STEREOSELECTIVE SYNTHESIS OF (+ -)-INDOLIZIDINES 167B AND 209D AND THEIR TRANSISOMERS BASED ON THE REDUCTIVE ALLYLBORATION OF PYRIDINE/

A general method for the synthesis of 5-substituted indolizidines base d on intramolecular cyclization of trans- and cis-2-allyl-6-R-1,2,3,6- tetrahydropyridines, obtained from pyridine and triallylborane, has be en elaborated. The closure of the five-membered ring is carried out by hydroboration-oxidation followed by cyclization of the resulting delt a-amino alcohols in the presence of the Ph3P-CBr4-Et3N system. (Pr2BH) (2) and Pr3B are used as the hydroborating reagents, and H2O2 in an ac id medium is used for the oxidation of 2-[3-(dipropylboryl)propyl]-Del ta(3)-piperideines formed. This method has been used for the synthesis of two natural alkaloids: indolizidine 209D (cis-5-hexylindolizidine) and its trans-isomer were prepared from cis- and trans-2-allyl-6-hexy l-1,2,3,6-tetrahydro respectively; indolizidine 167B and trans-5-propy lindolizidine were synthesized from cis- and trans-2,6-diallyl-1,2,3,6 -tetrahydropyridine, respectively.