Catalytic Synthesis of Xanthene and Unprecedented Evolution of Naphthopyrans Using Heteropoly Acid-Tantalum(V) Oxide Hybrid Composite as Promoter

Abstract

Xanthene derivatives are prepared by using tantalum(V) oxide (Ta₂O₅)-supported heteropoly acid (HPA), Keggin 12-phosphotungstic acid (PTA)-based heterogeneous catalyst PTA@Ta₂O₅ under neat conditions. The composite is prepared by the wetness impregnation method and is characterized by various techniques. Under optimized conditions, xanthenes are synthesized with prominent yields in remarkably short reaction times. The green chemistry metrics are appraised for the xanthene reaction. Surprisingly, a few novel naphthopyran derivatives are isolated instead of the conventional xanthene derivatives when cinnamaldehyde analogous are introduced under the same reaction protocol. Unprecedented naphtho[2,1-b]pyran-type derivatives of 3m, 3n, and 3o are isolated, depending on the specific substituted cinnamaldehyde used, and interestingly, the nature of the substituent in cinnamaldehyde decides the different reaction pathways leading to the formation of respective pyrans. Diverse possible mechanisms are encountered with the PTA@Ta₂O₅ catalyst based on the respective transformations. The solid-state structures of xanthenes and naphthopyrans are thoroughly investigated. Furthermore, some derivatives are studied in vitro to assess their antimicrobial activity, and the findings are compared with those of reference standard antibiotics.