Chiral oxabicyclic systems from ribonolactone

We describe the synthesis of (–)-5-O-diphenyl-t-butylsiloxymethyl-5H-furan-2-one (4) from D-ribonolactone; conversion of this chiral butenolide into (+)-(IR,6S,7S)-7-diphenyl-t-butylsiloxymethyl-8-oxabicyclo[4.3.0]non-3-en-9-one (10)(by means of a Diels-Alder reaction with butadiene); and thence via an efficient six-step sequence into (+)-(IR,2S,5S)-2-diphenyl-t-butylsiloxymethyl-3-oxabicyclo[3.3.0]octan-7-one (15). A structure proof for this final product was provided by X-ray structural analysis. The crystals are triclinic, space group P1 with Z= 2, a= 8.351(9), b= 17.400(8), c= 8.591 (8)Å; α= 77.8(1), β= 71.1(1), γ= 93.8(1)° 3423 Independent reflections were measured on a diffractometer and the structure has been refined to R 0.069. In the two independent molecules, the 3-oxabicyclo[3.3.0]octan-7-one fragments have strikingly different conformations.