Novel intramolecular cyclization of 2-(buta-1,3-dienyl)-3-methylpyrazines and 3-(buta-1,3-dienyl)-4-methyl-1,2,5-oxadiazoles into 5H-cycloheptapyrazines and 4H-cyclohepta-1,2,5-oxadiazoles

2-(Buta-1,3-dienyl)-3-methylpyrazines (5) and 3-(buta-1,3-dienyl)-4-methyl-1,2,5-oxadiazoles (10) were synthesized starting from base-induced reaction of 2,3-dimethylpyrazine or 3,4-dimethyl-1,2,5-oxadiazole with α,β-unsaturated carbonyls. The thus obtained heteroaromatics bearing a butadienyl moiety and a methyl at the adjacent position underwent intramolecular cyclization by the action of LDA to give the corresponding heteroaromatics fused with a seven-membered ring [7 (8)] and [11 (12)] in moderate to high yields.