Comparison of different catalysts for the synthesis of dihydropyrano[3,2-c]chromene derivatives.

Entry	Catalyst (mol%)	Reaction conditions	Time (min)	Yield (%)	References
1	SDS	Water/60 °C	150	88	48
2	Nano ZnO	Ethanol reflux	90	49	49
3	Nano Al(OH)3	Ethanol reflux	120	48	49
4	DAHP	Ethanol–H2O/25 °C	240	85	50
5	(S)-proline	Ethanol–H2O/100 °C	180	78	50
6	Nano Al2O3	Ethanol reflux	120	71	51
7	P 4 VPy	Ethanol–H 2 O/70 °C	5	95	47