Ring Contractions of 4-Oxoquinolizine-3-diazonium Tetrafluoroborates, by an Aza Wolff Rearrangement, to Alkyl Indolizine-3-carboxylates

The 1-substituted 3-amino-4H-quinolizin-4-ones 13, available in two steps from 10 and methyl (Z)-2-benzyloxycarbonylamino-3-(dimethylamino)propenoate (11), were diazotized to give the stable diazonium tetrafluoroborates 7a and 7b. Heating of these diazonium salts in alcohols gave mixtures of 3-unsubstituted quinolizine derivatives 8a and 8b and the alkyl indolizine-3-carboxylates 9a−h. The ratio of the two types of products 8 and 9 was dependent on the type of alcohol employed. Thus, treatment of 7a or 7b with 2-propanol predominantly resulted in the 3-unsubstituted quinolizinones 8, while treatment of 7a or 7b with primary alcohols gave the indolizine-3-carboxylates 9 as the major products in most cases. The transformation of the 4-oxoquinolizine-3-diazonium tetrafluoroborates 7a and 7b into the alkyl indolizine-3-carboxylates 9a−h represents the first example of a Wolff rearrangement in the fused cyclic α-diazoamide series.