Diels-Alder Addition of Acyclic Dienes to Chiral Naphthoquinones

Abstract

The Diels–Alder addition of naphthoquinone (2a) to 1-trimethylsilyloxybuta-1,3-diene (3) afforded a 1.4 : 1 ratio of Diels–Alder adducts (5a) and (6a) which were easily separated by flash chromatography. Individual treatment of adducts (5a) and (6a) with tin(IV) chloride afforded dihydrofuran (11a) as an inseparable 1 : 1 mixture of diastereomers. Diels–Alder addition of naphthoquinones (2a) and (2b) to 1-methoxy-1-trimethylsilyloxybuta-1,3-diene (4) followed by treatment with tin(IV) chloride afforded dihydrofurans (12a) and (12b) respectively, both as inseparable 1 : 1 mixtures of C 2 epimers. Removal of the pantolactone auxiliary from dihydrofuran (12a) was achieved by using lithium borohydride, affording aldehyde (13) which underwent oxidative rearrangement by using ceric ammonium nitrate to pyranonaphthoquinone (14). The inability to separate the individual C 2 epimers of dihydrofuran (12a) easily by flash chromatography did not allow extension of this method to an enantioselective synthesis of pyranonaphthoquinone (14). Pyranonaphthoquinone (14) is closely related to the 3C protease inhibitor thysanone (15).