Table 1 MS-conversion and isolated yields for phosphorothiolate esters peptides 4 and 5.

Entry	Phosphite^*	Solvent^†	Product	% Conversion^‡	% Yield^§	Product	% Conversion^‡	% Yield^§
1	3a	DMF	4a	20	—	5a	—	—
2	3b	DMF	4b	54	37	5b	—	—
3	3c	DMF	4c	6	—	5c	32	—
4	3d	DMF	4d	97	55	5d	—	—
5	3e	DMF	4e	22	27	5e	35	34
6	3a	MeCN:Tris (1:1)	4a	18	—	5a	—	—
7	3b	MeCN:Tris (1:1)	4b	59	43	5b	—	—
8	3c	MeCN:Tris (3:2)	4c	7	—	5c	18	—
9	3d	MeCN:Tris (3:2)	4d	36	27	5d	—	—
10	3e	Tris, pH 7.2	4e	2	—	5e	25	—
11	3e	Tris, pH 8.0	4e	3	—	5e	30	38

^*5 eq. of phosphite was used for each entry.
^†MeCN:Tris mixtures consist on MeCN and 25 mM Tris-HCl at pH 7.2.
^‡Conversion was determined by LC–MS analysis after 2a was depleted. The sum of areas under the XIC peaks corresponding to 1a, 2a, oxidized 1a, 4 and 5, for each entry, were counted as a 100%. All experiments were performed at least in duplicate, and the average values are presented.
^§Isolated yields of 4b, 4d, 4e and 5e products after semi-preparative HPLC. For synthetic procedures see Supplementary Methods.

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