Temperature-Gradient-Directed NMR Monitoring of a [3 + 3]-Cyclocondensation Reaction Between Alkynone and Ethyl 2-Amino-1H-indole-3-carboxylate Toward the Synthesis of Pyrimido[1,2-a]indole Catalyzed by Cs₂CO₃

Abstract

We describe a NMR strategy to resolve temperature-gradient-monitored real-time chemical reaction involving a [3 + 3]-cyclocondensation reaction between alkynone and ethyl 2-amino-1H-indole-3-carboxylate toward the synthesis of pyrimido[1,2-a]indole catalyzed by Cs₂CO₃. The in situ NMR study clearly indicates that the reactant undergoes [3 + 3]-cyclocondensation reaction through a concerted mechanism, resulting in the product formation. The detailed NMR spectroscopic data led to the optimization of the reaction conditions and quantitative analysis of the product accurately and efficiently.

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GRAPHICAL ABSTRACT

Keywords:

• Alkynone
• cyclocondensation
• heterogenous catalyst
• ¹H NMR
• HR-MAS
• magic angle
• nucleophile

ACKNOWLEDGMENT

S. G. and S. K. S. are thankful to the Council for Scientific and Industrial Research, New Delhi, for providing a fellowship.

Notes

CDRI Communication No. 8227.