The first enantioselective total syntheses of (−)‐secologanin (1), (−)‐5‐carboxystrictosidine (2), and (−)‐rubenine (3) were accomplished in 10, 9, and 14 steps, respectively. The key transformation in the synthesis of 1 was a sequential anti‐selective organocatalytic Michael reaction/Fukuyama reduction/spontaneous cyclization to form an optically active dihydropyran ring. In addition, the secologanin tetraacetate (16), which is a potential key intermediate for the bioinspired divergent syntheses of monoterpenoid indole alkaloids, was prepared in gram‐scale in seven steps. The total syntheses of 2 and 3, which are classified as glycosylated monoterpenoid indole alkaloids, were achieved through bioinspired transformations such as a diastereoselective Pictet–Spengler reaction, a site‐ and stereoselective epoxidation, and a site‐selective epoxide ring‐opening followed by a lactonization reaction.
A collective synthesis of glycosylated monoterpenoid indole alkaloids is reported. A highly diastereoselective Pictet–Spengler reaction with α‐cyanotryptamine and secologanin tetraacetate as substrates, followed by a reductive decyanation reaction, was developed for the synthesis of (−)‐strictosidine, which is an important intermediate in biosynthesis. This two‐step chemical method was established as an alternative to the biosynthetically employed strictosidine synthase. Furthermore, after carrying out chemical and computational studies, a transition state for induction of diastereoselectivity in our newly discovered Pictet–Spengler reaction is proposed. Having achieved the first enantioselective total synthesis of (−)‐strictosidine in just 10 steps, subsequent bioinspired transformations resulted in the concise total syntheses of (−)‐strictosamide, (−)‐neonaucleoside A, (−)‐cymoside, and (−)‐3α‐dihydrocadambine.
A collective synthesis of glycosylated monoterpenoid indole alkaloids is reported. A highly diastereoselective Pictet–Spengler reaction with α‐cyanotryptamine and secologanin tetraacetate as substrates, followed by a reductive decyanation reaction, was developed for the synthesis of (−)‐strictosidine, which is an important intermediate in biosynthesis. This two‐step chemical method was established as an alternative to the biosynthetically employed strictosidine synthase. Furthermore, after carrying out chemical and computational studies, a transition state for induction of diastereoselectivity in our newly discovered Pictet–Spengler reaction is proposed. Having achieved the first enantioselective total synthesis of (−)‐strictosidine in just 10 steps, subsequent bioinspired transformations resulted in the concise total syntheses of (−)‐strictosamide, (−)‐neonaucleoside A, (−)‐cymoside, and (−)‐3α‐dihydrocadambine.
A new pentacyclic monoterpenoid indole alkaloid glycoside named secorubenine (1) was isolated from the heartwood of Adina rubescens, collected in Indonesia. The structure was elucidated by spectroscopic analysis and chemical modification of isolated secorubenine (1). The bioinspired enantioselective total synthesis of 1 was accomplished in 12 steps, whereafter its structure was determined and the absolute stereochemistry was confirmed.
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