Journal of Medicinal Chemistry Scheme 3a aReagents and conditions: (a) (i) H2, Pd/C, ethanol, room temp, 144 h (23 and 13%), (ii) TFA in dichloromethane, room temp, 16 h (99% and 98%). imine with the Cbz-deprotected secondary amine, and a subsequent reduction provides the ethylated byproduct. The unsaturated enamine byproduct 12 (Scheme 4) was collected from a reductive amination reaction of 1-Boc-3- Scheme 4a aReagents and conditions: (a) NH3 in methanol, room temp, 72 h (93%); (b) TFA/dichloromethane, room temp, 12 h (97%). piperidone with methyl glycinate performed in a highly concentrated reaction mixture. The opportunity to use the unsaturated piperidine as a scaffold for the synthesis of analogues was taken. Attempts to Cbz-protect 12 using the conditions described for (S)-4 gave only unreacted starting material back. Boc-Deprotection by TFA in dichloromethane was performed, but subsequent coupling with Boc-L-leucine did not provide satisfying results. However, 12 was amidated by treatment with NH3 in methanol and subsequent Boc- deprotection gave pip(db)G (13) in 90% (Scheme 4). The synthetic route to the LeuGlyGlyPro-NH2 analogues (R)- and (S)-LpipGP (18), in which Gly236 in IgG is mimicked by the piperidine moiety, is shown in Scheme 5. Starting from the Cbz-protected core compounds (R)-and (S)-4, hydrolysis of the esters in excellent yields (95% and 99% of (R)- and (S)- 14, respectively) followed by coupling to L-proline amide afforded (R)- and (S)-15, respectively, in high yields (82% and Scheme 5a Article 78%). A small portion of (R)- and (S)-15 were deprotected to provide the GlyGlyPro-NH2 analogues (R)- and (S)-pipGP (16). Interestingly, the NMR spectra of the two diastereomers were highly similar. Furthermore, the coupling pattern in the 1H NMR spectra resembled the pattern seen for (R)- and (S)- pipG (3), implying that also here the monosubstituted piperidine ring adopts a defined chair conformation. To continue, removal of the Boc-protecting groups in (R)- and (S)-15 and subsequent couplings with Boc-L-leucine mediated by HOBt and EDC gave the epimers (R)- and (S)-17 in moderate to high yields (64% and 90%, respectively). The lower yield is due to the need of two purification steps. Also here, the NMR spectra recorded at different temperatures identified rotamers. The 1H NMR spectrum of (R)-17 showed five doublets assigned to the two methyl groups in the leucine side chain, implying that the molecule adopts several different conformations at room temperature. Finally, the two protecting groups were removed as described above to give the end products (R)- and (S)-LpipGP (18) in good yields (72% and 73%, respectively) both with excellent purities (100%) when analyzed by reversed phase HPLC. After the deprotection, the 1H NMR spectrum of both (R)- and (S)-LpipGP (18) showed two doublets assigned to the two nonequivalent methyl groups, as expected. The total yield for the synthesis of (R)- and (S)- LpipGP (18) over seven steps was 29% and 43%, respectively. The corresponding tetrapeptide analogues in which the piperidine moiety replaces Gly237 in IgG ((R)- and (S)-LGpipP (23)) were also synthesized (Scheme 6). Reductive amination of Boc-protected 3-piperidone with L-proline amide and NaBH(OAc)3 afforded 19 in 68% yield and a diastereomeric ratio of 60:40. The diastereomers were separated by preparative HPLC to give (R)- and (S)-19, and the absolute configurations were determined by chemical correlation.23 Boc-protected glycine was coupled to each deprotected diastereomer using EDC and HOBt to afford (R)- and (S)-20 in 45% and 47% yield, respectively. Deprotection with TFA in dichloromethane gave quantitative yields of (R)- and (S)- GpipP (21) as TFA salts. A subsequent coupling to Boc- protected L-leucine gave (R)- and (S)-22, respectively, in high yields (93% and 78%). Deprotection with TFA in dichloro- methane overnight afforded (R)- and (S)-LGpipP (23) in excellent yields. The total yield for the synthesis of (R)- and aReagents and conditions: (a) LiOH 1 M aq, THF/MeOH/H2O 3:1:1, 0 °C 30 min, room temp, 3 h (95 and 99%); (b) L-Pro-NH2, EDC, HOBt, Et3N, dichloromethane, room temp, 16 h (82% and 78%); (c) (i) H2, Pd/C, ethanol, room temp, 24 h, (ii) TFA, room temp, 4 h (quant yields over two steps); (d) (i) TFA, room temp, 16 h, (ii) Boc-L-Leu × H2O, EDC, HOBt, dichloromethane, molecular sieves, Et3N, room temp, 15 h (64% and 90% over two steps); (e) (i) H2, Pd/C, ethanol, room temp, 48 and 24 h, (ii) TFA, room temp, 4 h (72% and 73% over two steps). 2552 dx.doi.org/10.1021/jm201517a | J. Med. Chem. 2012, 55, 2549−2560