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Vedejs Edwin, Kongkittingam Chutima
Solution-phase synthesis of a hindered N-methylated tetrapeptide using Bts-protected amino acid chlorides: Efficient coupling and methylation steps allow purification by extraction
J. Org. Chem., 65 (2000) N 8, 2309-2318
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http://dx.doi.org/10.1021/jo9914115