Table 1. Selected Optimization Results^a.

Entry	R	Solvent	Temp	Ligand L	Yieldb
1	Tf (1a)	Toluene	105 °C	–	<5%
2	Tf	Toluene	105 °C	Boc-Val-OH	25%
3	Tf	Toluene	105 °C	Ac-Gly-OH	42%
4	Tf	Toluene	105 °C	Ac-Ala-OH	55%
5	Tf	Toluene	105 °C	Ac-Leu-OH	55%
6	Tf	Toluene	105 °C	Formyl-Val-OH	37%
7	Tf	Toluene	105 °C	Pro-Val-OH	52%
8	Tf	Toluene	105 °C	Ac-Val-OH	60%
9	Ms (1a′)	Toluene	105 °C	Ac-Val-OH	39%
10	Ns (1a″)	Toluene	105 °C	Ac-Val-OH	33%
11	Tf	p-Xylene	105 °C	Ac-Val-OH	49%
12c	Tf	THF	105 °C	Ac-Val-OH	53%
13	Tf	2-Me THF	85 °C	Ac-Val-OH	54%
14d	Tf	2-Me THF	85 °C	Ac-Val-OH	61%
15d,e	Tf	2-Me THF	85 °C	Ac-Val-OH	56%
16d,f	Tf	2-Me THF	85 °C	Ac-Val-OH	71%

^aConditions: 0.333 mmol of 1a, 0.167 mmol of allene 2a, 2 mL of solvent, under air, 2 equiv of Cu(OAc)₂·H₂O, 1.5 equiv of Cs₂CO₃, 16 h.

^bYields calculated based on 2a. Calculated by using an internal standard (entries 1–11). Isolated yields (entries 12–16).

^cReaction performed in sealed tube.

^d1 equiv of Cu(OAc)₂·H₂O and 1 equiv of Cs₂CO₃.

^e0.167 mmol of 1a, 0.167 mmol of allene 2a.

^fSlow addition over 1 h of 0.167 mmol of allene 2a in 1.5 mL of 2-Me THF to the reaction.