Polyannulated glycopyranosides via radical-mediated tandem reactions. Stereoselective synthesis of 6 center dot 5 center dot 6 dioxatricycles via 5-exo-trig, 6-endo-dig mode .3.

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 iodoalkoxylation of tri-O-acetylglycal. Thanks to the presence of the silyl group the resulting diastereomeric glycosides (S)A(1) and (R)A(1) were readily separated. Triethylborane/oxygen/ethyl iodide promoted iodine transfer afforded doubly annulated glycosides in a 5-eso-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. (C) 1997 Elsevier Science Ltd.