REACTION OF 3-ACETYL-7,12-DIHYDRO-2-METHYL-6H-INDOLO[2,3-ALPHA]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 Sa, 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 Sa, 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.