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 C2 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 C2 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).
Asymmetric Michael addition of 2-trimethylsilyloxyfuran to chiral naphthoquinones
作者:Margaret A Brimble、John F McEwan、Peter Turner
DOI:10.1016/s0957-4166(98)00088-3
日期:1998.4
various Lewis acids afforded the corresponding furofuran adducts 5–12. Moderate levels of diastereomeric excess were obtained using (R)-pantolactone, (S)-N-methyl-2-hydroxysuccinimide and (R)-(+)-4-benzyl-2-oxazolidinone as chiral auxiliaries. Low asymmetric induction was achieved using a camphorsultam auxiliary. X-Ray crystallographic analysis of the pantolactone adduct enabled determination of the absolute