Optimisation of the reaction conditions^a.

Entry	T (°C)	C (M)	B (mol%)	Time (h)	Conv.b (%)	5 c (%)	eed (%)
1	RT	0.5	10	16	100	20	64/66
2	RT	0.1	10	39	87	7	76/77
3	RT	0.1	7.5	48	94	10	76/76
4	RT	0.1	5	48	76	5	79/76
5	40	0.1	5	48	100	25	76/73
6	0	0.1	10	48	81	0	79/81
7	0	0.2	10	48	97	2	76/76
8	−20	0.2	10	72	86	0	79/78

^aUnless otherwise noted, all reactions were carried out with 0.1 mmol of 1a, 0.2 mmol of 2, and 0.05 mmol of (MeO)₃C₆H₃ in toluene; d.r. was determined from the reaction mixture by ¹H NMR spectroscopy and varied between 81 : 19 to 87 : 13; for full details see the ESI.

^bConversion depicts the amount of reacted 1a based on ¹H qNMR measurements using (MeO)₃C₆H₃ as an internal standard. Generally, the total amount of the formed 3a and retro-Michael product 5 corresponded to the amount of 1a depleted.

^cDescribes the extent of product decomposition through the retro-Michael reaction determined by ¹H qNMR, using the methylene proton signal of triethyl phosphonoacetate 5.

^dDetermined by HPLC analysis on a chiral stationary phase.