Simultaneous assay of four stereoisomers of diltiazem hydrochloride. Application to in vitro chiral inversion studies

Summary The simultaneous stereospecific assay of four stereoisomers of diltiazem hydrochloride in bulk drug and aqueous solution was developed using HPLC on a Chiralcel OF column. The four isomers were quantitated with good precision by the internal standard method. The chiral inversion of (+)-cis-diltiazem hydrochloride in vitro, stability of its (2S, 3S) configuration in the solid and aqueous states was examined by HPLC. Chiral inversion of (+)-cis-diltiazem hydrochloride was not observed in the solid state, and its (2S, 3S) configuration was stable to heat, humidity and light. Chiral inversion of (+)-cis-diltiazem hydrochloride (2S, 3S) was observed in aqueous solution under UV, but not in aqueous solution stored at 80°C for 5h nor under visible light for 10 h. The (+)-cis-diltiazem hydrochloride (2S, 3S) epimerized to (+)-trans-diltiazem hydrochloride (2R, 3S) with a half-life of 5h in aqueous solution under UV but the reverse chiral inversion of (+)-trans-diltiazem hydrochloride (2R, 3S) to (+)-cis-diltiazem hydrochloride (2S, 3S) was not observed.     

Comparative study of C(2) epimerization of d-glucose and d-mannose catalyzed by water soluble organometallic complexes with nitrogen ligands

Epimerization of d-glucose and d-mannose, catalyzed by the water soluble complexes of Cu(II), Ni(II), Co(II) and Cd(II) with bisnitrogen ligands 4–7, and by Mo(VI) complexes prepared in situ from ammonium heptamolybdate (AHM) with ligands 4–9 is compared. All examined complexes exhibit lower catalytic activity than AHM: strong coordination of the ligands by both (N,O) heteroatoms to metal ions, presumably affords catalytically less active species. Some free ligands and their metal (II) complexes catalyze both C(2) epimerization and isomerization of aldoses to d-fructose.     

Aryl vinyl cyclopropane Cope rearrangements

While the divinyl cyclopropane Cope rearrangement is well-known, and has been broadly applied in synthesis, examples of the aryl vinyl cyclopropane Cope rearrangement are less common and generally limited in scope or reaction yield. The aryl vinyl cyclopropane Cope rearrangement gives access to the benzocycloheptene scaffold, which is present in a variety of naturally occurring and medicinally relevant products. Herein we report a method to obtain either of two regioisomeric benzocycloheptene products via an aryl vinyl cyclopropane Cope rearrangement, featuring additive-controlled regioselectivity. Mechanistic studies indicate a dynamic equilibration of cyclopropane stereoisomers, followed by rearrangement of the cis diastereomer.     

A scalable synthesis of (1R,3S,5R)-2-(tert-butoxycarbonyl)-2-azabicyclo[3.1.0]hexane-3-carboxylic acid

A stereoselective and scalable synthesis of (1R,3S,5R)-2-(tert-butoxycarbonyl)-2-azabicyclo[3.1.0]hexane-3-carboxylic acid (3a) is described. Key to the success of the devised route was the realization that the stereoselectivity of a cyclopropanation step could be controlled by the composition of the functional group at C-α.     

Thiosugar Nucleosides. Effect of Sulfur in the Synthesis of Substituted Azido‐5‐Thio‐D‐Gluco‐ and Allopyranosyl‐N‐Nucleosides and New Isothionucleoside Derivatives Thereof

1‐(2,3,4‐tri‐O‐acety‐6‐azido‐6‐deoxy‐5‐thio‐β‐D‐glucopyranosyl)thymine 5 and the 6‐thio‐septanosylthymine analogue 7 were obtained via the intramolecular displacement of the corresponding tosylate 2 by azide. Alternatively, 5 was obtained from bromination of alcohol 1 in the presence of azide. Deblocking of 5 afforded the nucleoside 6. Glycosylation of the tetraacetate 11, obtained by acetolysis of 10 with thymine, afforded the 3‐O‐tosyl‐β‐D‐glucopyranosylthymine derivative 13, which furnished the 3‐azido‐3‐deoxy‐β‐D‐allopyranosyl‐thymine analogue 14 on reaction with azide ion. Alternatively, the glucoside 12 gave the corresponding gluco analogue 16 on treatment with azide. Acetolysis of 16 furnished the tetraacetate 17, which was subjected for glycosylation to give the gluco nucleoside 18. Deblocking of 14 and 18 afforded the free 3‐azido‐nucleosides 15 and 19, respectively. The isothionucleoside 21 was prepared from treatment of thymine with the 2,3‐epoxide derivative 20 in the presence of Ti(i‐PrO)₄ and triethyl amine. Mild acid hydrolysis of 21 afforded 22. Cycloaddition of the 2‐azido‐altroside 23 with dimethyl acetylenedicarboxylate gave the 1,2,3‐triazole derivative 24. Treatment of 24 with methanolic ammonia afforded the 4,5‐carboxamide analogue 25. The conformations of the new products were studied by NMR spectroscopy.     

