Table 1.

Optimization of the Annulation of (S_a)-1 with Benzaldehyde and tert-Butyl Carbamate

entry	Lewis acid	time (h)	solvent	yield of 2aa	drc
1	TMSOTf	48	DCM	9%	>20:1
2	TMSOTf	48	EtCN	41%	>20:1
3	TiCl4	48	DCM	<5%	>20:1
4	TiCl4	48	EtCN	33%	>20:1
5	TfOH	48	DCM	44%	>20:1
6	TfOH	48	EtCN	47%	>20:1
7	BF3 · OEt2	72	DCM	14%	>20:1
8	BF3 · OEt2	72	MeCN	56%	>20:1
9	BF3 · OEt2	72	EtCN	67%	>20:1

^aIsolated yields after purification over silica gel. All reactions run with 1.2 equiv Lewis acid at −78 to −40 °C except for entry 8 run at −40 °C.

^cDiastereomeric ratios were determined by ¹H NMR analysis on crude material.