Table 1.

Optimization of the reaction conditions for conversion of substrate 2a to 9a.

Entry	Brønsted acid	Solvent	Temp (oC)	Time (h)	Producta
1	1% TFA	DCM	rt	24	0
2	1% triflic acid	DMF	rt	24	0
3	p-TsOH	Toluene	120	24	0
4	AcOH	Ethanol	100	24	0
5	2% MSA	ACN	80	24	0
6	2% MSA	DMF	120	16	0
7	2% triflic acid	ACN	80	24	27
8	2% triflic acid	DMF	120	16	87
9	5% triflic acid	DMF	120	16	80
10	1% triflic acid	DMF	120	16	75
11	0.5% triflic acid	DMF	120	16	35

^abased on HPLC.