Heterocycles play important role in drug discovery research (natural, semi-synthetic and synthetic). Plenty of heterocyclic compounds are widespread in nature and many commercial drugs contain one or more heterocyclic pharmacophore in their structures. Pyrazole, a five-membered heterocyclic scaffold with two nitrogen atoms as the ring constituents, is present in several commercial drugs including sildenafil, rimonabant, fomepizole and celecoxib. On the other hand, 4-aryl/heteroaryl-2,4-dihydropyranopyrazole, a 6/5 fused pyrazole moiety having a phenyl or substituted aryl/heteroaryl group at the 4-poistion, has not been explored adequately from medicinal viewpoints. As a part of our ongoing research leading to the synthesis of bioprivileged heterocyclic scaffolds following environmentally benign route, we have developed a metal-free green synthesis of diversely substituted 4-aryl/heteroaryl-2,4-dihydropyranopyrazole derivatives. Controlled ultrasound energy was used as a green technique to develop this ecofriendly methodology and ethyl acetate was found to be the best green solvent to accomplish high yield of the products. Greener organocatalytic pathway was followed. The newly developed sonochemical procedure satisfies several aspects of green chemistry by (1) eliminating the use of metal catalyst, hazardous solvent, co-catalyst and additive; (2) preserving atom-economy; (3) succeeding easy work-up and isolation procedures; (4) using green source of energy; (5) maintaining high yield of the target compounds; (6) designing safer chemicals; (7) reducing/removing unwanted derivatization; (8) designing energy efficiency; (9) introducing inherently safer chemistry for accident prevention. This green method may find its application in future drug development research.

Highly efficient transition metal-free green synthesis of 4-aryl-2,4-dihydropyranopyrazoles

2019 Fall National Meeting

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