Table 1. Control experiments for the SGP-catalysed Mannich reaction^a.

Entry	Catalyst	Yield (%)b	dr (syn:anti)c	e.e. (syn) (%)d
1	protease type XIV from Streptomyces griseus (SGP)	66	85:15	82
2	no enzyme	28	41:59	0
3	albumin from chicken egg white	23	46:54	0
4	albumin from bovine serum	21	58:42	7
5e	SGP denatured with urea	87	48:52	8
6f	urea	28	39:61	0
7g	SGP pretreated with 2.5 mM Cu2+	62	88:12	82
8g	SGP pretreated with 25 mM Cu2+	61	87:13	77
9g	SGP pretreated with 250 mM Cu2+	51	43:57	9
10g	SGP pretreated with 2.5 mM Ag+	60	88:12	82
11g	SGP pretreated with 25 mM Ag+	65	82:18	74
12g	SGP pretreated with 250 mM Ag+	67	38:62	8

^aUnless otherwise noted, the reaction conditions were as follows: 4-nitrobenzaldehyde (0.5 mmol), aniline (0.55 mmol), cyclohexanone (5 mmol), deionised water (0.10 mL), MeCN (0.9 mL) and catalyst (50 mg) at 30°C for 96 h.

^bYield of the isolated product after silica gel chromatography.

^cThe dr was the syn:anti ratio, as determined by chiral HPLC analysis.

^de.e. value of the syn-isomer, determined by HPLC using a chiralpak AD-H column; the absolute configuration was assigned by comparison with the literature (for details, please see the Supplementary Information).

^eSGP (50 mg) in urea solution (0.42 M) [urea (50 mg) in deionised water (2 mL)] was stirred at 100°C for 24 h and then water was removed under reduced pressure before use.

^fUrea (50 mg) was used instead of SGP.

^gThe mixture of SGP (50 mg), deionised water (1 mL) and the specified amount of CuSO₄ (for entries 7–9) or AgNO₃ (for entries 10–12) was stirred at 30°C for 24 h and then water was removed under reduced pressure before use.