Phosphotungstic acid catalyzed benzylidene acetal formation.

Entry	SM	H3[P(W3O10)4] (equiv.)	Product	Yield
1	1d	0.5	4a	71%
2	1d	0.2	4a	58%
3	1d	0.25	4a	69%
4	1d	0.3a	4a	72%
5	1d	0.3b	4a	69%
6	3b	0.3	4b	84%
7	3c	0.15	4c	82%
8	3d	0.3	4d	65%
9	3e	0.3	4e	86%

^aThe benzaldehyde dimethyl acetal was used 2.0 equivalents.

^bThe benzaldehyde dimethyl acetal was used 3.0 equivalents.