Diels-Alder addition of acyclic dienes to chiral naphthoquinones

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-dien e (4) followed by treatment with tin( IV) chloride afforded dihydrofurans ( 12a) and (12b) respectively, both as inseparable 1 : 1 mixtures of C2 epime rs. Removal of the pantolactone auxiliary from dihydrofuran (12a) was achie ved by using lithium borohydride, affording aldehyde (13) which underwent o xidative rearrangement by using ceric ammonium nitrate to pyranonaphthoquin one (14). The inability to separate the individual C2 epimers of dihydrofur an (12a) easily by flash chromatography did not allow extension of this met hod to an enantioselective synthesis of pyranonaphthoquinone (14). Pyranona phthoquinone (14) is closely related to the 3C protease inhibitor thysanone (15).