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The total synthesis of a highly potent cytotoxic marine natural product, phorboxazole A, is described. Both a palladium(II)-mediated and a palladium(II)-catalyzed intramolecular alkoxycarbonylation were used as key steps for the preparation of the two cis-2,6-disubstituted tetrahydropyran rings (C22-C26 and C11-C15) of phorboxazole A.
The synthesis used a convergent approach involving five subunits, namely to prepare C3-C8 (195), C9-C19 (149), C20-C32 (131), C33-C41 (130) and C42-C46 (129). Assembly of two subunits, 130 and 131, employed Evans coupling protocol and provided 141 in excellent yield. The coupled product 141 was converted to aldehyde 215 in a six step sequence in which Julia-Kocienski olefination using a benzothiazole sulfone 129 was used to install the triene side chain. Allylsilylation of 149 with 195 afforded 202, which was linked by Wittig olefination with 215 to furnish 216. The C5-C9 trans-2,6-disubstituted tetrahydropyran ring was constructed from 216 in a three step sequence. Further transformations from 219 led to phorboxazole A (1) as an off-white solid. |
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