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. T he K2M(CO)(5)-'dianion' approach (M = Cr, Mo, W) provides access to metal i minoglycosylidenes. Stoichiometric metathesis with highly electrophilic pen tacarbonyl[(diphenyl)carbene]chromium is applied to the synthesis of furano sylidene 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 compl exes, which undergo stereoselective C-glycosidation and C-C-bond formation in the carbohydrate backbone. An aminolysis/Mitsunobu recyclisation sequenc e provides a straightforward route to C-4 stereoinverted metal iminofuranos ylidenes. Vinylchromate intermediates allow for stereoselective labeling an d alkylation at C-2. Insertion of electron-rich alkynes into the metal-carb ene carbon bond leads to novel organometallic C-glycosides. Carbohydrate-de rived vinylcarbene complexes undergo diastereoselective Diels-Alder and Mic hael reactions. In addition to these ligand-centered reactions the chromium carbonyl fragment may serve as a template for [3 + 2 + 1] benzannulation, c yclopropanation and photocarbonylation. This methodology has been applied t o the metal-mediated stereoselective synthesis of densely functionalized ch romans, novel anomeric spirocyclopropanes and C-disaccharides. (C) 2001 Els evier Science B.V. All rights reserved.