The total synthesis of okaramine alkaloids

Abstract

The okaramines are a series of indole alkaloids isolated from Penicillium simplicissimum fermented on okara, the residue from ground soybeans. The first total synthesis of okaramines J and C, and studies towards the synthesis of okaramine A are described. Okaramines J and C were synthesised 6om derivatives of L-tryptophan. Alkylation of a sterically hindered hexahydropyrroloindoline with an alkynyl bromide, followed by hydrogenation gave a key N - reverse prenyl intermediate. A facile acid-catalysed N-reverse prenyl to C-prenyl aza-Claisen rearrangement of this intermediate led to the total synthesis of okaramine J. Alternative conditions were found that avoided this rearrangement to enable the synthesis of okaramine C. Two strategies have been explored for the formation of an 8-membered ring like that present in okaramine Acid-catalysed cyclisation of an alkene onto an indole has been investigated and the cyclisation achieved in low yield with AICI3. A general route to vinyl amides has been developed using ethanolamine as the alkene precursor. This methodology has been used to prepare substrates for ring-closing metathesis, which has been investigated as an alternative means of forming an 8-membered ring

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