Facile Access to Unnatural Dipeptide-Alcohols Based on cis-2,5-Disubstituted Pyrrolidines

Well-defined unnatural dipeptide-alcohols based on a cis-2,5-disubstitued pyrrolidine backbone were synthesized from commercially available starting materials meso-diethyl-2,5-dibromoadipate, (S)-(−)-1-phenylethylamine, and phenylalaninol. The structures of these unnatural dipeptide-alcohols are supported by HRMS, 1H- and 13C-NMR spectroscopy. These unnatural dipeptide-alcohols can act as building blocks for peptidomimetics.


Results and Discussion
The monoacid cis-1 can be obtained by both chemical and enzymatic protocols via the monohydrolysis of diethyl cis-1-[(S)-1-phenylethyl]pyrrolidine-2,5-dicarboxylate [20]. In the presence of KOH/EtOH, monoacid cis-1 was obtained in 76% yield as a light yellow slurry. Using the conventional peptide-synthetic protocol, the coupling reactions of L-and D-phenylalaninol with monoacid cis-1 were investigated, respectively. The couplings were performed smoothly by using 1.5 equiv. of dicyclohexylcarbodiimide (DCC) as a coupling reagent in dry CH2Cl2 at room temperature (rt) (Scheme 1). Both the diastereomers cis-2 and cis-3 were obtained in good yields (up to 80%). Interestingly, the diastereomeric mixture cis-2 prepared from L-phenylalaninol and monoacid cis-1 was easily separated to be (−)-4a and (+)-4b by a flash column chromatography (FC) on silica gel, however, the diastereomeric mixture cis-3, the coupling product obtained from monoacid cis-1 and D-phenylalaninol instead could not be separated as two compounds by flash column chromatography (Scheme 2). The diastereomeric ratio of the major and the minor component in cis-3 is 2/1 which was deduced from its 1 H-NMR spectrum.

Scheme 2. Synthetic routes to (−)-5a and (−)-5b.
In the presence of catalytic quantity of Pd(OH)2/C and under H2 atmosphere, compounds (−)-4a and (+)-4b were converted to be the corresponding deprotected dipeptide-alcohols (−)-5a and (−)-5b with one protected carboxylic group and one C-terminal hydroxyl group, respectively (Scheme 2). The dipeptide-alcohols (−)-5a and (−)-5b containing a cis-pyrrolidine backbone with one free N-terminal at pyrrolidine ring and one C-terminal hydroxyl group in the side-chain can be used as valuable building blocks for connection of other amino acids to furnish complex peptide-alcohols.
Hydrolysis of compounds (−)-4a and (+)-4b using solid KOH in THF/H2O afforded the corresponding dipeptide-alcohols (−)-6a and (−)-6b with free C-terminal carboxylic acid and hydroxyl groups (Scheme 3). These free carboxylic acid and hydroxyl groups can enable the coupling with other amino acids to yield complex unnatural peptide-alcohols. The other two unnatural dipeptide-alcohols

General Information
Melting points are uncorrected and expressed in °C. 1 H-and 13 C-NMR spectra were measured in CDCl3, MeOD or DMSO-d6 solution on a Bruker AV-300 or AV-500 spectrometers (Bruker, Fällanden, Switzerland) using TMS as an internal reference. Multiplicities are designated by the following abbreviations: s, singlet; d, doublet; t, triplet; q, quartet; br, broad; m, multiplet. Optical rotations analyses were performed on a Model 343 Polarimeter (Perkin-Elmer, Waltham, MA, USA). High-resolution mass spectra were performed on a VG Micromass 7070F Mass Spectrometer (VG Instruments, St Leonards-on-Sea, UK) with ES ionization (ESI). All commercially available reagents were used as received. Products were purified by flash column chromatography on silica gel purchased from Qingdao Haiyang Chemical Co. Ltd. (Qingdao, China). All reactions involving air or moisture sensitive species were performed in oven-dried glassware under inert atmosphere. The monoacid cis-1 was prepared following the reported procedures in the previous literature [17].

Typical Procedure for cis-2 or cis-3
To a mixture of monoacid cis-1 (2.90 g, 10.0 mmol) and L-phenylalaninol (1.60 g, 10.5 mmol) in dry CH2Cl2 (50 mL), DCC (3.20 g, 15.5 mmol) and DMAP (125 mg, 1.0 mmol) added at 0 °C, and the mixture was stirred for 0.5 h at this temperature and stirred overnight at rt. After the reaction was finished, it was filtered on a Celite pad. The solvents was evaporated to give cis-2 (diastereomeric mixture) as a yellow oil which was purified by a flash column chromatography on silica gel to afford (−)-4a and (+)-4b. The coupling product cis-3 (diastereomeric mixture) was obtained by the similar procedure from D-phenylalaninol (0.79 g, 5.2 mmol) and monoacid cis-1 (1.46 g, 5.0 mmol), and it could not be separated by a flash column chromatography.

Typical Procedure for (−)-5a or (−)-5b
In the presence of Pd(OH)2/C (0.20 g), the compound (−)-4a (0.50 g, 1.17 mmol) in MeOH (10.0 mL) was stirred overnight under 1.0 atm H2 at rt. After the reaction was finished, it was filtered on a Celite pad to remove catalyst. The filtrate was evaporated to give the desired product (−)-5a without further purification. Compound (−)-5b was obtained from (+)-4b by the similar procedure. The compound (−)-4a (1.0 g, 2.35 mmol) in THF/H2O (1:1) (15 mL) was added by KOH pellets (0.33 g, 4.7 mmol) and the mixture was stirred 2 h at rt. After the reaction was finished, the solvent was evaporated and the acidity of the aqueous residue was adjusted to be pH = 2.0 by 6.0 M HCl, then it was extracted by ethyl acetate (3 × 10 mL), the combined organic layer was washed by H2O (2 × 5 mL) and brine (10 mL), dried (Na2SO4). The solvent was evaporated under reduced pressure to give the desired product (−)-6a without further purification. Compound (−)-6b was obtained from (−)-4b by the similar procedure. In the presence of Pd(OH)2/C (0.23 g), the compound (−)-6a (0.60 g, 1.5 mmol) in MeOH (8 mL) was stirred overnight under 1.0 atm H2 at rt. After the reaction was finished, it was filtered on a Celite pad to remove catalyst. The filtrate was evaporated to give the desired product (−)-7a without further purification. Compound (−)-7b was obtained from (−)-6b by the similar procedure.