Table 2. Catalyst Screening in the 1,3-Dipolar Cycloaddition between Aromatic Nitrone 1a and 3-Phenylpropionaldehyde 2.

entry	Cat (mol %)	T (°C)	t (h)	Ca (%)	Yb (%)	S/Ra
1	Pyr (10)	rt	24	30		50:50
2	Pyr·HCl	rt	1.5	>99	97	50:50
3c	A (5)	–10	24	>99	99	95:5
4	B (10)	–10	24	23	20	89:11
5	C (10)	rt	<0.2	>99	99	97:3
6	D (10)	rt	0.5	>99	96	96:4
7	E (10)	rt	<0.2	>99	99	97:3
8	C (10)	–10	1	>99	99	99:1
9	D (10)	–10	8	>99d	95	99:1
10	E (10)	–10	1	>99	99	99:1

^aDetermined by HPLC.

^bIsolated yields.

^cData from ref (19a).

^dConversion was 37% after 1 h.