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
.