Efficient Approach to Androstene-Fused Arylpyrazolines as Potent Antiproliferative Agents. Experimental and Theoretical Studies of Substituent Effects on BF3-Catalyzed Intramolecular [3+2] Cycloadditions of Olefinic Phenylhydrazonest

Highly diastereoselective Lewis acid induced intramolecular 1,3-dipolar cycloadditions of alkenyl phenylhydrazones (containing various substituents on the aromatic ring) obtained from a D-secopregnene aldehyde were carried out under fairly mild conditions to furnish androst-5-ene-fused arylpyrazolines in good to excellent yields. The ability of phenylhydrazones to undergo cyclization was found to be affected significantly by the electronic features of the substituents on the aromatic moiety. The rates of the ring-closure reactions were observed to be increased by electron-donating and decreased by electron-withdrawing groups. The experimental findings on the BF3-catalyzed transformations were supported by calculations of the proposed mechanism at the BLYP/6-31G(d) level of theory, indicating a noteworthy dependence, mainly of the initial complexation step, and hence of the whole process, on the character of the substituent. The cycloaddition was estimated to occur via a zwitterionic intermediate rather than involving a pure concerted mechanism. The antiproliferative activities of the structurally related pyrazoline derivatives were tested in vitro on three malignant human cell lines (HeLa, MCF7, and A431): the microculture tetrazolium assay revealed that several compounds exerted marked cell growth-inhibitory effects. The highest cytotoxic activities, displayed by the p-methoxyphenylpyrazoline derivative 7d (IC50 values: 2.01, 2.16, and 1.41 mu M on HeLa, MCF7, and A341 cells, respectively), were better than those of cisplatin (IC50 values: 12.43, 9.63, and 2.84 mu M, respectively).