Table 1. Screenings of the Acid Loadings^a.

entry	loading (mol %)	conv. (%)b	entry	loading (mol %)	conv. (%)b
1	0.005	68	14	30	91
2	0.01	76	15	60	89
3	0.03	91	16	100	84
4	0.05	93	17	120	78
5	0.10	95	18	300	59
6	0.20	94	19	600	6
7	0.25	94	20c	0.10	93
8	0.30	95	21d	0.10	93
9	0.5	92	22e	0.10	96
10	1.0	92	23f	0.10	93
11	3.0	92	24g	0.10	91
12	6.0	92	25h	0.10	92
13	12	91	26i	0.10	91

^aUnless otherwise specified, a mixture of aldehydes 1 (2 mmol) and hydrochloric acid in methanol was stirred at an ambient temperature for 20 min.

^bDetermined by gas chromatography (GC) analyses of the crude reaction mixture.

^cAt 50 °C for 3 min.

^dAt 30 °C for 10 min.

^eAt 0 °C for 2 h.

^fAt −20 °C for 2 h.

^gAt −60 °C for 24 h.

^h8 mol/L HCl was used.

ⁱ4 mol/L HCl was used.