetylide addition reactions and palladium-catalyzed Sonogashira couplings.

Synthesis of orthogonally protected (R)- and (S)-2-methylcysteine via an enzymatic desymmeterization and Curtius rearrangement

A synthesis of differentially protected (R)- and (S)-2-methylcysteines is described. Monomethylation of dimethylmalonate followed by alkylation with tert-butylchloromethyl sulfide gave an achiral diester. Desymmeterization by selective hydrolysis of one ester with pig-liver esterase gave the acid in 97% chemical yield and 91% enantiomeric excess. Heating this acid with diphenylphosphoryl azide followed by 4-methoxybenzyl alcohol gave protected (R)-2-methylcysteine. Alternately, the acid and ester groups were interchanged and heated with diphenylphosphoryl azide followed by 4-methoxybenzyl alcohol, giving protected (S)-2-methylcysteine.

Synthesis of the apratoxin 2,4-disubstituted thiazoline via an intramolecular aza-Wittig reaction

[reaction: see text] In developing a synthetic entry to the thiazoline-containing domain of the apratoxin natural products, we converted vicinal azido-thiolesters into 2,4-disubstituted thiazolines via sequential one-pot Staudinger reduction/aza-Wittig reaction. This method of de novo thiazoline formation provides a mild and versatile process that is particularly well suited to acid-sensitive substrates.

Stereoselective synthesis of conformationally constrained glycosylated amino acids using an enzyme-catalyzed desymmetrization

As part of an effort to probe the mechanism by which glycosyltransferases recognize glycoproteins and assemble the core structures of O-linked oligosaccharides, constrained glycopeptides, compounds 2 and 3, based on the alpha-N-acetylgalactosaminyl serine substructure 1, were designed. In this paper we describe a stereoselective preparation of protected versions of these compounds. A pig liver esterase-catalyzed enzymatic desymmetrization of a diacetate substrate, 10, was employed as a key component in the synthesis.

Synthesis of thiazolines linked to a difluoromethylphosphonate diester via dithioester chemistry

[reaction: see text] A two-step, high-yielding synthesis of delta(2)-thiazolines containing a difluoromethylphosphonate diester moiety has been devised using a building block approach. Racemic or chiral beta-amino alcohols and diols were coupled with methyl difluoro(diethoxyphosphono)dithioacetate to give predominantly the corresponding beta-hydroxythioamides, which were then cyclized to provide a series of novel substituted delta(2)-thiazolines.

Synthesis of aldehyde building blocks protected as acid labile N-Boc N,O-acetals: toward combinatorial solid phase synthesis of novel peptide isosteres

The synthesis of (RS)-3'-tert-butoxycarbonyl-perhydro-1', 3'-oxazine-2'-yl acetic acid and the syntheses of the simple and C-2 substituted 3'-tert-butoxycarbonyl-perhydro-1',3'-oxazine-2'(RS)-yl propionic acids from simple starting materials are presented. The simple compounds were prepared from 1,3-propanediol and 1, 4-butanediol, respectively, via a short series of facile steps, in 70% overall yield in both cases. For the syntheses of the C-2 substituted compounds of the longer homologue, (RS)-3'-tert-butoxycarbonyl-perhydro-1',3'-oxazine-2'-yl propionic acid, a malonic ester route was selected, thus allowing easy incorporation of various side chains. The stability of the novel aldehyde protection group, the N-Boc N,O-acetal moiety, under various acidic conditions was investigated, and it was found to cleanly and rapidly yield the aldehyde under strong acidic conditions or, if desired, slower under less harsh conditions. As a demonstration of the use of the building blocks, one building block was coupled to a solid support and, after unmasking of the aldehyde, submitted to three different types of nucleophilic reactions (Pictet-Spengler condensation, reductive amination, Horner-Wadsworth-Emmons olefination) followed by further chemical modification, and the identity of the structures were verified after cleavage from the resin.

Titanium(IV)-mediated tandem deprotection-cyclodehydration of protected cysteine N-amides: biomimetic syntheses of thiazoline- and thiazole-containing heterocycles

The scope and limitations of TiCl(4)-mediated Delta(2)-thiazoline synthesis via tandem deprotection-dehydrocyclization of trityl-protected cysteine N-amides is presented. While chemical yields are acceptable (53-96%), the stereochemical outcomes vary on the basis of structural considerations and reaction conditions (22-99% ee). Racemization at the C(2)-exomethine position limits the utility of this method for the formation of a thiazoline within a peptide. Treatment of a tritylated Cys-Cys dipeptide with TiCl(4) afforded the corresponding thiazole-thiazoline heterocycle 12 (38% yield, 97% ee).