Phorboxazole B synthetic studies: construction of C(1–32) and C(33–46) subtargets

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Abstract
The convergent syntheses of the C(1–32) and C(33–46) domains of phorboxazole B are described. An iterative cyclocondensation strategy exploited the Jacobsen hetero-Diels–Alder (HDA) reaction as a platform for the synthesis of both the C(5–9) and C(11–15) tetrahydropyran rings. The use of 2-silyloxydiene coupling partners bearing an increasing resemblance to the phorboxazole skeleton was found to lead to a reduction in diastereoselectivity, however, in the case of the C(11–15) ring. The coupling of aldehyde 21 and 2-silyloxydiene 20 by this route provided a C(1–32) fragment which was elaborated to the macrolide core of phorboxazole B. The synthesis of the C(33–46) domain involved a Nozaki–Kishi coupling of aldehyde 31 and vinyl iodide 39. The syntheses of 31 and 39 were highly diastereoselective: an Evans [Cu(Ph-pybox)](SbF6)2-catalysed Mukaiyama aldol reaction formed the cornerstone of the synthesis of 31 whilst a Nagao–Fujita acetate aldol reaction provided a convenient means of installing the sole stereogenic centre of 39.