Reaction of 3‐Acetyl‐7,12‐dihydro‐2‐methyl‐6 H ‐indolo[2,3‐ a ]quinolizinium Chloride with Aromatic Aldehydes Forming “Dimers” under Kinetic Control; Stereoselective Acetal versus Aldol Ring Closure

3‐Acetyl‐7,12‐dihydro‐2‐methyl‐6 H ‐indolo[2,3‐ a ]quinolizinium chloride ( 1 ) forms “dimers” ( 5a , b ) by condensation with benzaldehyde and by means of kinetically controlled acetal ring closure (piperidine catalysis). In 5a , b the two quinolizinium units are connected by intercalation of a hydrogenated chroman system whose reactivity is attributed to the presence of a pyranoid hemiacetal structure. On standing or heating of their solution 5a , b are converted via the intermediate 4 and aldol ring closure into the thermodynamically more stable diastereoisomers 2a , b with a bicyclo[3.3.1]nonadiene system. All the ring closures occur with diastereofacial selectivity. In the case of the condensation of 1 with salicylaldehyde the Schiff base of this aldehyde with aniline replaces the aldehyde moiety as well as the piperidine catalyst and yields 10a , b under kinetically controlled conditions. By sodium tetrahydridoborate reduction in methanol 5a is converted into 7 , the X‐ray structural analysis of which confirms structure and stereochemistry.