Quassinoids. 2. Synthesis of an ABCD Ring Precursor Involving a Lactonization Induced by Iodotrimethylsilane

The Diels-Alder reaction of 3-formyl-4a,5,6,7,8,8aα-hexahydro-7β-(benzyloxy)-4aβ,8α-dimethylnaphthalen-2(LH)-one with l-methoxy-3-(trimethylsilyloxy)-l,3-butadiene in benzene at 25 °C furnished an extremely acid-labile Diels-Alder adduct, 3β-(benzyloxy)-8β-formyl-14α-methoxy-12-((triniethylsilyl)oxy)-19-nor-9β-podocarp-12-en-7-one. Stereoselective reduction of this adduct with sodium bis(2-methoxyethoxy)aluminum hydride in toluene at 0 °C and subsequent acid hydrolysis provided 3β-(benzyloxy)-7α-hydroxy-8,β-(hydroxymethyl)-19-nor-9β-podocarp-13-en-12-one. An X-ray crystallographic study of the monobenzoate derivative, 3β-(benzyloxy)-7α-hydroxy-8β-((benzoyloxy)methyl)-19-nor-9β-podocarp-13-en-12-one, of the aforementioned product confirmed the relative stereochemistry of this ABC ring intermediate. Sequential treatment of this monobenzoate derivative with chloroacetic anhydride and sodium iodide furnished an α-iodo ester, 3β-(benzyloxy)-8β-((benzoyloxy)-methyl)-7α-(iodoacetoxy--19-nor-9β-podocarp-13-en-12-one. Cyclization of this α-iodo ester using iodotrimethylsilane provided 3β-(benzyloxy)-20-hydroxy-21-nor-9β-picrasan-12-one benzoate, a tetracyclic ABCD ring progenitor of the quassinoids.