Synthesis of the glucuronide metabolite of ABT-751

A linear synthesis of the glucuronide metabolite of ABT-751 was replaced with a convergent synthesis that features direct glycosidic coupling between the aglycone and a trichloroacetimidate glucuronyl donor. Structural elucidation of a unique and unexpected difluoroboron complex of the desired glycosidic coupling product along with optimization of the synthetic steps is described.     

Total synthesis of leustroducsin B via a convergent route

Total synthesis of leustroducsin B was achieved via a convergent route, which includes Julia coupling reaction of segment A with segment B followed by Stille coupling reaction of segment C.     

酶催化法合成CCK-8的一个四肽片段

用酶催化法合成了缩胆囊素(CCK-8)的1个四肽片段(Phac-Met-Gly-Trp-Met-OEt).该四肽片段是依次通过Phac-Met-OEt与Gly-OMe·HCl,Trp-OMe和Met-OEt偶合得到的,这3步偶合反应分别用α-糜蛋白酶、木瓜蛋白酶和α-糜蛋白酶作为催化剂,并对影响反应的因素进行了研究,反应中间体和产物均经质谱(FAB-MS)分析证实.     

“Armed-disarmed” glycosidation strategy based on glycosyl donors and acceptors carrying phosphoroamidate as a leaving group: A convergent synthesis of globotriaosylceramide

A stereocontrolled synthesis of globotriaosylceramide with three different glycosidic linkages has been accomplished by linear and convergent routes exploiting “armed-disarmed” glycosidation methodology based on glycosyl donors and acceptors carrying tetramethylphosphoroamidate as a leaving group. In particular, the convergent strategy featuring a coupling of a galactosyl-(1→4)-galactosyl donor with a glucosylceramide derivative has proven to be extremely efficient.     

Toward the total synthesis of disorazole A(1) and C(1): asymmetric synthesis of a masked southern segment

[reaction: see text] A highly convergent asymmetric synthesis of the masked southern segment of the antimitotic agent disorazole A(1) involves a Sonogashira coupling between a C1'-C10' enyne and a suitably protected C11'-C19' vinyl iodide. The central E,Z,Z-triene moiety is masked as a more stable ynediene.     

Practical routes to the triarylsulfonyl chloride intermediate of a β3 adrenergic receptor agonist

A β₃ adrenergic receptor agonist was prepared on a multi-kilogram scale in high yield and purity via a convergent synthesis. A key intermediate in this synthesis was an arylthiazolylbenzenesulfonyl chloride. The triaryl segment of this sulfonyl chloride was assembled at the thiazole ring via coupling of α-haloketone and thiobenzamide precursors (Hantzsch synthesis). Three strategies for introducing the para-sulfonyl chloride moiety were developed and evaluated. The sulfonation/chlorination and diazotization/chlorosulfonylation routes were found the most efficient.     

Promising general solution to the problem of ligating peptides and glycopeptides

Our global goal is that of synthesizing complex polypeptides and glycopeptides in homogeneous form. Chemistry-derived access to homogeneous biologics could well have useful consequences in the discovery of drugs and vaccines. The key finding in this study is that thio acids can become highly competent acyl donors following even trace levels of oxidative activation, thereby undergoing amide bond formation upon reaction with N-terminal peptides. Though our data set does not establish the specific mechanism of this reaction, a framework to account for the fact that minute levels of oxidation actuate amide bond formation with high turnover is offered. An apparently general coupling of thio acids (including complex peptide thio acids with N-termini of complex peptides) has thus been realized. These ligations are conducted with minimal α-epimerization in the C-terminal group and allow for the coupling of N-terminal and C-terminal glycopeptides en route to homogeneous glycoproteins.     

‘O-Acyl isopeptide method’: racemization-free segment condensation in solid phase peptide synthesis

We disclosed a novel ‘racemization-free segment condensation’ based on the ‘O-acyl isopeptide method’ in which an N-segment including C-terminal O-acyl isopeptide structure with urethane-protected Ser/Thr residue was employed for the segment condensation, suggesting that the use of this method contributes to the effective convergent synthesis of long peptides/proteins.     

The Total Synthesis of Starfish Ganglioside GP3 Bearing a Unique Sialyl Glycan Architecture

The total synthesis of ganglioside GP3, which is found in the starfish Asterina pectinifera, has been accomplished through stereoselective and effective glycosylation reactions. The sialic acid embedded octasaccharide moiety of the target compound was constructed by [4+4] convergent coupling. A tetrasaccharyl donor and acceptor that contained internal sialic acid residues were synthesized with an orthogonally protected N‐Troc sialic acid donor as the key common synthetic unit, and they underwent highly stereoselective glycosidation. The resulting sialosides were subsequently transformed into reactive glycosyl acceptors. [4+4] coupling furnished the octasaccharide framework in 91 % yield as a single stereoisomer. Final conjugation of the octasaccharyl donor and glucosyl ceramide acceptor produced the protected target compound in high yield, which underwent global deprotection to successfully deliver ganglioside GP3.     

