Studies on the Michael addition of naphthoquinones to sugar nitro olefins: first synthesis of polyhydroxylated hexahydro-11H-benzo[a]carbazole-5,6-diones and hexahydro-11bH-benzo[b]carbazole-6,11-diones

Abstract

A strategy for the synthesis of the novel (6bR,7R,8S,9S,10S,10aR)-8-(benzyloxy)-7,9,10-trihydroxy-6b,7,8,9,10,10a-hexahydro-11H-benzo[a]carbazole-5,6-dione is reported. The key steps were the Michael addition of 2-hydroxy-1,4-naphthoquinone to 1-nitrocyclohexene or 3-O-benzyl-5,6-dideoxy-1,2-O-isopropylidene-6-nitro-alpha-D-xylo-hex-5-enefuranose and the diastereoselective intramolecular Henry reaction of 3-O-benzyl-5,6-dideoxy-S-C-(3'-hydroxy-1',4'-naphthoquinon-2'-yl)-1,2-O-isopropylidene-6-nitro-alpha-D-glucofuranose to give the key (1S,25,35,4R,5R,6R)-3-(benzyloxy)-1,2,4-trihydroxy-5-(3'-hydroxy-1',4'-naphthoquinon-2'-yl)-6-nitrocyclohexane. When 2-hydroxy-1,4-naphthoquinone was replaced by (1,4-dimethoxynaphthalen-2-yl)lithium, the novel (1R,2S,35,4R,4aS,11bS)-2-(benzyloxy)-1,3,4-trihydroxy-1,2,3,4,4a,5-hexahydro-11bH-benzo[b]carbazole-6,11-dione was obtained. (C) 2011 Elsevier Ltd. All rights reserved.