Structural Analysis and Reactivity of Unusual Tetrahedral Intermediates Enabled by SmI2‐Mediated Reduction of Barbituric Acids: Vinylogous N‐Acyliminium Additions to α‐Hydroxy‐N‐acyl‐carbamides.

Structural characterisation and reactivity of new tetrahedral intermediates based on a highly modular barbituric acid scaffold, formed via chemoselective electron transfer using the SmI2–H2O reagent, are reported. Lewis acid promoted cleavage of bicyclic α-amino alcohols affords vinylogous N-acyliminium ions, which undergo selective (>95:5, 1,4 over 1,2) capture with a suite of diverse nucleophiles in a practical sequence to biologically active uracil derivatives.