Pentafluoropropionic Acid: An Efficient and Metal-Free Catalyst for the One-Pot Synthesis of Tetrahydrobenzo[b]pyran Derivatives

Table 1

Optimizing the reaction conditions^a.

Entry	Catalyst (mol%)	Solvent	Condition	Time (min)	Yield (%)^b
1	—	EtOH	Reflux	150	30
2	—	H2O	Reflux	150	32
3	—	CHCl3	Reflux	180	25
4	—	CH3CN	Reflux	150	28
5	—	EtOH : H2O (1 : 1)	Reflux	120	34
6	—	EtOH	r.t	180	Trace
7	—	H2O	r.t	180	Trace
8	—	CHCl3	r.t	180	Trace
9	—	CH3CN	r.t	180	Trace
10	—	EtOH : H2O (1 : 1)	r.t	180	Trace
11	(PFPA) 30	—	r.t	100	55
12	(PFPA) 35	—	r.t	100	66
13	(PFPA) 40	—	r.t	100	64
14	(PFPA) 30	EtOH : H2O (1 : 1)	r.t	120	85
15	(PFPA) 35	EtOH : H2O (1 : 1)	r.t	80	90
16	(PFPA) 40	EtOH : H2O (1 : 1)	r.t	80	90
17	(PFPA) 35	EtOH	r.t	100	65
18	(PFPA) 35	H2O	r.t	100	68
19	(PFPA) 35	CHCl3	r.t	120	47
20	(PFPA) 35	CH3CN	r.t	100	56
21	(TFAA) 30	—	r.t	150	41
22	(TFAA) 35	—	r.t	150	52
23	(TFAA) 40	—	r.t	150	51
24	(TFAA) 30	EtOH : H2O (1 : 1)	r.t	150	73
25	(TFAA) 35	EtOH : H2O (1 : 1)	r.t	120	77
26	(TFAA) 40	EtOH : H2O (1 : 1)	r.t	120	79
27	(TFAA) 35	EtOH	r.t	120	36
28	(TFAA) 35	H2O	r.t	120	33
29	(TFAA) 35	CHCl3	r.t	180	24
30	(TFAA) 35	CH3CN	r.t	120	29

aBenzaldehyde (1 mmol), dimedone (1 mmol), and malononitrile (1.2 mmol).
^bIsolated yields.