SYNTHESIS OF THE PSEUDOPTERANE 2,5-FURANOCYCLIC RING-SYSTEM BY [2,3] WITTIG RING CONTRACTION OF BRIDGED FURAN AND DIHYDROFURAN PROPARGYLIC ETHERS

Two routes to the 2,5-furanocyclic ring system of the pseudopterane family of natural products are described. Both employ [2,3] Wittig ring contraction of a 15-membered allylic propargylic ether as the key step. The first route utilizes the bridged 2,5-dihydro furanocyclic ether 24 as the immediate precursor. Treatment with BuLi in THF-pentanes at -78 degrees C affords a 70:30 mixture of trans,anti and trans,syn diastereomers 29 and 31 in 68% yield. The acetate derivatives 30 and 32 are dehydrogenated to furan 33 by MnO2 in ether. In the second route, the furano bridged (Z)-allylic propargylic ether 40Z rearranges to furanocycle 41 in 73% yield upon treatment with LiTMP in; THF-pentane at -78 degrees C.