Alkaloid carbazolequinones

In 1993, Eurukawa ei al. reported the isolation of bis-2-hydroxy-3-methylcarba-zole (213), bismurrayaquinone-A (215), and bikoeniquinone-A (216) from the acetone extract of the root bark of M. koenigii (82). Bismurrayaquinone-A and bikoeniquinone-A were the first examples of dimeric carbazolequinone alkaloids isolated from Nature. The absolute configurations of these alkaloids are not known (82). However, two years later, Bringmann et al. described the enantiomeric resolution and the chiroptical properties of bismurrayaquinone-A (215) (167) (Schemes 2.51 and 2.52). 

The tricyclic carbazolequinones represent an important class of carbazole alkaloids with a quinone moiety in the A-ring of the carbazole nucleus. This family of... 

Ito, C., Thoyama, Y., Omura, M., Kajiura, I. and Furukawa, H. (1 993) Alkaloidal constituents of Murraya koenigii. Isolation and structural elucidation of novel binary carbazolequinones and carbazole alkaloids. Chemical and Pharmaceutical Bulletin 41, 2096-21 00. 

A synthesis of the carbazolequinone alkaloid murrayaquinone-B, previously isolated from the root bark of Murraya euchrestifolia, is described and the compound was obtained in 13% yield (88JCS(P1)241). Photolysis of 2-amino-1,4-naphthoquinone afforded the pentacyclic quinone 61 (R = H) and some of its N-substituted derivatives (88TL591). From 2,3-dibromo-5,6-dimethyl-l,4-benzoquinone, ethyl acetoacetate, and pyridine, a derivative of the indoiequinone system related to 65 was prepared (87IJC180). 

Recently syntheses of several carbazolequinone alkaloids, isolated from Murraya euchrestifolia, have been reported. 

Prior to the discovery of the aryl-Heck reaction (Chapter 72), the direct Pd-promoted oxidative cyclization of diaryl amines to carbazoles was well known. In 1975 Akennark reported this reaction (Scheme 1, eqnation 1) [1], In addition, A -phenylanthranUic acid gave carbazole-l-carboxylic acid (60%). Miller and Moock used Pd(OAc)j to cyclize 6-anilino-5,8-dimethylisoquinoIine to eUipticine in low yield [2]. The second advance in this chemistry was reported independently by Bittner [3] and Furukawa [4], who described the Pd-mediated (stoichiometric) oxidative conversion of 2-anilino-l,4-benzoquinones and 2-anilino-l,4-naphthoquinones to the corresponding carbazole-l,4-diones and benzo[ ]carbazole-l,6-diones (equations 2, 3). Furukawa s studies included syntheses of several carbazolequinone alkaloids. In 1995 Akermark and colleagues developed catalytic versions (i.e., using tert-butyl hydrogen peroxide [TBHP] or oxygen) of this cyclization (equation 3) [5,6], which elevated the importance of this palladium oxidative cyclization, mainly because of the expense of Pd(OAc)2. Somewhat earlier, Knbiker used cupric acetate as a reoxidant in a synthesis of carbazole-l,4-quinones [7]. 

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