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.     

Rational design of bacitracin A derivatives by incorporating natural product derived heterocycles

Heterocycles display common structural motifs in nonribosomally produced peptides with an enormous impact on their bioactivity. In the case of the branched cyclic Bacitracin A, the thiazoline moiety is manufactured during NRPS peptide chain elongation. Here we describe a method to selectively alter the heterocyclic metal binding subunit of Bacitracin A by the synthesis of heterocyclic building blocks that were successfully coupled to the linear decapeptide and subsequently cyclized using the excised bacitracin PCP-TE bidomain. Utilization of this cyclase allowed the first generation of branched cyclic bacitracin derivatives containing thiazole and oxazoles. The generated bacitracin derivatives showed bactericidal activity, indicating the possibility of altering the biological important heterocyclic subunit and overcoming existing limitations for the application of bacitracin.     

Total Synthesis of the Cyclic Depsipeptide Vioprolide D via its (Z)‐Diastereoisomer

The first total synthesis of vioprolide D was accomplished in an overall yield of 2.0 % starting from methyl (2S)‐3‐benzyloxy‐2‐hydroxypropanoate (16 steps in the longest linear sequence). The cyclic depsipeptide was assembled from two building blocks of similar size and complexity in a modular, highly convergent approach. Peptide bond formation at the C‐terminal dehydrobutyrine amino acid of the northern fragment was possible via its (Z)‐diastereoisomer. After macrolactamization and formation of the thiazoline ring, the (Z)‐double bond of the dehydrobutyrine unit was isomerized to the (E)‐double bond of the natural product. The cytotoxicity of vioprolide D is significantly higher than that of its (Z)‐diastereoisomer.     

Rational design of a bimodular model system for the investigation of heterocyclization in nonribosomal peptide biosynthesis

Cyclization (Cy) domains in NRPS catalyze the heterocyclization of cysteine and serine/threonine to thiazoline and oxazoline rings. A model system consisting of the first two modules of bacitracin synthetase A fused to the thioesterase (Te) domain of tyrocidine synthetase was constructed (BacA1-2-Te) and shown to be active in production of the heterocyclic IleCys(thiazoline). Based on this model system, the feasibility of Cy domain module fusions was investigated by replacing the BacA2 Cy-A-PCP-module with modules of MbtB and MtaD from the biosynthesis systems of mycobactin and myxothiazol, revealing the formation of novel heterocyclic dipeptides. To dissect the reaction sequence of the Cy domain in peptide bond formation and heterocyclization, several residues of the BacA1-2-Te Cy domain were analyzed by mutagenesis. Two mutants exhibited formation of the noncyclic dipeptide, providing clear evidence for the independence of condensation and cyclization.     

Total Synthesis of the Antimitotic Marine Natural Product (+)-Curacin A

The structurally novel antimitotic agent curacin A was prepared in 15 steps and in 2.6% yield for the longest linear sequence. Key steps in our synthesis are the use of a hydrozirconation-transmetalation protocol for the preparation of divinyl alcohol 8, the stereoselective formation of the acyclic triene segment 11 via enol triflate chemistry, and a second hydrozirconation of the conjugated triene followed by an isocyanide insertion. For the preparation of the heterocyclic moiety of curacin A, the oxazoline --> thiazoline conversion offered an efficient access to the sensitive marine natural product.     

Solid‐phase Total Synthesis of (−)‐Apratoxin A and Its Analogues and Their Biological Evaluation

Two approaches for the solid‐phase total synthesis of apratoxin A and its derivatives were accomplished. In synthetic route A, the peptide was prepared by the sequential coupling of the corresponding amino acids on trityl chloride SynPhase Lanterns. After cleavage from the polymer‐support, macrolactamization of 10 , followed by thiazoline formation, provided apratoxin A. This approach, however, resulted in low yield because the chemoselectivity was not sufficient for the formation of the thiazoline ring though its analogue 33 was obtained. However, in synthetic route B, a cyclization precursor was prepared by solid‐phase peptide synthesis by using amino acids 13 – 15 and 18 . The final macrolactamization was performed in solution to provide apratoxin A in high overall yield. This method was then successfully applied to the synthesis of apratoxin analogues. The cytotoxic activity of the synthetic derivatives was then evaluated. The epimer 34 was as potent as apratoxin A, and O‐methyl tyrosine can be replaced by 7‐azidoheptyl tyrosine without loss of activity. The 1,3‐dipolar cycloaddition of 38 with phenylacetylene was performed in the presence of a copper catalyst without affecting the thiazoline ring.     

Solid-phase total synthesis of trunkamide A(1).

