Finally, we applied the combination of B 4b and MTHP for the reductive alkylation of amino acids and peptide derivatives in the presence of H2 (Figure ). Aminosalicylic acids 1b and 1c were effectively alkylated under the optimized conditions, and 3ba and 3ca were obtained in 90% and 93% yield, respectively; 60 atm of H2 was used in the latter case. For comparison, under a pressure of 80 atm of H2 in THF, 3ba and 3ca were furnished in 47% and 70% yield in the presence of 10 mol% and 15 mol% B 1a , respectively, which again demonstrates the advantages of the present system using B 4b and MTHP in terms of synthetic efficiency and sustainability. The reductive alkylation of anthranilic acid (1d), which is also known as vitamin L1, 5-aminoisophthalic acid (1e), and 4-aminophenylacetic acid (1f) afforded 3da, 3ea, and 3fa in excellent yield using 40-60 atm of H2. In contrast, we recognized that aliphatic amino acids (or their imine derivatives) and substrates insoluble in MTHP were not suitable. For example, aspartic acid (1g) is insoluble in MTHP, and no reaction took place when 1g was employed under otherwise identical conditions. Esterification of the carboxy group in 3-amino-4,4-dimethylpentanoic acid effectively improved its solubility in MTHP. Moreover, the hydrogenation of the imine derived from 1h and 2a did not occur. Based on these results, we prepared alanine-and S-methylcysteine-based peptides 1i and 1j, and subjected them to the optimal reaction conditions; alkylated peptides 3ia and 3ja were obtained in 95% and 55% yield, respectively. In terms of the scope of aldehydes, 3,5-bis(trifluoromethyl)benzaldehyde (2b) furnished 3ab in 93% yield, but 3,5-di-tert-butylbenzaldehyde (2c) gave 3ca in merely 38%. In the latter case, a significant amount of 2c remained unreacted, indicating difficulties associated with the formation of the imine intermediate due to the decreased electrophilicity of the aldehyde moiety. A comparable result was obtained when p-tolualdehyde (2d) was used with respect to the case using 2a, and 3da was afforded in 80% yield.