Chemistry of insect antifeedants from Azadirachta indica(part 12): use of silicon as a control element in the synthesis of a highly functionalized decalin fragment of azadirachtin

Abstract

A diastereoselective synthesis of a highly functionalized decalin fragment 61 of the insect antifeedant azadirachtin 1 is described. An intramolecular Diels–Alder reaction of triene 15 and subsequent intramolecular aldol reaction were employed to assemble the basic carbon skeleton. A high degree of stereocontrol in the cycloaddition step was achieved by using a dimethyl(phenyl)silyl group to effect endo-selectivity. The silyl group enabled further diastereoselective elaboration and later stereospecific introduction of C(3) hydroxy functionality through oxidation with peracid. Both enantiomers of the intermediate alcohol 40 were obtained by optical resolution. The C(9)–C(10) tetrahydrofuran hemiketal moiety was introduced using a novel 6→5 ring-contraction protocol with initial formation of the δ-lactone 49 by an intramolecular Michael addition. Subsequent α-hydroxylation and methanolysis furnished the tetrahydrofuran hemiketal 50, which was converted in two steps into the fully protected enantiopure target molecule 61. Single-crystal X-ray analyses of compounds 18, 50, and (+)-53 have been carried out and the absolute configuration of compound (+)-53 determined