Marine organisms are a rich source of novel, biologically active compounds. Herein, the solid-phase total synthesis of trunkamide A, currently in preclinical trials, is presented. Trunkamide A contains a thiazoline heterocycle and two residues of Ser and Thr with the hydroxy function modified as reverse prenyl (rPr). Cornerstones of the synthesis are as follows: (i) solid-phase peptide chain elongation using a quasi-orthogonal protecting scheme with tert-butyl and fluorenyl based groups, on a chlorotrityl resin; (ii) concourse of HOAt-based coupling reagents; and (iii) cyclizations in solution. Furthermore, the following synthetic steps are discussed: (i) preparation of the reverse prenyl derivatives of Ser and Thr; (ii) introduction of precursor of thiazoline as a protected amino thionoacid derivative; and (iii) formation of the thiazoline ring with DAST. All these features make this strategy particularly suitable for the large-scale synthesis of trunkamide A and other peptides containing the same motifs.     

Synthesis of cyclo-PLAI using a combination of solid- and solution-phase methods

Cyclo-PLAI was successfully synthesized using a combination of solid- and solution-phase methods. This current synthesis was found to be faster than the previously reported synthesis for the cyclic peptide. The linear precursor was synthesized on 2-chlorotrityl resin with Fmoc/t-Bu strategy. HATU/HOAt was employed as the coupling reagent in the amide bond formation on the resin. Cyclization of the linear precursor was experimented with HATU/HOAt reagents with different conditions. However, the linear precursor was best cyclized using HATU reagent in DIPEA by stirring the reaction mixture at 0?°C for 1?h and followed by stirring the reaction mixture at room temperature for 30?min, giving the cyclic product in 70% yield (calculated from the linear peptide). Both linear and cyclic products were characterized using HR-TOF-ESMS, ¹H-NMR, ¹³C-NMR, and compared with previously reported spectral data for the cyclic product.     

The bacitracin biosynthesis operon of Bacillus licheniformis ATCC 10716: molecular characterization of three multi-modular peptide synthetases

Background: The branched cyclic dodecylpeptide antibiotic bacitracin, produced by special strains of Bacillus, is synthesized nonribosomally by a large multienzyme complex composed of the three bacitracin synthetases BA1, BA2 and BA3. These enzymes activate and incorporate the constituent amino acids of bacitracin by a thiotemplate mechanism in a pathway driven by a protein template. The biochemical features of these enzymes have been studied intensively but little is known about the molecular organization of their genes.Results: The entire bacitracin synthetase operon containing the genes bacA-bacC was cloned and sequenced, identifying a modular structure typical of peptide synthetases. The bacA gene product (BA1, 598 kDa) contains five modules, with an internal epimerization domain attached to the fourth; bacB encodes BA2 (297 kDa), and has two modules and a carboxy-terminal epimerization domain; bacC encodes BA3, five modules (723 kDa) with additional internal epimerization domains attached to the second and fourth. A carboxy-terminal putative thioesterase domain was also detected in BA3. A putative cyclization domain was found in BA1 that may be involved in thiazoline ring formation. The adenylation/thioester-binding domains of the first two BA1 modules were overproduced and the detected amino-acid specificity coincides with the first two amino acids in bacitracin. Disruption of chromosomal bacB resulted in a bacitracin-deficient mutant.Conclusions: The genes encoding the bacitracin synthetases BA1, BA2 and BA3 are organized in an operon, the structure of which reflects the modular architecture expected of peptide synthetases. In addition, a putative thiazoline ring formation domain was identified in the BA1 gene.     

A facile synthesis of a complex type N-glycan thiazoline as an effective inhibitor against the antibody-deactivating endo-β-N-acetylglucosaminidases

Endo-β-N-acetylglucosaminidases are a class of endoglycosidases that deglycosylate N-glycans from glycoproteins. We describe here a facile synthesis of a complex type N-glycan thiazoline as a new mechanism-based inhibitor for this class of enzymes. The synthesis started with the readily available sialoglycopeptide (SGP) and its conversion into the glycan thiazoline through several enzymatic and chemical reactions. The synthetic glycan thiazoline showed potent inhibitory activity against several endoglycosidases including the two antibody-deactivating enzymes, Endo-S and Endo-S2, from human pathogen Streptococcus pyogenes, which would be useful as tools for structural and functional studies of these enzymes.     

Synthesis of a family of cyclic peptide-based anion receptors

We report here the design and synthesis of a family of novel backbone modified cyclic peptides, bearing dipicolylamine side chains for metal complexation and subsequent anion binding studies. Two approaches to the cyclic peptides were investigated. Initially, a stepwise approach was employed, involving solid-phase assembly of oxazole-based building blocks, followed by solution-phase macrolactamisation of the resulting linear precursor. The alternative strategy involved the formation of linear bisoxazole fragments in solution-phase, followed by a cyclodimerisation reaction. The zinc(II) complexes of these receptors bind selectively to di- and tri-phosphate ions over hydrogenphosphate.     

Synthesis of nitrophenyl-substituted 1,3-thiazoline-2-thiones by oxirane ring opening with several dithiocarbamates

Thiazolo-2(3H)-thiones have been prepared by the reaction of triethylamine dithiocarbamates and 4-nitrophenyloxirane via formation of intermediate 2-hydroxy-2-(4-nitrophenyl)ethyldithiocarbamates with simultaneous dehydrogenation of the thiazolidine ring to a thiazoline. It was found that both the basicity of the starting amines and the temperature influenced the course of the reaction.     

