Enantioselective Synthesis of α‑Quaternary Mannich Adducts by Palladium-Catalyzed Allylic Alkylation: Total Synthesis of (+)-Sibirinine
A catalytic enantioselective method for the synthesis of α-quaternary Mannich-type products is reported. The two-step sequence of (1) Mannich reaction followed by (2) decarboxylative enantioselective allylic alkylation serves as a novel strategy to in effect access asymmetric Mannich-type products of "thermodynamic" enolates of substrates possessing additional enolizable positions and acidic protons. Palladium-catalyzed decarboxylative allylic alkylation enables the enantioselective synthesis of five-, six-, and seven-membered ketone, lactam, and other heterocyclic systems. The mild reaction conditions are notable given the acidic free N–H groups and high functional group tolerance in each of the substrates. The utility of this method is highlighted in the first total synthesis of (+)-sibirinine.
© 2015 American Chemical Society. This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes. Received: November 26, 2014. Publication Date (Web): January 11, 2015. This manuscript is dedicated to Professor Teruaki Mukaiyama in celebration of the 40th anniversary of the Mukaiyama Aldol reaction. The authors wish to thank NIH-NIGMS (R01GM080269), Amgen, the Gordon and Betty Moore Foundation, the Caltech Center for Catalysis and Chemical Synthesis, and Caltech for financial support. Y.N. thanks Toray Industries Inc. for a postdoctoral fellowship. B.P.P. thanks the NSF for a predoctoral fellowship. K.C. acknowledges the Japan Society for the Promotion of Science for a postdoctoral fellowship. The authors thank Mona Shahgholi and Naseem Torian for mass spectrometry assistance and Dr. Scott Virgil (Caltech) for instrumentation assistance.
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