D ‐Erythronolactone and 2,3‐ O ‐Isopropylidene‐ L ‐erythrose as C 4 Building Units: An Efficient Synthesis of both Enantiomers of endo ‐Brevicomin and its 7‐Vinyl Analogues

Both enantiomers of endo ‐brevicomin (+)‐ and (−)‐ 3a were synthesized from D ‐erythronolactone ( 1 ) and 2,3‐ O ‐isopropylidene‐ L ‐erythrose ( 2 ), respectively. In the course of the synthesis also the 7‐vinyl analogues (+)‐ and (−)‐ 3b of endo ‐brevicomin are available. The key step of the synthesis is the diastereoselective acetalization of the diols 1 and 10 with 4‐(phenylsulfonyl)‐2‐butanone ( 4 ) and boron trifluoride‐diethyl ether complex as a catalyst. The described procedure yields the key compounds 5a and ent ‐ 7b in good yields and with high diastereomeric purity.