Table 2.

Optimization of the aromatisation reaction to give phthalides. All reactions with 4‐exo, unless stated otherwise.

Entry	Solvent	Catalyst	Loading [equiv]	Ac2O	T [°C][a]	Yield [%][b]
1	Neat	MsOH	13	20 vol %	20	75
2			0.5	4 equiv	20/80	97[c]
3			0.5	4 equiv	80	98
4			0.1	4 equiv	80	95
5			0.02	4 equiv	80	94
6			0.01	4 equiv	80	39
7		TfOH	0.01	4 equiv	80	95
8		H2SO4	0.01	4 equiv	80	80
9		Amberlyst 15	0.1	4 equiv	80	82
10	EtOAc		0.1	4 equiv	80	78
11			0.1	2 equiv	80	89 (79)[d]
12[e]	Neat	MsOH	0.5	8 equiv	80	60[f]
13	Neat	MsOH	13	0	20	66
14	d8‐Toluene	MsOH	0.1	0	80	66
15		Hf(OTf)4	0.1	0	80	60
16		TfOH	0.1	0	80	63
17		TfOH	0.01	0	80	63
18		TfOH[g]	0.01	0	80	58
19	CDCl3	TfOH	0.1	0	80	79
20	CH3COOH	TfOH	0.1	0	80	56

[a] Acid/Ac₂O added at 0 °C and then warmed to 20 or 80 °C as specified. [b] Determined by quantitative ¹H NMR. [c] Reaction mixture heated to 80 °C for 1 h after 22 h at 20 °C. [d] Yield using recycled Amberlyst catalyst. [e] Using 12‐exo as the substrate. [f] Isolated yield. [g] Silica‐supported TfOH acid. ^[38]