Synthesis of (+)-1,3:2,5-dianhydroviburnitol ((+)-(1R,2R,3S,5R,6S)-4,7-dioxatricyclo[3.2.1.03,6]octan-2-ol)

Epoxidation of (?)-(1R,2R,4R)-2-endo-cyano-7-oxabicyclo2.2.1]hept-5-en-2-exo-yl acetate ((?)-5) followed by saponification afforded (+)-(1R,4R,5R,6R)-5,6-exo-epoxy-7-oxabicyclo2.2.1]heptan-2-one ((+)-7). Reduction of (+)-7 with diisobutylaluminium hydride (DIBAH) gave (+)-1,3:2,5-dianhydroviburnitol ( = (+)-(1R,2R,3S,4R,6S)-4,7-dioxatricyclo3.2.1.0^3,6]octan-2-ol; (+)-3). Hydride reductions of (±)-7 were less exo-face selective than reductions of bicyclo2.2.1]heptan-2-one and its derivatives with NaBH₄, AlH₃, and LiAlH₄ probably because of smaller steric hindrance to endo-face hydride attack when C(5) and C(6) of the bicyclo-2.2.1]heptan-2-one are part of an exo oxirane ring.