Antitumor Agents - 183 - Syntheses, Conformational Analyses, and Antitubulin Activity of Allothiocolchicinoids

7-O-Substituted analogues of allothiocochicine were synthesized and evaluated for their inhibitory effects on tubulin polymerization in vitro. Ketone 6, a key compound in this study, was derived from thiocolchicone 5 by ring contraction. The structure of 6 was determined from spectral data. Optically active alcohols 7a and 7b were obtained by reduction of ketone 6 followed by chemical resolution including a separation of the camphanate diastereomers 8a and 8b and basic hydrolysis. The aR,7R configuration of 8b was verified by X-ray crystallographic analysis. Almost all compounds had strong inhibitory effects on the tubulin polymerization reaction, with IC50 values from 1.7 to 9.0 mu M. The camphanates, cyclohexanates, and, most notably, the 7S-benzoate ester (10a), were inactive with IC50 values >40 mu M. Compounds 6 and 7a also showed potent antitumor activity with GI(50) values at nM concentration range for most cell lines in NCI's in vitro screening. Generally, the 7S enantiomers of colchicinoids with a troplone C-ring showed greater activity than the 7R enantiomers. In the current allothiocolchicinoid (with a benzenoid C-ring) study, only small differences occurred between the two active enantiomers of each pair. The acyl esters with a 7S configuration were slightly more active than the 7R isomers. However, the aroyl ester with a 7S configuration was less active than the 7R isomer. NMR, optical rotation, and molecular modeling studies revealed two conformers in a solvent-dependent equilibrium for both 7S and 7R isomers. In polar solvents, the molecular chirality in esters with a 7-O-aroyl substituent was reversed from aS to aR or from aR to aS at an intensified rate. [References: 22]

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