Stereoselective Hydrogen Transfer Reactions Involving Acyclic Radicals. Tandem Substituted Tetrahydrofuran Formation and Stereoselective Reduction: Synthesis of the C17-C22 Subunit of Ionomycin
摘要:
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.
A convergent stereocontrolled totalsynthesis of (+)-tautomycin (1), a specific inhibitor of protein serine/threonine phosphatases, has been achieved through an esterification of the C1.–C7. fragment A′ 7 4 with the C1–C26 fragment B′ 7 6 by a modified Yamaguchi method and an aldol reaction of the C17–C26 fragment C 5 with the C1–C16 fragment D 6 using LDA as key steps. The fragments 5 and 6 have been
Synthesis of the tetraene fragment 2, the C1–C12 portion, of calyculins (1f) has been accomplished by use of the Stille coupling of the vinyl iodide (C6-C12) unit 4 with the nitrile (C1–C5) unit 3 followed by converting to the vinyl iodide function according to the Barton's procedure.
The synthesis of Segment B corresponding to the C26–C17 portion of tautomycin was accomplished by coupling reaction between the epoxide (Sub-segment B-1) and the dithiane (Sub-segment B-2). The degradation product of tautomycin corresponding to the C26–C19 portion was also synthesized from Sub-segment B-1.