Table 2. Brønsted base screen for DAL with diene 13d, acid chloride 5, and (S)-(–)-BTM as catalyst.

Entry	Base	de a (%)	ee b , c (%)	Conversion d (%)
1	Et3N	71	60	>95(60)
2	Pyridine	76	85	>95(46)
3	Hünig's base [EtN(iPr)2]	71	65	>95(55)
4	2,6-Lutidine	60	99	>95(68)
5	2,6-Di-tert-butylpyridine [DTBP]	95	99	>95(43)

^aDetermined by ¹H NMR (500 MHz) analysis of crude reaction mixtures.

^bDetermined by chiral HPLC analysis (see ESI for details).

^cEnantiomeric excess of the major (–)-14f (endo) diastereomer (ee values for the exo diastereomer were similar).

^dYields in parentheses refer to isolated, purified yields of cycloadducts.