Tandem one-pot synthesis of flavans by recyclable silica–HClO4 catalyzed Knoevenagel condensation and [4 + 2]-Diels–Alder cycloaddition

Abstract

An efficient one-pot multi-component synthesis of flavans using perchloric acid supported on silica as a recyclable heterogeneous catalyst has been described. This is the first report of direct one-step construction of a flavan skeleton from a phenolic precursor. The method involves a Knoevenagel-type condensation leading to in situ formation of transient O-quinone methide which further undergoes [4 + 2]-Diels-Alder cycloaddition with styrene to yield a flavan skeleton. The method provides easy access to a wide range of bio-active natural products viz. flavonoids, anthocyanins and catechins.

ortho-Amidoalkylation of Phenols via Tandem One-Pot Approach Involving Oxazine Intermediate

“…The solution was allowed to slowly warm to room temperature, and stirring was continued for a further 4 h, during which time a salmon colored precipitate formed. This precipitate was then filtered off and washed with more water, to get 2,4-diformyl phloroglucinol 2a ( 15 General Procedure for Preparation of Diacylphloroglucinols 2b−2e. A solution of phloroglucinol (10 g, 79.36 mmol) and acetic acid or propionic acid/isovaleric acid/butyric acid (3 equiv) in BF 3 − etherate (100 mL) was refluxed at 100°C for 2.5 h. Reaction mixture was cooled to room temperature, poured into crushed ice and extracted with ethyl acetate (100 mL × 3).…”

A convenient synthesis of coumarinyl chalcones using HClO4–SiO2: A green approach

“…In this context, HClO 4 -SiO 2 is a suitable candidate for various organic reactions due to its inherent properties such as high acidity, efficiency, stability, inexpensiveness, recyclability, selectivity, operational simplicity, non-corrosiveness, moisture-insensitivity and easy handling etc (Das et al, 2006, Chakraborti andGulhane, 2003). It has been successfully employed for numerous organic reactions, such as protection of hydroxyl groups (Shaterian et al, 2007), acetylation of phenols, thiols, alcohols, amines (Chakraborti and Gulhane, 2003), synthesis of xanthenes / substituted coumarins (Maheswara et al, 2006)/ chromenyl pyridines (Ghashang et al, 2014)/ β-keto enol ethers / quinoxalines and dihydropyrazines / flavans (Bharate et al, 2012)/ enaminones and enamino esters / acylals (Kamble et al, 2006) and chemoselective carbon sulfur bond formation (Khatik et al, 2007). It has been also applied in Hantzsch (Dasri et al, 2011), Winkler (Heydari and MaMani, 2008), Mannich and Biginelli (Maheswara et al, 2008) reactions.…”

Trifluoroacetic acid catalyzed thiophenylmethylation and thioalkylmethylation of lactams and phenols via domino three-component reaction in water

“…5 Further, the 3CC of phenols with formaldehyde and lactam gave amidoalkyl products 2 through Mannich-type condensation. 6 As a continuation of these results, herein we investigated the reactivity of thiophenols in these 3CC reactions.…”