Table 1.

Reaction optimisation.^a

entry	catalyst	acid	yield (%)b	ee (%)c
1	3b	HCl	10	5
2	3b	HOTf	73	32
3	3b	FSO3H	78	19
4	3b	2,4-diNBSA	79	63
5	3b	4-NBSA	72	73
6	–	4-NBSA	7	n/a
7	3a	4-NBSA	16	12
8	3c	4-NBSA	84	84
9	3d	4-NBSA	80	85
10	3e	4-NBSA	93	93
11	3f	4-NBSA	91	91
12	3g	4-NBSA	97	88
13	4e	4-NBSA	98	92

^aOptimisation reactions were performed on 0.05 mmol scale.

^bIsolated yields of material purified chromatographically.

^cEnantiomeric excesses (ee’s) determined by HPLC analysis.

MS = molecular sieves; 2,4-diNBSA = 2,4-dinitrobenzenesulfonic acid; 4-NBSA = 4-nitrobenzenesulfonic acid.