Studies Directed toward the Stereochemical Structure Determination of the Naturally Occurring Glucosidase Inhibitor, Kotalanol: Synthesis and Inhibitory Activities against Human Maltase Glucoamylase of Seven-Carbon, Chain-Extended Homologues of Salacinol

The synthesis of new seven-carbon, chain-extended sulfonium salts of 1,4-anhydro-4-thio- d-arabinitol, analogues of the naturally occurring glycosidase inhibitor salacinol, are described. These compounds were designed on the basis of the structure activity data of chain-extended analogues of salacinol, with the intention of determining the hitherto unknown stereochemical structure of kotalanol, the naturally occurring seven-carbon chain-extended analogue of salacinol. The target zwitterionic compounds were synthesized by means of nucleophilic attack of the PMB-protected 1,4-anhydro-4-thio- d-arabinitols at the least hindered carbon atom of two 1,3-cyclic sulfates differing in stereochemistry at only one stereogenic center. The desired cyclic sulfates were synthesized starting from d-glucose via Wittig olefination and Sharpless asymmetric dihydroxylation. Deprotection of the coupled products by using a two-step sequence afforded two sulfonium sulfates. Optical rotation data for one of our compounds indicated a correspondence with that reported for kotalanol. However, comparison of (1)H and (13)C NMR spectral data of the synthetic compounds with those of kotalanol indicated discrepancies. The collective data from this and published work were used to propose a tentative structure for the naturally occurring compound, kotalanol. Comparison of physical data of previously synthesized analogues with those for the recently isolated six-carbon chain analogue, ponkoranol or reticulanol, also led to elucidation of this structure. Interestingly, both our compounds inhibited recombinant human maltase glucoamylase (MGA), as expected from our previous structure activity studies of lower homologues, with K i values of 0.13 +/- 0.02 and 0.10 +/- 0.02 microM.