Tetrahydroquinolizinium ylides: preparation and 1,3-dipolar cycloaddition

By the Cu(II)-catalyzed reaction of 2-(4-diazo-3-oxoalkyl)pyridines (2), 4-alkoxycarbonyl (or 4-acyl)-3-oxo-1,2,3,4-tetrahydroquinolizinium ylides (3) were obtained in high yields. From the cycloaddition reaction of 3 with acetylenic esters (propynoates or acetylenedicarboxylates) the labile [2 + 3] cycloadducts, 3-oxo-3H-2a,4,5,8a-tetrahydropyrrolo[2,1,5-de]quinolizine-2a-carboxylates (8 or 12), were identified, which further reacted with DMAD (dimethyl acetylenedicarboxylate) to afford azocine derivatives (15 or 16) and produced pyrrolodihydroquinolizines (9 or 20) by dealkoxycarbonylation.