Polyannulated glycopyranosides via radical-mediated tandem reactions, stereoselective synthesis of 6-5-6 dioxatricycles via 5-exo-trig, 6-endo-dig mode - III.

Article abstract of DOI:10.1016/S0040-4020(97)00518-8

A series of C-silylated enynols, e.g. 6-methyl-1-trimethylsilyl-hept-5-en-1-yn-4-ol (rac-A) was prepared and submitted to N-iodosuccinimide mediated iodoalkoylation of tri-O-acetylglycal. Thanks to the presence of the silyl group tile resulting diastereomeric glycosides (S)AI and (R)AI were readily separated. Triethylborane/oxygen/ethyl iodide promoted iodine transfer afforded doubly annulated glycosides in a 5-exo-trigonal, 6-endo-digonal cascade. The required re-protection of the three acetoxy groups was carried out orthogonally. The newly installed iodocyclohexene moiety served as site for further functionalization, which was accomplished by metal-halogen exchange followed by electrophilic capture or by reduction/epoxidation.