STEREOSELECTIVE HYDROGEN-TRANSFER REACTIONS INVOLVING ACYCLIC RADICALS - TANDEM SUBSTITUTED TETRAHYDROFURAN FORMATION AND STEREOSELECTIVE REDUCTION - SYNTHESIS OF THE C-17-C-22 SUBUNIT OF IONOMYCIN

The tandem iodoetherification reaction and stereoselective reduction of acyclic radicals has been used in the stereocontrolled synthesis of substituted tetrahydrofurans. Such a tetrahydrofuran intermediate is regioselectively cleaved using Me(2)BBr to reveal the acyclic array 2 which represents the C-17-C-22 subunit of ionomycin. In experiments that provide for a better understanding of hydrogen transfer reactions involving acyclic radicals, a significant improvement in the stereoselectivity is observed when the two substituents at the stereogenic center alpha to the radical are imbedded in a cycle (''cycle effect''). A mechanistic rationale is discussed.