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Cercosporin-Photocatalyzed [4+1]- and [4+2]-Annulations of Azoalkenes Under Mild Conditions
Published on: July 17, 2020
Synthetic and quantum mechanical studies into the N-heterocyclic carbene catalyzed (4 + 2) cycloaddition
Sarah J Ryan1, Andreas Stasch, Michael N Paddon-Row
1School of Chemistry, Monash University, Clayton 3800, Victoria, Australia.
Abstract:
The N-heterocyclic carbene catalyzed (4 + 2) cycloaddition between α,β-unsaturated acid fluorides and TMS dienol ethers provides cyclohexene fused β-lactone intermediates stable below -20 °C. These can be intercepted reductively or with organolithium reagents to produce diastereomerically pure cyclohexenes (>20:1 dr) with up to four contiguous stereocenters. The mechanism has been investigated using theoretical calculations and by examining secondary kinetic isotope effects. Together these studies implicate the formation of a diastereomerically pure β-lactone intermediate by a stepwise (4 + 2) cycloaddition involving Michael addition, aldol cyclization, and lactonization.
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