Table 3.

Synthesis of N³-acylated 7.

entry	R	acylating reagent	6	yieldc	7	yieldc
1	Me	3a	6a	83	7a	48
2	Et	3b	6b	84	7b	44
3	Pr	3c	6c	85	7c	37
4	iPr	3d	6d	67	7d	40
5	TBSO(CH2)4	3e	6e	62	7e	50
6		3f	6f	56	7f	25

^aThis step was carried out with compound 5 (1.0 equiv) and an anhydride in neat or an NHS ester (1.5 equiv) in DMF.

^bThis operation was executed with 6 (1.0 equiv), BOP (1.3 equiv) and DBU (1.5 equiv) for 4 h. Then NH₃ (7 N in MeOH) was added.

^cIsolated yields.