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 o f acyclic radicals has been used in the stereocontrolled synthesis of substituted tetrahydrofurans. Such a tetrahydrofuran intermediate is r egioselectively cleaved using Me(2)BBr to reveal the acyclic array 2 w hich represents the C-17-C-22 subunit of ionomycin. In experiments tha t provide for a better understanding of hydrogen transfer reactions in volving acyclic radicals, a significant improvement in the stereoselec tivity is observed when the two substituents at the stereogenic center alpha to the radical are imbedded in a cycle (''cycle effect''). A me chanistic rationale is discussed.