Indole continued ring synthesis

The synthesis continued with the formation of the 2-azabicyclo[3.3.1]nonane ring (D, E-rings), and epimerization of the cyano group at C-20 to an axial position. Closure of the C ring using a Bischler-Napieralski cyclization gave the quaternary indole alkaloid, ( )-melinonine-E (159). A closely related analog, ( )-strychnoxanthine, was also synthesized by a similar method [67]. 

The intramolecular condensation of ort/ro-(2-oxoalkyl)analines continues to provide reliable access to the indole ring system. Kerr and Jackson prepared the indole core of herbindole B 89 in three steps (80%) via oxidation of the Diels-Alder adduct 81 (Plieninger indolization) as illustrated below <07JOC1405>. A similar strategy was applied by the same group in the total synthesis of ( )-decursivine <07EJOC237>. 

Cycloaddition reactions continue to find application in the synthesis of natural products and related substances containing indole and carbazole rings. Moody has published a summary of syntheses of carbazole alkaloids in which cycloadditions of pyrano[3,4-b]indol-3-ones figure prominently. <94SL681>... 

Many examples of Diels-Alder reactions have been carried out on quinodimethanes derived in a variety of ways from pyrrole and indole precursors. This area continues to be widely investigated because of its generality and its synthetic utility. In the case of pyrrole derivatives, this methodology leads to the synthesis of indoles, and particularly those substituted in the benzene ring. 

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