Synthesis of docosasaccharide arabinan motif of mycobacterial cell wall

Mycobacterial arabinan is a common constituent of both arabinogalactan (AG) and lipoarabinomannan (LAM). In this study, synthesis of β-Araf containing common arabinan docosasaccharide motif (22 Araf monomer units) of mycobacterial cell wall was achieved. Our synthetic strategy toward arabinan involves (1) the stereoselective β-arabinofuranosylation using both 3,5-O-TIPDS-protected and NAP-protected arabinofuranosyl donors for straightforward intermolecular glycosylation and intramolecular aglycon delivery (IAD), respectively, and (2) the convergent fragment coupling with branched fragments at the linear sequence using thioglycoside donor obtained from the corresponding acetonide at the reducing terminal of each fragment through a three-step procedure. Because the acetonide at the reducing terminal of all fragments would be converted to thioglycoside as the glycosyl donor, and mainly Bn ether protections were used, our strategy will be readily applicable to the synthesis of more complex arabinan, arabinogalactan, and arabinomycolate derived from mycobacterial CWS.     

Total synthesis of the antitumor acetogenin mosin B: desymmetrization approach to the stereodivergent synthesis of threo/trans/erythro-type acetogenins

A total synthesis of the threo/trans/erythro-type acetogenin mosin B and one of its diastereomers has been achieved. The carbon skeleton is assembled in a convergent fashion from two segments (a THF ring segment and a gamma-lactone segment) through the Nozaki-Hiyama-Kishi reaction. The THF ring segment was stereoselectively constructed by a stereodivergent synthesis starting from a common intermediate (4-cyclohexene-1,2-diol) based on a desymmetrization strategy. The gamma-lactone segment was synthesized by coupling a triflate and a chiral alpha-sulfenyl gamma-lactone. By virtue of these synthetic results, we suggest that the absolute configuration of natural mosin B is 1 a. Antiproliferative effects of 1 a and 1 b were also investigated.     

Formal synthesis of (-)-apicularen A

[reaction: see text] A formal synthesis of (-)-apicularen A has been completed. The synthesis features a cyanohydrin acetonide coupling as a convergent approach to the C9-C18 segment and an intramolecular Diels-Alder addition sequence to create both the 10-membered macrocycle and the aromatic ring.     

A convergent synthesis of the C1-C16 segment of goniodomin A via palladium-catalyzed organostannane-thioester coupling

A convergent synthesis of the C1-C16 segment of goniodomin A, an actin-targeting marine polyether macrolide natural product, has been achieved via a 2-fold application of palladium-catalyzed organostannane-thioester coupling.     

Convergent synthesis of a GPI containing an acylated inositol

A GPI of sperm CD52 was synthesized by a highly convergent procedure, representing the first chemical synthesis of a complex GPI having an acylated inositol. The presence of a large acyl group resulted in unusual properties and reactions of the relevant intermediates, which gave rise to a number of problems. To overcome the problems and achieve the target molecule, a new synthetic strategy was developed. First, the pseudodisaccharide of 2-O-palmitoylinositol was phospholipidated, and then the trimannose segment and the phosphoethanolamine group were sequentially attached. Global deprotection eventually afforded the sperm CD52 GPI. The method may be useful for the synthesis of other GPIs having an acylated inositol.     

Trypsin-catalyzed synthesis of dipeptide containing alpha-aminoisobutyric acid using p- and m-(amidinomethyl)phenyl esters as acyl donor

Two series of inverse substrates, p- and m-(amidinomethyl)phenyl esters derived from N-(tert-butyloxycarbonyl)amino acid, were prepared as acyl donor components for enzymatic peptide synthesis. They were found to be readily coupled with an acyl acceptor such as L-alanine p-nitroanilide to produce dipeptide. An alpha-aminoisobutyric acid containing dipeptide was especially obtained in satisfactory yield. Streptomyces griseus trypsin was a more efficient catalyst than the bovine trypsin. The optimum condition for the coupling reaction was studied by changing the organic solvent, pH, and acyl acceptor concentration. It was found that the enzymatic hydrolysis of the resulting product was negligible.     

Synthesis of nonadjacently linked tetrahydrofurans: an iodoetherification and olefin metathesis approach

[reaction: see text] A convergent approach to the synthesis of the bis-tetrahydrofuran (THF) components of the nonadjacently linked THF-containing acetogenins is illustrated in the synthesis of 2, a potential intermediate for the antitumor agent bullatanocin (squamostatin C). The plan centers on the olefin cross-metathesis of THF allylic alcohol derivatives 3 and 4 as the key segment coupling step and the assembly of 3 and 4 through the iodoetherification of 1,2-O-isopropylidene-5-alkene precursors.     