Total synthesis of apratoxin A

[reaction: see text]. We have achieved a total synthesis of apratoxin A in which thiazoline formation was accomplished from the moCys containing amide 4 using PPh3(O)/Tf2O. Deprotection of the Troc and allyl ester in 17, coupling with tripeptide 3, and deprotection of the allyl ester and the Fmoc, followed by macrolactamization provided apratoxin A (1).     

Ring — Chain tautomerism of S-acylalkyl-substituted imidazoles and annelated imidazole systems

The ring-chain tautomerism of S-acylalkyl-substituted 2-mercapto-4,5-diphenylimidazoles, 2-mercaptobenzimidazoles, 2-mercaptonaphth[1,2-d]imidazoles, 8-mercaptopurines, 8-mercaptotheophyllines, and 2-mercaptoimidazolines was studied by means of PMR and IR spectra. It was established that, depending on the structure of the aldehyde or ketone residue, the character of the heteroring conden sed with the imidazole ring, the aggregate state, and the solvent used, these substances exist as open heterylmercapto aldehydes (ketones), cyclic 3-hydroxy derivatives of imidazothiazoline systems, or as mixtures of the indicated tautomeric forms. When there is a substituent in the 2 position of the thiazoline ring, the cyclic compounds exist as mixtures of two diastereoisomeric forms.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1125–1131, August, 1972.     

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.     

Conversion of Heterocycles of The Dihydrothiazine–Thiazoline Series in Aqueous Solutions

When heterocycles of the dihydrothiazine–thiazoline series are treated with an aqueous solution of base, they undergo ring opening, leading to formation of ureidoalkanethiols. Study of solvolysis of the heterocycles when treated with ammonia permitted us to observe a novel heterocyclic ring opening reaction, occurring with formation of 2(3)-guanidinoalkanethiols. We have developed a novel preparative method for obtaining 2(3)-guanidinoalkanethiols.     

Solid-phase synthesis of biaryl cyclic peptides containing a histidine-tyrosine linkage

A solid-phase strategy for the synthesis of biaryl cyclic peptides containing a side-chain to side-chain His-Tyr linkage was developed. The key step was the macrocyclization of a linear peptidyl resin incorporating a 5-bromohistidine and a 3-boronotyrosine via the formation of the biaryl bond by means of a microwave-assisted Suzuki-Miyaura reaction. This method allowed direct access to biaryl cyclic peptides containing a 3- or 5-amino acid ring and bearing the histidine residue at the N- or the C-terminus, being especially conducive for analogues in which this amino acid is located at the C-terminus. This study also served to establish a strategy for the synthesis of biaryl cyclic peptides derived from the two hemispheres of the natural biaryl bicyclic peptides aciculitins.     

Oxidation of 4-carboxylate thiazolines to 4-carboxylate thiazoles by molecular oxygen

A facile and environment-benign oxidation by molecular oxygen was applied for the conversion of 4-carboxylate thiazolines to 4-carboxylate thiazoles. The substituent effect on thiazoline ring was investigated. It was found that electron-poor group on the thiazoline ring could facilitate the oxidation.     

Topological Polymer Chemistry Enters Surface Science: Linear versus Cyclic Polymer Brushes

The cyclic polymer topology strongly alters the interfacial, physico‐chemical properties of polymer brushes, when compared to the linear counterparts. In this study, we especially concentrated on poly‐2‐ethyl‐2‐oxazoline (PEOXA) cyclic and linear grafts assembled on titanium oxide surfaces by the “grafting‐to” technique. The smaller hydrodynamic radius of ring PEOXAs favors the formation of denser brushes with respect to linear analogs. Denser and more compact cyclic brushes generate a steric barrier that surpasses the typical entropic shield by a linear brush. This phenomenon, translates into an improved resistance towards biological contamination from different protein mixtures. Moreover, the enhancement of steric stabilization coupled to the intrinsic absence of chain ends by cyclic brushes, produce surfaces displaying a super‐lubricating character when they are sheared against each other. All these topological effects pave the way for the application of cyclic brushes for surface functionalization, enabling the modulation of physico‐chemical properties that could be just marginally tuned by applying linear grafts.     

Cyclic PNA-based compound directed against HIV-1 TAR RNA: modelling, liquid-phase synthesis and TAR binding

A cyclic molecule including a hexameric PNA sequence has been designed and synthesized in order to target the TAR RNA loop of HIV-1 through the formation of a "kissing complex". For comparison, its linear analogue has also been investigated. The synthesis of the cyclic and linear PNA has been accomplished following a liquid-phase strategy using mixed PNA and fully N-protected (aminoethylglycinamide) fragments. The interactions of this cyclic PNA and its linear analogue with TAR RNA have been studied and the results indicate clearly that no interaction occurs between the cyclic antisense PNA and TAR RNA, whereas a tenuous interaction has been detected with its linear PNA analogue.