Optimization of the sequence^a.

Entry	H-acceptor	Acid (equiv.)	t (h)	2b : 3 : 4b ratiob (%)
1	5a	—	16	100 : 0 : 0
2	5a	HCl (1)	16	82 : 18 : NDc
3	5a	HCl (5)	16	100 : 0 : 0
4	5a	AcOH (1)	16	72 : 28 : NDc
5	5a	AcOH (5)	16	0 : 100 : NDc
6	5b	AcOH (5)	16	0 : 100 : 100
7	5b	AcOH (5)	5	0 : 100 : 100

^aAll reactions were carried out with 1 mmol of 1b in toluene/H₂O 9/1 at 105 °C, 4 h. 1/5b/6c = 1/2/0.05 and then addition of acid (5 equiv.), 105 °C.

^bDetermined by ¹H NMR spectroscopy.

^c4b is detected in trace amounts.