Table 1. Different Types of Catalysts for the Synthesis of 2,2′-Aryl-methylenebis(3-hydroxy-2-cyclohexene-1-one) Derivatives.

entry	reaction condition	reuse (times)	refs
1	PANI-PTSI, 25 wt %, water reflux, 6 h, 84%	6	(52)
2	PPA–SiO2, 10 mol %, solvent-free, 140 °C, 0.5 h, 71%		(37)
3	glycerol, 2.0 g, solvent-free, 90 °C, 2.5 h, 94%		(36)
4	SbCl3/SiO2, 10 mmol %, solvent-free, 120 °C, 45 min, 94%	3	(53)
5	Al(DS)3, 20 mol %, solvent-free, 80 °C, 10 min, 80%	5	(54)
6	SD–OS3H, 0.0061 mmol, EtOH reflux, 35 min, 90%	3	(32)
7	FeNp@SBA-15, 0.5 mol %, solvent-free, 80 °C, 20 min, 99%	12	(31)
8	ZNPS, 10 mol %, deionized water, 60–70 °C, 90 min, 60%		(34)
9	BAILS, 10 mol %, solvent-free, 100 °C, 45 min, 95%	5	(33)
10	ZrOCl2·8H2O, 2 mol %, solvent-free, 85 °C, 30 min, 87%		(30)
11	PAC–silica gel, 0.018 mol %, solvent-free, ball milling, 24 min, 92%	6	this work