Domino Reactions of Amidines with Methyl 2-Chloro-2-cyclopropylideneacetate as an Efficient Access to Cyclobutene-Annelated Pyrimidinones

[reaction: see text] An efficient one-step synthesis of 2,4-diazabicyclo[4.2.0]octa-1(6),2-dien-5-ones 3 from methyl 2-chloro-2-cyclopropylideneacetate (1) and amidines 2a-c as well as N,N-dimethylguanidine (2d) is described. Similar to the benzocyclobutenes, the cyclobutene-annelated pyrimidones 3 undergo thermal ring opening and the resulting o-quinodimethane analogues readily cycloadd dienophiles to yield tetrahydroquinazolone derivatives.