Metal glycosylidenes: novel organometallic tools for C-glycosidation: Part 19. Organotransition metal modified sugars

Different synthetic methodologies have been developed for the introduction of the ‘Fischer-type carbene functionality’ into a carbohydrate skeleton. The K2M(CO)5-‘dianion’ approach (M=Cr, Mo, W) provides access to metal iminoglycosylidenes. Stoichiometric metathesis with highly electrophilic pentacarbonyl[(diphenyl)carbene]chromium is applied to the synthesis of furanosylidene complexes; chromium pyranosylidenes are accessible by addition of lithioglucals to the donor-stabilized pentacarbonylchromium fragment. Sugar-derived propynols have been applied to the synthesis of vinylcarbene complexes, which undergo stereoselective C-glycosidation and CC-bond formation in the carbohydrate backbone. An aminolysis/Mitsunobu recyclisation sequence provides a straightforward route to C-4 stereoinverted metal iminofuranosylidenes. Vinylchromate intermediates allow for stereoselective labeling and alkylation at C-2. Insertion of electron-rich alkynes into the metalcarbene carbon bond leads to novel organometallic C-glycosides. Carbohydrate-derived vinylcarbene complexes undergo diastereoselective Diels–Alder and Michael reactions. In addition to these ligand-centered reactions the chromiumcarbonyl fragment may serve as a template for [3+2+1] benzannulation, cyclopropanation and photocarbonylation. This methodology has been applied to the metal-mediated stereoselective synthesis of densely functionalized chromans, novel anomeric spirocyclopropanes and C-disaccharides.