SYNTHESIS OF 2,5-SUBSTITUTED PIPERIDINES AND THEIR BICYCLIC PIPERAZINE ANALOGS - THE 2,7-SUBSTITUTED OCTAHYDRO-2H-PYRIDO[1,2-A]PYRAZINES

Partial and complete reduction of the key compound 1-benzyl-5-(ethylenedioxy)-2-piperidinecarbonitrile (1) was applied to generate the corresponding aldehyde 2 and primary amine 3. These were transformed into bicyclic 7-(ethylenedioxy)-2(R)-octahydro-2H-pyrido[1,2-a]pyrazines 7 (R = H) and 15 (R = aryl) through the following sequence: (i) chloroacetylation of 3 and of arylamines derived from 2, (ii) cyclization to give the intermediate lactams 5 and 14, and (iii) reduction with LiAlH4. Deprotection of the N-aryl compounds 15 yielded the corresponding ketone model compounds 16. From amino acetal 7, a complementary ketone synthon 11 was prepared via N-benzylation and cleavage of the acetal group, providing a general route to piperidine-bridged analogues of 1,4-substituted piperazine drugs.