Organotransition metal-modified sugars: Part 25. Spiro-C-glycosidation and C2-homologization of carbohydrates via Fischer glycosylidene complexes: a strategy to novel organometallic disaccharides and a rare example of atropisomerism

Fischer-type glycosylidene complexes are shown to bear considerable potential for the synthesis of C-glycosidic compounds. Insertion of nucleophilic alkynes into the chromium glycosylidene bond as shown for mannosylidene complex 2 affords C2-homologuous carbohydrates still bearing the metal carbene functionality. The insertion of O-ethynyl-carbohydrates leads to novel organometallic disaccharides in which the sugar moieties are linked by a chromium carbene spacer. Ynamine insertion proceeds with virtually complete E-selectivity, and the insertion products reveal a remarkable irreversible atropisomerism. The sugar metal carbenes also serve as glycosylidene sources as demonstrated by the cyclopropanation of electron-deficient alkenes. The mannosylidene complex 2 is further applied to the diastereoselective synthesis of anomeric spirocyclopropyl-C-glycosides which bear a conformationally rigid scaffold.