Synthesis of the carbazole alkaloid murrayaquinone-B

Abstract

A synthesis of the carbazolequinone alkaloid murrayaquinone-B (2) is described. The route to the 1-methoxycarbazole (14a) involves a regioselective Claisen rearrangement to give the 7-(3-methylbut-2-enyl)indole (9a), and formation of the third ring of compound (14a) by cyclisation of the aldehyde (13a), which is prepared from the lactone (11a)(Scheme 1). The methoxycarbazole (14b), which lacks three methyl groups, was prepared by a similar route. Photo-oxidation of the carbazole (14a) gave murrayaquinone-B (2), although a similar photo-oxidation of the pyranocarbazole (21), prepared in an analogous manner (Scheme 2), failed to yield the related carbazolequinone alkaloid, pyrayaquinone-B (6).