Unexplored reactivity of 2-oxoaldehydes towards Pictet–Spengler conditions: concise approach to β-carboline based marine natural products

Abstract

Novel reactions under Pictet-Spengler conditions between tryptophan methyl ester/tryptamine and 2-oxoaldehydes have been developed and successfully utilized for the total synthesis of Merinacarboline (A and B), Eudistomin Y1, Pityriacitrin B, Pityriacitrin, Fascaplysin and analogues.2-Oxoaldehydes (OA) are among a few precursors that have been used extensively to synthesize a large variety of heterocyclic compounds. 1 Pictet-Spengler is one of the various reactions reported on OA leading to the synthesis of b-carbolines. 2-11 A large number of naturally occurring b-carbolines with acyl substitution at the C-1 position have shown promising antiinammatory, 2 anti-malarial, 12 anti-cancer, 3,13 anti-phospholipase A2, 14 anti-microbial, 15 and anti-bacterial activities. 16 In view of these biochemical observations, convenient synthetic methods for the synthesis of such constructs are desirable. Even though the Pictet-Spengler reaction of tryptamine/tryptophan/ tryptophan methyl ester with OA for generation of 1-substituted b-carbolines has been explored, 2,6 a few areas are still untouched. As we know in contemporary organic synthesis, coupled domino reactions, wherein two or more domino processes occur sequentially in the same reaction, are considered to be most effective for the synthesis of complex organic compounds using simple and readily available building blocks. 17,18 In this context, we developed a few unexplored reactions between tryptophan methyl ester/tryptamine and 2-oxoaldehydes with a focus on establishing a multicoupled domino strategy for the synthesis of various marine based natural products and their analogues.We initiated the present study with a reaction of 2,4-dimethoxy acetophenone 1 with tryptophan methyl ester 2 in the presence of iodine in DMSO. The reaction of 1 (1 equiv.) and 2 (1 equiv.) with I 2 (1 equiv.) in DMSO at 90 C for 1.5 h afford the desired product in low yield (38%, Table 1, entry 15). To improve upon the yields of desired product, a preliminary set of reactions between tryptophan methyl ester (1 equiv.) and acetophenone (1 equiv.) under different condition were carried out (Table 1). Table 1 Optimization studies for synthesis of 3a employing 2,4-dimethoxy acetophenone as building block a Entry (Equiv.) Temp. Time Yield b (%)

Pityriacitrin marine alkaloids as novel antiviral and anti‐phytopathogenic‐fungus agents

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