Concise Synthesis of Arnottin I and (−)-Arnottin II

Application of the Buchwald protocol to the coupling of o-bromobenzoates and 1-tetralones directly
affords benzodihydronaphthopyrones with a fused tetracyclic system. Aromatization of the 7,8-dimethoxy-2,3-methylenedioxy derivative yielded arnottin I, whereas oxidation with dioxirane afforded dihydroarnottin
II composed of a spiro phthalide−tetralone system. Sharpless asymmetric dihydroxylation using AD-mix yielded optically active dihydroarnottin II with good enantioselectivity. The absolute stereochemistry
of the stereogenic center in the (+)-spiro product was determined to be R by X-ray crystallographic
analysis of the dibromo derivative. (+)-Dihydroarnottin II was subjected to successive bromination and
dehydrobromination to prepare (−)-arnottin II. The R-configuration of natural (−)-arnottin II, previously
assigned by application of the exciton chirality method to the Cotton effects observed in the CD spectrum,
was confirmed by asymmetric synthesis.