SYNTHESIS OF BIS-2,5-LINKED TETRAHYDROFURANS VIA IODOETHERIFICATION

Iodoetherification of alcohol 4 and several ether derivatives 12-15 af forded bis-tetrahydrofurans 8 and 9. The ratio of 8:9 depended on the nature of the ether substituent with 8 being the major product for R=H , SiMe(3), and Si(t)BuPh(2) whereas 9 was favoured when R=dichlorobenz yl. Attempts to effect ring expansion of 8 afforded ketone 16 wherein hydride migration had occurred. Iodoetherification of trisubstituted h ydroxyalkene 17 afforded predominantly iodoether 21. Attempts to incre ase the amount of iodoether 22 formed led to a 1:1 ratio of 21:22 usin g trimethylsilyl ether 23. Treatment of iodoether 21 and 22 with Ag2CO 3 in aqueous acetone proceeded stereospecifically affording the ring e xpansion products 27 and 28 respectively. Pyran 28 possesses the same stereochemistry as that present in the polyeher antibiotic salinomycin .