Intramolecular Palladium(II)-Mediated Alkoxy Carbonylation as a Route to Functionalized Tetrahydropyrans. Synthesis of the C9−C32 Segment of Phorboxazole A

Abstract

[reaction: see text] Hydroxy alkene 12, synthesized stereoselectively from 2-methyloxazole-4-carboxaldehyde, underwent intramolecular methoxy carbonylation in the presence of palladium(II) acetate to give 13 in which all five stereogenic centers around the tetrahydropyran correspond to those in ring C of phorboxazole A. Aldehyde 15, derived from 13, was linked to hydroxy alkene 23 via a Wittig coupling, and the composite 25 was subjected to a second palladium(II) acetate mediated methoxy carbonylation to yield 26, accompanied by acetoxy ester 27.