2,4-Bis(methylsulfanyl)pyrimidine o-quinodimethane: a versatile building block for functionalized fused pyrimidines

Two alternative methods for the preparation of new pyrimidine Diels–Alder cycloadducts from the readily available 2,4-bis(methylsulfanyl)-5,6-dihydrocyclobuta[d]pyrimidine are presented. In the first method, the in situ generated pyrimidine ortho-quinodimethane reacts with various dienophiles to form the respective cycloadducts bearing two methylthio groups, which can be easily replaced by other functional groups. In the second method, one or both of the methylsulfanyl groups of the starting pyrimidine are replaced first and the resulting functionalized pyrimidines are able to undergo Diels–Alder cyclization with different dienophiles to form pyrimidine cycloadducts. These alternative synthetic strategies provide access to a wide variety of pyrimidine cycloadducts with a different substitution pattern on the pyrimidine ring. Yield data indicate that the electronic nature of the functional groups strongly influence the efficiency of the cycloaddition reaction.