With the optimal reaction conditions in hand, large-scale reaction on 4 mmol of
N-acylimine under the standard condition delivered oxazoline
3aa in 90% yield with no loss of the enantioselectivity (98%
ee), demonstrating the robustness of this asymmetric [4 + 1] annulation. Consequently, a series of
N-acylimines with 3-chlorooxindoles were selected to examine the tolerance of substitution patterns and functional groups in this enantioselective formal [4 + 1] annulation. This methodology was shown to be efficient for a variety of
N-acylimines (
Scheme 2). All of the evaluated aromatic
N-acylimines bearing electron-donating and electron-withdrawing groups could provide the desired chiral oxazoline products
3ba-
3ka in high yields with excellent diastereoselectivities and enantioselectivities, regardless of substitution patterns on the aromatic ring. The imines with benzo[
d][
1,
3]dioxol-5-yl and 2-thienyl substitution also gave good yields (86% yield for
3la, 87% for
3ma) and high enantioselectivities (97%
ee for
3la, 96%
ee for
3ma). It is noted that aliphatic imines failed to provide the targeted oxazolines. Various alkyl-substituted imines, such as methyl,
tert–butyl and cyclohexyl were tested, however decomposition results were obtained. This might be due to the isomerization of aliphatic imines to the corresponding enamines. In addition, aliphatic imines are more unstable and susceptible to hydrolysis compared to aromatic imines. The scope of the reaction with respect to variation of acyl part of
N-acylimines was then explored. Electron-rich, electron-poor and halide substitution provided access to oxazolines in excellent yields and enantioselectivities.
Ortho-,
meta- and
para-substitution and naphthyl-substitution on the aryl ring were tolerated well (
3na-3va), demonstrating good tolerance to steric crowding. Reactions of heteroatom-substituted acyl part took place smoothly, leading to the corresponding oxazolines
3wa-3c'a in high yields and excellent enantioselectivities. It is noteworthy that
S, N-oxazoline ligands containing oxazoline functionality and thiophene, benzothiophene or dibenzothiophene as an auxiliary donor ligands [
69,
70] could be obtained through this method. Moreover, the R2 group of
N-acylimines could be ferrocenyl, furnishing chiral ferrocenyl-oxazoline
3d'a, a type of new Ferrox ligand [
71,
72], in 61% yield with 94%
ee.