Amino Acids and Peptides in Ball Milling - Ball Milling Towards Green Synthesis: Applications, Projects, Challenges - page 142

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20

15

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0 5 10 15 20 25 30 35 40

Time (min)

Ball-milling, ML = 22.5 mg/mL, EtOAc (η = 1.4 μL/mg)
Ball-milling, ML = 5.9 mg/mL, solvent-free (η = 0 μL/mg)

Figure 6.5 Influence of the composition of the reaction mixture on the hydrolysis of

Boc-Phe-NCA. 45

ofdipeptidetobeobtainedinasmall10mLjarwith80%yield(Table6.2,
entry 11). Producing this amount of peptide enabled the synthesis of much
longer peptides by using this approach. On this perspective, the Boc pro-
tecting groups of the mechanosynthesized dipeptides were cleaved using
gaseous HCl treatment. This procedure has the advantages of being solv-
ent-free and of furnishing the deprotected peptides as hydrochloride salts
in quantitative yields.
The corresponding dipeptides were then engaged in another coupling step
with activated a-amino acid derivatives. A wide variety of tripeptides were
isolated with good to excellent yields starting from either Boc-AA-NCA or
Boc-AA-OSu (Table 6.2, entries 12-19). Thus, hydrophobic Boc-Leu-Leu-Leu-
OBn and Boc-Phe-Phe-Phe-OMe were produced in 79% and 86% yield re-
spectively (Table 6.2, entries 12 and 13). This approach perfectly tolerates
the presence of an heteroatom in the side chain of the reactants as Boc-
Lys(Boc)-Gly-Pro-OBn and Boc-Trp-Val-Phe-OMe were isolated in 79% and
86% (Table 6.2, entries 14 and 15). Interestingly, sterically challenging Boc-
Ile-NCA furnished both Boc-Ile-Leu-Pro-OBn and Boc-Ile-Ile-Ile-OMe with
86% yield when treated with HCl H-Leu-Pro-OBn and HCl H-Ile-Ile-OMe
respectively (Table 6.2, entries 16 and 17). Similarly, less encumbered Boc-
Gly-OSu also produced Boc-Gly-Gly-Phe-OMe and Boc-Gly-Phe-Leu-OBn with
good yields (Table 6.2, entries 18 and 19). The latter peptide could be pro-
duced at the gram scale as 937 mg of this tripeptide (94% yield) could be
isolated by performing the corresponding reaction with a high milling load
of 152.7 mg mL 1 (Table 6.2, entry 19). After having been deprotected by
gaseous HCl, two tripeptides could be engaged in another mechano-medi-
ated peptide bond coupling. The corresponding tetrapeptide Boc-Leu-Leu-
Leu-Leu-OBn and Boc-Gly-Gly-Phe-Leu-OBn were both isolated with an
excellent yield of 96% (Table 6.2, entries 20 and 21). Once again, increasing