Table 2. Selected optimization of the reaction conditions ^a .

Entry	Variation from standard conditions	Yield b (%)
1	None	90(65)
2 c	Without PdCl2	ND e
3 c	Without Cu(OAc)2	ND
4 c	Without PivOH	42
5 c	AcOH instead of PivOH	55
6 c	Cs2CO3 instead of PivOH	ND
7 c	Pd(OA)2 instead of PdCl2	Trace
8 c	Pd(TFA)2 instead of PdCl2	Trace
9 c	CuCl2 instead of Cu(OAc)2	Trace
10 c	Cu(acace)2 instead of Cu(OAc)2	45
11 c	AgOAc as oxidant	Trace
12 c	K2S2O8 as oxidant	Trace
13 c	Toluene instead of mixed solvent	Trace
14 c	i-PrOH instead of mixed solvent	Trace
15 c	DMA instead of DMF	35
16	110 °C instead of 100 °C	82
17	90 °C instead of 100 °C	55
18	Boc–Phe–OH	80
19	Fomc–Val–OH	72
20	CO/O2 = 4 : 1	35
21 d	Add H2O (10 equiv.)	17

^a 4a (0.1 mmol), PdCl₂ (10 mol%), Cu(OAc)₂ (30 mol%), Boc–Val–OH (30 mol%), PivOH (1.0 equiv.), toluene/dioxane (5 : 1, 1.2 mL), V(CO)/V(O₂) (2 : 1), 100 °C, 60 h.

^bDetermined by GC-MS, isolated yield is given in the parenthesis.

^cToluene/DMF (5 : 1, 1.2 mL), 120 °C.

^d100 °C, 48 h.

^eND = not determined.