Preparation of Silica-HClO₄ catalyst: Perchloric acid (1.25 g, as a 70% aqueous solution) was added to the suspension of silica gel (23.75 g, mesh 230–400) in Et₂O. The mixture was concentrated and the residue was heated at 100 °C for 72 h under vacuum to afford HClO₄–SiO₂ as a free flowing powder. http://dx.doi.org/10.1039/B304178F

To the solution of 2,4-diacetyl phloroglucinol (100 mg, 1 mmol) in acetonitrile (10 mL) were added formaldehyde (0.04 mL, 3 mmol), styrene (74 mg, 1.5 mmol) and 50 %w/w silica-HClO₄ (50 mg). The mixture was then heated to reflux at 80 °C for 6 h. Completion of the reaction was monitored by TLC (by disappearance of 2,4-diacetyl phloroglucinol on TLC). After completion of the reaction, the mixture was cooled to room temperature and was filtered through filter paper. The filtrate was concentrated on a rotary evaporator to give the crude product as a colourless oil. Purification by silica gel (mesh 100-200) column chromatography using EtOAc: Hexane - 1:1 as eluent gave product as white crystals (84% yield).

• Disubstituted phloroglucinols showed better reactivity compared with mono-substituted followed by non-substituted phloroglucinol (For details, see: Org. Biomol. Chem. 2012, 10, 5143–5150  http://dx.doi.org/10.1039/C2OB25376C).
• The method involves 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 flavan skeleton.
• Key features of the protocol are (a). no protection/deprotection steps required; (b). inexpensive, easy to prepare, non-hazardous, easy to separate from reaction mixture, reusable catalyst and (c). diversity-oriented synthesis.

6,8-Diacetyl-5,7-dihydroxyflavan: Yield: 84%; white crystalline solid; m.p. 101-103 °C

¹H NMR (CDCl₃, 500 MHz) δ  ppm 16.15 (s, 1H), 15.18 (s, 1H), 7.40 (m, 5H), 5.15 (dd, J = 2.3, 10.4 Hz, 1H), 2.76 (m, 1H), 2.72 (s, 3H), 2.66 (m, 1H), 2.53 (s, 3H), 2.24 (m, 1H), 2.02 (m, 1H)

¹³C NMR (CDCl₃, 125 MHz) δ ppm 204.5, 203.4, 171.6, 170.9, 162.6, 140.0, 128.8, 128.5, 127.8, 125.8, 104.6, 103.9, 101.1, 79.99, 33.34, 33.10, 28.54, 18.49

IR (CHCl₃) 3400, 2924, 1615, 1423, 1364, 1169, 1105, 1024 cm^-1

ESI-MS m/z 327 [M+1]⁺