Review on Synthesis and Medicinal Application of Dihydropyrano[3,2-<i>b</i>]Pyrans and Spiro-Pyrano[3,2-<i>b</i>]Pyrans by Employing the Reactivity of 5-Hydroxy-2-(Hydroxymethyl)-4<i>H</i>-Pyran-4-One

Abstract

Pyrano[3,2-b]pyrans are an important class of heterocyclic compound containing fused 4H-pyrans rings that are a structural constituent of many natural, non-natural, drug candidates, synthetic compounds and plays a significant role in the field of medicinal and pharmaceutical chemistry because of their wide-ranging biological activities. The natural γ-pyrone, 5-hydroxy-2-(hydroxymethyl)-4H-pyran-4-one, popularly known as kojic acid have emerged as the most effective substrates for the synthesis of several useful dihydropyrano[3,2-b]pyrans and their corresponding spiro-pyrano[3,2-b]pyran derivatives. Furthermore, the attractiveness of 5-hydroxy-2-(hydroxymethyl)-4H-pyran-4-one and its derivatives has drastically increased over the past decades due to their vast applications in medicinal chemistry, food industry, cosmetic products, and chemical industry. Benefited from the diversity and application in various fields of the starting materials, an extensive array of synthetic strategies for the construction of dihydropyrano[3,2-b]pyrans and spiro-pyrano[3,2-b]pyran derivatives has been developed successfully over the last decades. This review will summarize all the synthetic strategies developed for the construction of dihydropyrano[3,2-b]pyran and spiro-pyrano[3,2-b]pyrans using the reactivity of 5-hydroxy-2-(hydroxymethyl)-4H-pyran-4-one from 1997 to 2020 and focused on their medicinal application. In order to furnish a complete and understandable overview, this review article is organized based on the nature of the catalyst employed.