Preparative and theoretical study on chain length-dependence and substrate selectivity in the cycloalkylation of condensed [1,2,4]triazolo[4,3-b]pyridazine-6(5H)-one-3(2H)-thiones

Cyclization of condensed [1,2,4]triazolo[4,3-b]pyridazine-3(2H)-6(5H)-ones with α,ω-dibromoalkanes afforded a series of novel ring systems including zwitterions and isomeric tetracyclic lactams. The product distribution is controlled by the chain-length of the reagent, the polarity of the solvent and the structure of ring A in the substrate. The observed substrate selectivity was interpreted on the basis of amide-I IR frequencies and by the results of ab initio B3LYP DFT calculations carried out at 6-31G and 6-31G(d) basis sets using IPCM solvation model. The cycloalkylation with 1,4-dibromobutane gave also macrocyles as detectable by-products which underwent ring contraction yielding lactams on attempted chromatographic separation.