Table 1.

Minimizing enamine induced epimerization^a

entry	Dr (13a:13b)	solvent	T (°C)b		equiv BF3·OEt3	equiv Et3SiH	combined yieldc	drd (20a:20b)	%eee 20a	%eee 20b
			[O]	[R]
1	68:32	CH2CI2	−78	−10	3.2	3.2	52	60:40	94	86
2b	68:32	CHCI3	−65	−65	2.2	2.0	48	66:34	94	88
3	>97:3	CH2CI2	−78	−10	3.2	3.2	61	92:8	93	90
4b	>97:3	CHCI3	−65	−65	2.2	2.0	68	>97:3	93	-
5	3:>97	CH2CI2	−78	−10	3.2	3.2	66	8:92	88	85
6b	3:>97	CHCI3	−65	−65	2.2	2.0	64	3:>97	-	85

^aOzonolysis reactions run at 4.0 equiv dimethyl sulfide.

^bAll reactions proceed to 23 °C.

^cIsolated yields.

^ddr determined by ¹H NMR of crude reaction mixture.

^eee determined by HPLC analysis after purification.

^fReactions run by adding half the equiv of HSiEt₃ and BF₃.OEt₂ at t₀ and t_30·.