Organotransition metal-modified sugars, Part 10 - Chromium iminoglycosylidenes: Synthesis and application to photoinduced C-glycosidation

The iminoglycosylidene complexes 3a,b, 9, and 16a,b are conveniently prepar ed from the sugar lactams 2a,b, 8, and 15a,b by reaction with K2Cr(CO)(5) a nd subsequent deoxygenation with trimethylsilyl chloride (TMSCl). The Cr(CO )(5)-stabilized carbene moiety of the imino-D-ribo-pyranosylidene complexes 9 and 16a,b has been exploited in the photoinduced generation of ketene-li ke species on irradiation with UV light. These intermediates were trapped w ith methanol to produce the methyl 2,6-imino-D-allonates 10 and 17a,b. The C-glycosidation is p-selective and has been applied further to the preparat ion of the galactosyl 2,6-imino-D-allonate 19. Solvent effects suggest that the diastereoselectivity originates in the chromium fragment, which shield s the re face of the proposed ketene intermediate.