Reaction of 3-Acetyl-7,12-dihydro-2-methyl-6H-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‐6H‐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.