α-Chymotrypsin-catalyzed peptide synthesis in frozen aqueous solution using N-protected amino acid carbamoylmethyl esters as acyl donors

A kinetically controlled peptide synthesis catalyzed by α-chymotrypsin was performed in frozen aqueous solution (ice, −24 °C). The yield of the peptide was significantly improved by the use of the carbamoylmethyl (Cam) ester as the acyl donor instead of the conventional ethyl ester. The peptide yield increased up to ca. 90% when N-benzyloxycarbonyl (CBZ)-Phe-OCam and H-Phe-NH₂ were used as the acyl donor and nucleophile, respectively. Such an improvement of the peptide yield in ice was also observed in the coupling of other CBZ-amino acid Cam esters as acyl donors. Furthermore, this approach was applied to the synthesis of peptides containing d-amino acids. The peptides such as CBZ-d-Phe-Phe-NH₂, CBZ-Phe-d-Phe-NH₂ and CBZ-d-Phe-d-Phe-NH₂ were also obtained in excellent to moderate yields in ice. A high diastereoselectivity towards the l–l peptide was observed when the racemic amino acid Cam ester was used as the acyl donor in ice.     

Substrate mimetics in protease catalysis: characteristics,kinetics, and synthetic utility

This article reviews the latest developments in protease-catalyzed peptide synthesis focusing on the use of substrate mimetics. The substrate mimetics approach takes advantage of the characteristic of this novel type of substrates to direct the enzyme to recognize an alternative site on the acyl donor, i.e. the site-specific ester leaving group, mediating the acceptance of originally poorly reactive acyl moieties. At first the kinetics and catalytic mechanism of substrate mimetics-mediated reactions are discussed on the basis of hydrolysis, peptide synthesis, protein-ligand docking, and molecular dynamics studies. By the example of the Glu-specific V8 protease and the aromatic amino acid-specific chymotrypsin both the empirical and computer-aided design of specific substrate mimetics is described. The influence of the leaving group specifically recognized by the enzyme is also considered. The benefits of these artificial substrates over common acyl donor components are illustrated by selected synthesis reactions of small peptides, peptide isosteres, non-peptidic carboxylic acid amides, and the coupling of peptide fragments at non-specific ligation sites resulting in biologically active peptide products. Finally, this review focuses on critical syntheses that uses specific-amino acid-containing peptides as the reactants of ligation. Based on these, the restrictions of the substrate mimetics approach is critically discussed and techniques to their overcoming are presented.     

Catalytic Asymmetric Total Synthesis of Exiguolide

The catalytic asymmetric total synthesis of (−)-exiguolide, a complex 20-membered macrolide embedded with a bis(tetrahydropyran) motif, is reported. The convergent synthesis involves the construction of the C1–C11 tetrahydropyran segment via catalytic asymmetric allylation and Prins cyclization, and the formation of the C12–C21 phosphonate segment via catalytic asymmetric cyclocondensation reaction and Johnson–Claisen rearrangement. The synthesis of 15-epi-exiguolide is also described.     

Insights into long-range structural effects on the stereochemistry of aldol condensations: a practical total synthesis of desoxyepothilone F.

A processable total synthesis of a potent antitumor agent, desoxyepothilone F (dEpoF, 21-hydroxy-12,13-desoxyepothilone B, 21-hydroxyepothilone D), has been accomplished. The route is highly convergent. The new technology has also been applied to a total synthesis of 12,13-desoxyepothilone (dEpoB). The crucial point of departure from previous syntheses of dEpoB and dEpoF involves presentation of the C1-C11 sector for Suzuki coupling with C3 in reduced form. Hitherto, the required S stereochemistry at C3 had been implemented via reduction of a keto function after Suzuki coupling. Whereas that chemistry worked quite well in a synthesis of dEpoB, it was not transferable to a high-yielding synthesis of dEpoF. The reduction of the keto group at C3 via a Noyori protocol after Suzuki coupling had proved to be very difficult. In our current approach, two consecutive aldol reactions are used to fashion the acyl sector. In the first aldol condensation, C6 becomes attached to C7. Following protection at C7, a two-carbon acetate equivalent is used to join C2 and C3 with very high asymmetric induction at C3. Only after this center has been implemented is the Suzuki reaction conducted. This major advance allowed us to synthesize dEpoF in a straightforward fashion. These findings found ready application in the total synthesis of dEpoB. Another part of the study involved analysis of the factors associated with aldol condensations joining C6 to C7. In the work described herein, the consequences of the status of C3 in promoting the C6-C7 aldol coupling are probed in detail. Dramatic stereochemical long-range effects uncovered during the study are described, and a working model to explain these effects has emerged.