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

The 1‐substituted 3‐amino‐4 H ‐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.