Spirocyclic Restriction of Nucleosides. An Analysis of Protecting Group Feasibility while Accessing Prototype anti-1-Oxaspiro[4.4]nonanyl Mimics

The first spirocyclic nucleoside featuring a β-hydroxyl (anti) at C5‘ has yielded to synthesis. While the OMOM functionality proved to be sensitive to the conditions necessary to incorporate heterocyclic bases, PMB protection of the carbinol was readily accommodated. The remarkably similar minimum-energy conformations of the title compounds relative to natural thymidine as deduced by Amber calculations in the gas phase are noted.