Enantioselective Total Syntheses of (R)- and (S)-Naphthotectone, and Stereochemical Assignment of the Natural Product

Both isomers of naphthotectone, an isoprenoid quinone from Verbenaceae Tectona grandis possessing interesting biological activities, were enantioselectively obtained by two different synthetic routes in which the carbon side-chain of the naphthoquinone core was introduced using either a Sonogashira or a Heck coupling reaction. In both cases, the naphthoquinone core of the final products was obtained by a late-stage anodic treatment. (R)-Naphthotectone was obtained in six steps from leuconaphthazarin with an overall yield of 38 % and an enantiomeric excess of 86 %. This compound was found to have the same absolute configuration as the natural product at its C-3' stereogenic center. (S)-Naphthotectone was obtained in five steps from leuconaphthazarin with an overall yield of 36 % and an enantiomeric excess of 80 %.