Reductive reaction of enamine 11 to racemates 13^a.

Entry	Reduction	Additive	Conversion (%)
1	NaBH4	None	0
2	NaBH4	BF3 diethyl etherate	95
3	NaBH4	MsOH (methanesulfonic acid)	93
4	NaBH4	Acetic acid	10
5	NaBH4	TFA (trifluoroacetic acid)	2

^aAll reactions were carried out as follows: THF (50 mL) was firstly cooled to −10 °C. NaBH₄ (2.33 g, 61.68 mmol) and an additive such as MsOH (5.9 g, 61.68 mmol) were added dropwise at −10 to −5 °C, and then enamine 11 (5.0 g, 12.34 mmol) and isopropanol (30 mL) were added. The reaction mixture was aged at −15 °C for 4.5 h, monitored by HPLC. Isolated yield after ethyl acetate extraction.