New synthesis with acetylene biscobalthexacarbonyl complex (1)

Syntheses of multi-functional natural and unnatural compounds have been of great significance in organic chemistry not only as the target molecules but also as leading field for conceptual and methodological developments. Particularly those compounds in relation to biology have collecting attentions from various field such as structure-activity relationship, asymmetric synthesis, drug design for chemotherapy and agricultural control, etc. We have to keep looking for new chemistry for these purposes; thus, we became interesting in the acetylene biscobalthexacarbonyl complex from its synthetic point of views. It started from our previous C-glycosidation with silylacetylenes providing sugar acetylenes, in which the alkynyl group at the anomeric position of a pyranose ring can be epimerized through a biscobalthexacasbonyl complex by treatment with trifluoromethanesulfonic acid. Three steps involving cobalt complexation, acidic transformation and decomplexation afford overall epimerization and thus one can obtain either the alpha- or beta-alkynyl C-glycoside as desired. Ring opening of a dihydropyran derivative using Nicholas-type cation intermediates was also an accomplished part of this study. Several decomplexation conditions of endo-type acetylene-cobalt complexes provide various olefins possessing potential utilities for synthesis. These methodologies have been utilized for the synthesis of polyoxygenated natural products and derivatives. Basis of these chemistry is described in this issue, and the application and more examples are to be described in the next issue.