Table 1.

Nucleophile-catalyzed cycloaddition of a ketene with a nitrosoarene: Effect of reaction parameters.

Entry	Change from the "standard" conditions	Yield of 3 (%)[a]	ee of 3 (%)[b]	Yield of 4 (%)[c]
1	none	90	79	3
2	no (−)-1	9	-	30
3	5% (−)-2, instead of (−)-1	16	−22	33
4	5% (+)-5, instead of (−)-1	19	−35	61
5	5% (+)-6, instead of (−)-1	33	<2	17
6	toluene, instead of CH2Cl2	89	66	2
7	THF, instead of CH2Cl2	71	78	1
8	−20 °C	81	73	6
9	r.t.	89	76	4

^[a]Determined by ¹H NMR spectroscopy versus an internal standard.

^[b]A negative ee value signifies that the opposite enantiomer of 3 is formed preferentially.

^[c]Yield of isolated product (average of two experiments).