Tandem sigmatropic shifts in [4 + 2] cycloaddition reactions of 1,3-diazabuta-1,3-dienes with butadienylketene: Synthesis of pyrimidinone derivatives

The reactions of 4-dialkylamino substituted 1,3-diazabuta-1,3-dienes 1 with butadienylketene 2, are shown to undergo [4 + 2] cycloadditions to yield 5-(buta-1′,3′-dienyl)pyrimidinone 4 and tandem [1,5]H and [1,5]SCH₃ shifts are shown to accompany the [4 + 2] cycloaddition reactions of 4-dialkylamino-4-methylthio substituted 1,3-diazabuta-1,3-dienes 5 with 2. The regioselective reactions of N-arylamino-1,3-diazabuta-1,3-dienes 11 and 14 with butadienyl-ketene 2 are reported to yield 5-(buta-1′,3′-dienyl)-2-dialkylaminopyrimidin-4(3H)-one 13 and a mixture of 5-(buta-1′,3′-dienyl)-2-methylthiopyrimidin-4(3H)-one 17, 2-methylthio-5-[1′-(N-phenylamino)but-2′-enyl]pyrimidin-4(3H)one 19 and 2-methylthio-5-[3′-(N-phenylamino)but-1′-enyl]pyrimidin-4(3H)-one 20, respectively. Tandem [1,5]H, [1,3]NHPh and [1,5]NHPh shifts are involved in the formation of pyrimidinones 19 and 20. The Diels-Alder reactions of the 5-dienylpyrimidinones with dimethyl acetylenedicarboxylate (DMAD) and 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD) yielded corresponding cycloadducts.