C13-Methylation of methyl pheophorbide a and stereoselective preparation of methyl (13R)-methylpyropheophorbide a

The (13²R)-methoxycarbonyl group of methyl pheophorbide a, one of the chlorophyll-a derivatives, was converted to a methyl group through methylation at the C13² position followed by removal of the methoxycarbonyl group. The methylation of the C13² carboanion gave a 4:1 mixture of methyl 13²-methyl-pheophorbide a and its 13²-epimer. The successive pyrolysis of the major methylated product afforded methyl (13²R)-methyl-pyropheophorbide a with a small amount of its (13²S)-epimer. The substitution effects at the C13² position including stereochemistry were discussed on the basis of 1D/2D NMR, UV–vis absorption, and circular dichroism spectroscopic analyses as well as molecular modeling simulation.     

Proline-catalyzed aldol reactions of cyclic diketones: fluorine modifies pathways as well as transition states

Proline-catalysed aldol reactions are central to the development of organocatalysis, and have been the subjects of many studies. List’s results on the effect of fluorine substitution on proline catalysts for an intramolecular aldol condensation provide a perfect test set for computational analysis, as subtle changes in the catalyst structure lead to clear changes in the product ratios. The results show that the carbon-carbon bond forming transition states for the Hajos-Parrish-Wiechart reaction do not account for the observed selectivity in all cases. However, if an analysis of post transition-state epimerization pathways is also included, together with the effect of water, it is possible to account for all of the experimental data.     

Improved Methodology for the Synthesis of a Cathepsin B Cleavable Dipeptide Linker,Widely Used in Antibody-Drug Conjugate Research

Antibody-drug conjugates (ADCs) represent an emerging class of biopharmaceutical agents that deliver highly potent anticancer agents (payloads) selectively to tumors or components associated with the tumor microenvironment. The linker, responsible for the connection between the antibody and payload, is a crucial component of ADCs. In certain examples the linker is composed of a cleavable short peptide which imparts an additional aspect of selectivity. Especially prevalent is the cathepsin B cleavable Mc-Val-Cit-PABOH linker utilized in many pre-clinical ADC candidates, as well as the FDA approved ADC ADCETRIS^® (brentuximab vedotin). An alternative route for the synthesis of the cathepsin B cleavable Mc-Val-Cit-PABOH linker is reported herein that involved six steps from l-Citrulline and proceeded with a 50% overall yield. In this modified route, the spacer (a para-aminobenzyl alcohol moiety) was incorporated via HATU coupling followed by dipeptide formation. Importantly, this route avoided undesirable epimerization and proceeded with improved overall yield. Utilizing this methodology, a drug-linker construct incorporating a potent small-molecule inhibitor of tubulin polymerization (referred to as KGP05), was synthesized as a representative example.     

Synthesis of aspergillide A from a synthetic intermediate of aspergillide B

The first synthesis of aspergillide A, a cytotoxin produced by a marine-derived fungus, has been achieved from a synthetic intermediate of aspergillide B by using a proline-mediated epimerization of a 2,6-trans-substituted tetrahydropyran-2-acetaldehyde intermediate into the corresponding cis-isomer via a retro-oxy-Michael/oxy-Michael sequence as the key transformation.     

Epimerization and hydrolysis of 3,6-dimethyl-2,3,5,6-tetrahydro[1,2,4]thiadiazino[6,5,4-hi]indole 1,1-dioxide

In this study, configurational and chemical stability of (R,R),(S,S),(R,S),(S,R)-3,6-dimethyl-2,3,5,6-tetrahydro[1,2,4]thiadiazino[6,5,4-hi]indole 1,1-dioxide (1) were investigated by dynamic and stopped-flow HPLC methods. Single epimeric mixtures (R,R),(R,S)-1 and (S,S),(S,R)-1 were obtained combining synthetic and chromatographic strategies. Separation of (R,R)-1 and (R,S)-1 was achieved by chiral chromatography and absolute configuration of eluted epimers has been assigned basing on molecular modelling calculations. Epimerization and hydrolysis of (R,R),(R,S)-1 have been studied by classical off-column, dynamic HPLC and stopped-flow HPLC methods. The influence of different parameters, such as temperature, pH and dielectric constant was evaluated. The data obtained indicate that (R,R),(R,S)-1 undergoes to a rapid epimerization in aqueous solvent and hydrolysis in acidic conditions. Moreover, epimerization and hydrolysis were investigated in presence of an artificial membrane and in physiological buffers (pH 2.2 and 7.0 at 37.5°C) to simulate in vivo conditions.