Spirocyclic restriction of nucleosides. An analysis of protecting group feasibility while accessing prototype anti-1-oxaspiro[4.4]nonanyl mimics

[GRAPHICS] The first spirocyclic nucleoside featuring a beta -hydroxyl (anti) at C5' h as yielded to synthesis. While the OMOM functionality proved to be sensitiv e to the conditions necessary to incorporate heterocyclic bases, PMB protec tion of the carbinol was readily accommodated. The remarkably similar minim um-energy conformations of, the title compounds relative to natural thymidi ne as deduced by: Amber calculations in the gas phase are noted.