The one step synthesis of 2,4-dialkyl and 2,4-diaryl substituted 5,6-dihydrocyclobuta[d]pyrimidines (12a,b) as new precursors for pyrimidine ortho-quinodimethanes and their trapping with different dienophiles and C¢,0 is reported. © 1997 Elsevier Science Ltd. o-Quinodimethane (o-QDM) 1, firstly reported by Cava and Napier, 1 are valuable species in organic synthesis and have been widely used for synthetic applications as highly reactive dienes in cycloaddition reactions, z In contrast, heterocyclic o-quinodimethanes 2 have received less attention despite their potential synthetic interest, 3 and only very recently this chemistry has attracted a great deal of attention. 4 A variety of synthetic alternatives for the generation of different nitrogen containing heterocyclic oquinodimethanes permit to access to this class of reactive intermediates. In this regard, pyridine derived oquinodimethanes 3 and 4 are available from o-bis(cloromethyl)pyridines and subsequent 1,4-elimination. 5 The pyrimidine nucleus occurs widely in compounds with biological and pharmaceutical activity and therefore, pyrimidine o-quinodimethanes and adducts derived from them are of particular interest. It has also been recently described that pyrimidine ortho-quinodimethanes 5 can be generated in a multistep synthetic procedure by thermal extrusion of sulfur dioxide from pyrimidine fused-3-sulfolenes 7 as precursors. 6 Sulfolenes were obtained in four steps by reaction of amidines with 3-methoxycarbonyl-4-oxotetrahydrothiophene followed by conversion into 4-chloropyrimidines, oxidation at sulfur and subsequent displacement of the chlorine atom by nucleophiles. More recently pyrimidine and pyrimidone derivatives of [60]fullerene were formed from the respective heterocyclic o-quinodimethanes. 7 Chung et al. s have reported the synthesis in two steps of 7,8disubstituted quinoxalino-fused sultines 8 and their applications in Diels-Alder reactions with alkenes and