Table 3.

Optimization of reaction conditions.

entry	product	reagent	additive	base	time	yielda
1	15a	DCC	HOBt	Et3N	24 h	63%
2	15a	EDCI	HOBt	Et3N	18 h	86%
3	15a	EDCI	OxymaPure	DIPEA	18 h	88%
4	15a	COMU	–	DIPEA	2 h	92%
5	15b	COMU	–	DIPEA	3 h	91%
6	15c	COMU	–	DIPEA	3 h	94%
7	15d	COMU	–	DIPEA	3 h	92%
8	15e	COMU	–	DIPEA	3 h	92%

^aYield of dipeptide analogues 15 after isolation.