Initial screening of CPAs^a.

Entry	CPA	Time (h)	Yield of 3aab (%)	endo/exoc	eec (%)
1	(R)-4a	12	95	81 : 19	71/88
2	(R)-4b	36	95	88 : 12	5/−12
3	(R)-4c	36	67	87 : 13	5/−3
4	(R)-5	20	95	74 : 26	71/NDd
5	(S)-6	74	62	99 : 1	88/NDd
6e	(S)-6	5 d	91	99 : 1	92/NDd

^aUnless otherwise noted, all reactions were carried out using 0.1 mmol of 1a, 0.12 mmol of 2a, and 0.010 mmol of CPA catalyst (10 mol%) in toluene (1 mL).

^bIsolated yield.

^cDiastereomeric and enantiomeric excess were determined by chiral stationary phase HPLC analysis.

^dND: not determined.

^eMS 5A (50 mg) was used as an additive.