Journal of Medicinal Chemistry Scheme 1a Article aReagents and conditions: (a) ethyl glycinate, NaBH(OAc)3, AcOH, molecular sieves, dichloromethane, room temp, 20 h (62%); (b) preparative HPLC; (c) ethyl glyoxylate, NaBH(OAc)3, AcOH, dichloromethane, room temp, 4 h (56% and 62%). Scheme 2a aReagents and conditions: (a) NH3 in methanol, room temp, 72 h (92 and 90%); (b) TFA, room temp, 12 h (quant yields); (c) Cbz-OSu, Et3N, dichloromethane, 0 °C → room temp, 17 h (82% and 84%); (d) (i) TFA, room temp, 16 h, (ii) Boc-L-Leu × H2O, EDC, HOBt, dichloromethane, molecular sieves, Et3N, room temp, 15 h (91% and 79% over two steps); (e) NH3 in methanol, room temp, 72 h (65% and 73%); (f) (i) H2, Pd/C, ethanol, room temp, 72 h, (ii) TFA, room temp, 4 h (84% and 75% over two steps); (g) (i) TFA, room temp, 16 h, (ii) Boc-L-Leu × H2O, EDC, HOBt, dichloromethane, molecular sieves, Et3N, room temp, 15 h (88 and 91%, over two steps); (h) NH3 in methanol, room temp, 168 h (63% and 86%); (i) (i) H2, Pd/C, ethanol, 3 h, (ii) TFA, room temp, 14 h (90% and 92% over two steps). isolated enantiomers (R)- and (S)-1 were protected using Cbz- succinimide (Cbz-OSu) to afford (R)- and (S)-4, respectively, in high yields (82% and 84%). The 13C NMR spectra recorded at ambient temperature showed rotamers from both the Boc- and Cbz-groups, however the signals coalesced in spectra recorded at 50 °C. The enantiomers (R)- and (S)-4 were Boc- deprotected followed by coupling with Boc-L-leucine to afford high yields of (R)- and (S)-5, respectively (91% and 79%). Aminolysis afforded amides (R)- and (S)-6, which were deprotected to the end products (R)- and (S)-LpipG (7), respectively (yields: 84% and 75% over two steps). Boc- Deprotection of (R)- and (S)-5 and coupling to Boc-L-leucine gave (R)- and (S)-8, respectively, in high yields (88% and 91%). A subsequent aminolysis afforded (R)- and (S)-9. The reaction times were rather long and purification was needed which resulted in lower yields (63% and 86%). Finally, the Cbz- and Boc-protecting groups were removed by catalytic hydro- genation (Pd/C) and TFA hydrolysis, respectively, to afford the end products (R)- and (S)-LLpipG (10) in high yields (90% and 92%). The total yield for the synthesis of (R)- and (S)-LLpipG (10) over eight steps from (R)- and (S)-1, respectively, was 37% and 48%. N-Ethylated byproducts were formed during the Cbz- deprotection to (R)- and (S)-LpipG (7) as well as (R)- and (S)-LLpipG (10) using catalytic hydrogenation (Pd/C 10%). Isolation of the byproducts after prolonged reaction time and subsequent TFA hydrolysis gave (R)- and (S)-LLpipEtG (11) (Scheme 3). Already after three hours, byproduct formation was observed. To verify the structure of the byproduct, Cbz- deprotected (R)-9 was converted to the corresponding ethylated product in 76% yield via reductive amination using acetaldehyde and NaBH(OAc)3 (reaction not shown). The NMR spectra of the synthesized compound and that of (R)- LLpipEtG (11) were identical. It has been proposed that a Pd- catalyzed oxidation of ethanol to acetaldehyde can occur during the hydrogenation reaction.21,22 The acetaldehyde forms an 2551 dx.doi.org/10.1021/jm201517a | J. Med. Chem. 2012, 55, 2549−2560