Masaru Kojima

Inserting a sulfur atom into the 1,2-dithiolane ring of lipoic acid (LA racemate) is a promising approach for improving the diversity of lipoic acid (LA racemate). For this purpose, we prepared 1,2,3-trisulfur–lipoic acid derivatives (trisulfur lipoic acid (R-enantiomer, TR-LA, trisulfur lipoamide, racemate TLPA)) by the reaction of R-LA (lipoic acid R-enantiomer) or lipoamide (LPA) and H₂S and performed precise stereochemical studies. As a result, the 6-membered-1,2,3-trisulfur ring (TR-LA and TLPA) showed a completely different profile from that of the five-membered dithiolane compounds (R-LA and LPA) in ¹H- and ¹³C-NMR spectroscopy. Raman spectroscopy of TR-LA and TLPA showed a different profile to that of LA and LPA, which also indicates the uniqueness of the 6-membered 1,2,3-trisulfur ring chromophore. The regeneration of lipoic acid from 1,2,3-trisulfur lipoic acid (TR-LA) was achieved using a phosphine derivative and sodium cyanide while maintaining the stereochemistry of the chirality center, with an almost quantitative yield.

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In the course of our studies on the regioselective carbon-oxygen bond cleavage of the benzylidene acetal group of hexopyranosides with a reducing agent, we found that a combination of a Lewis acid and a reducing agent triggered a ring-opening reaction of the pyranose ring of methyl α-D-allopyranosides. The formation of an acyclic boronate ester by the attachment of a hydride ion at C-1 indicated that the unexpected endocyclic cleavage of the bond between the anomeric carbon atom and the pyranose ring oxygen atom proceeded via an oxacarbenium ion intermediate produced by the chelation between O5/O6 of the pyranoside and the Lewis acid, followed by nucleophile substitution with a hydride ion at C1.