4,6-Disubstituted quinolizidines stereochemistry

The 5,8-disubstituted indolizidines and 1,4-disubstituted quinolizidines are the more common structural patterns found in amphibian skin[21]. None of these alkaloids has so far been reported from any other source. In addition, the biological activity of only a few 5,8-disubstituted indolizidines has been investigated due to the isolation in minute quantities from the skin. Among them, the relative stereochemistry of quinolizidine 2071 was anticipated to be 75 by our chiral synthesis of 76[35] followed by stereocontrolled synthesis of 75[36]. A sample of synthetic racemate of 75 had produced the best separations on GC analysis with (3-dextrin chiral column[36]. 

All the currently known indolizidine and quinolizidine alkaloids isolated from ants are illustrated in Fig. 6. (+ )-Monomorine I (427), the well-studied trail pheromone constituent of the Pharaoh ant Monomorium pharaonis), and the analogs 428-431 were described in the earlier volumes in this series. These five compounds are 3,5-disubstituted indolizidines bearing short saturated or mono-unsaturated hydrocarbon chains. The relative stereochemistry in monomorine VI (430) still remains unknown, while that of 431 was atypical for the class at the time of its isolation 380). The structural resemblance of fiiis group of alkaloids to the... 

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