We report herein an asymmetric Pictet–Spengler reaction of α‐ketoesters. In the presence of a catalytic amount of simple alanine‐derived squaramide and p‐nitrobenzoic acid, reaction of tryptamines with methyl 2‐oxoalkanoates afforded the corresponding 1‐alkyl‐1‐methoxycarbonyl tetrahydro‐β‐carbolines (THBCs) in high yields and ee values. A primary kinetic isotope effect (KIE=4.5) using C2‐deteurium‐labelled tryptamine indicates that rearomatization through deprotonation of the pentahydro‐β‐carbolinium ion could be the rate‐ and enantioselectivity‐determining step. A concise enantioselective total synthesis of (+)‐alstratine A, a hexacyclic cagelike monoterpene indole alkaloid, featuring this reaction as a key step, was subsequently accomplished. Remeasurement of the [a]D value of the natural product indicates that natural alstratine A is dextrorotatory rather than levorotatory as it was initially reported in the isolation paper.
We report herein an asymmetric Pictet-Spengler reaction of α-ketoesters. In the presence of a catalytic amount of simple alanine-derived squaramide and p-nitrobenzoic acid, reaction of tryptamines with methyl 2-oxoalkanoates afforded the corresponding 1alkyl-1-methoxycarbonyl tetrahydro-β-carbolines (THBCs) in high yields and ee values. A primary kinetic isotope effect (KIE = 4.5) using C2-deteurium-labelled tryptamine indicates that rearomatization through deprotonation of the pentahydro-β-carbolinium ion could be the rate-and enantioselectivity-determining step. A concise enantioselective total synthesis of (+)-alstratine A, a hexacyclic cagelike monoterpene indole alkaloid, featuring this reaction as a key step, was subsequently accomplished. Remeasurement of the [a] D value of the natural product indicates that natural alstratine A is dextrorotatory rather than levorotatory as it was initially reported in the isolation paper.
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