Amino Acids and Peptides in Ball Milling - Ball Milling Towards Green Synthesis: Applications, Projects, Challenges - page 141

100

80

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40

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0

0 5 10 15 20 25 30 35 40

Time (min)

Ball-milling, ML = 22.8 mg/mL, EtOAc (η = 1.4 μL/mg)
Ball-milling, ML = 5.9 mg/mL, solvent-free (η = 0 μL/mg)
Ball-milling, ML = 22.8 mg/mL, solvent-free (η = 0 μL/mg)
Round-bottom flask with classical stirring, DMF (η = 1.4 μL/mg)
Round-bottom flask with classical stirring, EtOAc (η = 1.4 μL/mg)

Figure 6.4 Influence of the composition of the reaction mixture and the type of

agitation on the conversion into Boc-Phe-Leu-OMe. 45

epimerization occurred during the coupling as no traces of Boc- D- Phe-Leu-
OMe could be observed by HPLC analysis. Other NCAs such as Boc-
Leu-NCA, Boc-Val-NCA or Boc-Ile-NCA reacted eciently with differently
substituted a-amino esters (Table 6.2, entries 2-5). Indeed, replacing either
the hydrochloride amino ester salt with a p-tosylate salt or the methyl ester
with a benzyl ester did not affect the e ciency of the reaction as p-TsOH H-
Leu-OBn and HCl H-Pro-OBn were transformed into Boc-Leu-Leu-OBn and
Boc-Leu-Pro-OBn with 93% and 90% yield, respectively (Table 6.2, entries 2
and 3). Similarly, sterically challenging Boc-Val-NCA and Boc-Ile-NCA
reacted with HCl H-Phe-OMe and HCl H-Ile-OMe to produce Boc-Val-Phe-
OMe and Boc-Ile-Ile-OMe with high yields (Table 6.2, entries 4 and 5).
Functionalities other than N-carboxyanhydride can be used to activate
a-amino acid substrates. Indeed, N-hydroxysuccinimide esters of Boc-
protected glycine, tyrosine or phenyalanine (Boc-AA-OSu) led to the ecient
production of the corresponding peptides (Table 6.2, entries 6-11). This
a-amino acid activation presents the advantage of producing the low toxi-
city and water-soluble N-hydroxysuccinimide (HOSu) as a side-product
during the course of the reaction. Afterwards, HOSu can easily be elimin-
ated during extractions. To evaluate the ecacy of liquid-assisted grinding
on the elimination of the mass transfer limitations, the milling load of
the reaction producing Boc-Phe-Phe-OMe from Boc-Phe-OSu and
p-TsOH H-Leu-OBn was raised substantially to 188.8 mg mL 1 . While
being slightly slower than with a milling load of 28.4 mg mL 1 , performing
the reaction with a milling load of 188.8 mg mL 1 allowed close to 800 mg