Synthesis of Both Enantiomers of Chiral Phenylalanine Derivatives Catalyzed by Cinchona Alkaloid Quaternary Ammonium Salts as Asymmetric Phase Transfer Catalysts

A practical synthesis of both enantiomers of unnatural phenylalanine derivatives by using two pseudoenantiomeric phase transfer catalysts is described. Through asymmetric α-alkylation of glycine Schiff base with substituted benzyl bromides and 1-(bromomethyl)naphthalene under the catalysis of O-allyl-N-(9-anthracenmethyl) cinchoninium bromide (1f) and O-allyl-N-(9-anthracenylmethyl)cinchonidium bromide (1i), respectively, a series of both (R)- and (S)-enantiomers of unnatural α-amino acid derivatives were obtained in excellent yields and enantioselectivity. The synthetic method is simple and scalable, and the stereochemistry of the products is fully predictable and controlled: the cinchonine-type phase transfer catalyst 1f resulted in (R)-α-amino acid derivatives, and the cinchonidine-type phase transfer catalyst 1i afforded (S)-α-amino acid derivatives.


Introduction
Unnatural α-amino acids are important building blocks for synthesis of peptides, pharmaceutical molecules and natural products.In particular, unnatural α-phenylalanine derivatives have been the subject of numerous investigations for their extensive distribution in biological active compounds.For example, CPD-15A5, which is a small-molecule negative allosteric modulator (antagonist) for the β 2 -adrenergic receptor (β 2 AR) [1], contains a (S)-3,5-dibromophenylalanine subunit (Figure 1).Levothyroxine, used for the treatment of hypothyroidism [2][3][4], has a (S)-3,5-diiodophenylalanine backbone.ADEP 4, which shows potent antibacterial activity against multidrug-resistant pathogens [5,6], has a (S)-3,5-difluorophenylalanine sidechain.In addition, LY355703, a potent and broad spectrum antitumor agent [7,8], is partially composed of (R)-(3-chloro-5-methoxy)phenylalanine.We are particularly interested in unnatural α-phenylalanine derivatives [9] because they are the key building blocks for synthesizing a series of dipeptides as allosteric antagonists of the β 2 -adrenergic receptor (β 2 AR) [1] in one of our ongoing research projects.We need both the (R)-and (S)-enantiomers of α-phenylalanine derivatives for structure-activity relationship studies.Although many different methods for the synthesis of α-phenylalanine derivatives have been reported in the literature [10][11][12], these methods have significant drawbacks, such as the use of very costly catalysts, low yields, and/or poor enantioselectivity for some derivatives.Asymmetric phase-transfer catalysis has been widely used for the synthesis of chiral α-amino acids because of its operational simplicity, mild reaction conditions, and reduced environmental impact [13][14][15][16][17]. Quaternary cinchona alkaloid catalysts, discovered by O'Donnell et al. [18] and further improved by Lygo [19] and Corey [20], have been the most useful and practical chiral phase-transfer catalysts for the synthesis of α-amino acids.On the other hand, only a few examples have been reported for asymmetric synthesis of disubstituted α-phenylalanine derivatives by using quaternary cinchona alkaloid catalysts.Furthermore, some of the reported procedures are not suitable for wide range of substrates.For example, McAlister et al. prepared a series of substituted 2-nitrophenylalanine derivatives through asymmetric alkylation of N-(dibenzylidene)glycine tert-butyl ester with substituted 2-nitrobenzyl bromides using a cinchonidine phase transfter catalyst.5-Methyl-2-nitrobenzyl bromide gave 5-methyl-2-nitrophenylalanine derivative with 100% ee; however, when the methyl group was replaced with a trifluoromethyl group, the ee value decreased to 90%, and with a chloro group replacement, the corresponding product has only 75% ee [21].
Our group recently synthesized some biologically important compounds via asymmetric phase transfer catalysis [1,22].We predicted that both enantiomers of the disubstituted α-phenylalanine derivatives could be obtained by using two pseudoenantiomeric quaternary cinchona alkaloids as the phase transfer catalysts.Our objective was to develop a straightforward preparative-scale method for synthesizing the unnatural α-phenylalanine derivatives with high chemical and optical purities in sufficient quantities to permit rapid preparation of the dipeptides for laboratory bioassays and animal studies.Herein we report a convenient synthesis of both the (R)-and (S)-enantiomers of α-phenylalanine derivatives, including several disubstituted unnatural α-phenylalanine derivatives which have not been reported in literature, with excellent yield and excellent enantioselectivity through asymmetric phase-transfer catalysis.The described procedure is simple, mild and scalable, and its usefulness has been demonstrated with the synthesis of a dipeptide derivative of the β 2 AR allosteric antagonist CPD-15A5 by using an enantiomer-enriched 3-chlorophenylalanine derivative.

Substrate Expansion
With the optimal reaction conditions in hand, we investigated the scope and limitations of the asymmetric α-alkylation of glycine Schiff base 2. A variety of disubstituted and monosubstituted benzyl bromides 3a-o as well as 1-(bromomethyl)naphthalene (3o) were tested for the alkylation reaction with 2, and the results are outlined in Table 2.A variety of substituents, such as halo (F, Cl, Br and I), electron-withdrawing (nitro and difluoro groups), electron-donating (dimethoxy group) and α-naphthyl groups, were well tolerated under the alkylation conditions, affording the desired products 4a-4n.Eight disubstituted unnatural α-phenylalanine derivatives 4a-4h (Table 2, entries 1-8) were obtained with satisfactory yields and enantioselectivity.When the benzyl bromides containing strong electron-withdrawing groups were used, the corresponding α-phenylalanine derivatives were prepared with excellent enantioselectivity.Under the same alkylation conditions, 1-(bromomethyl)naphthalene was reacted smoothly with 2, affording (R)-tert-butyl 2-((diphenylmethylene)amino)-3-(1-naphthyl)propanoate (4o) with 85% yield and 97% ee (Table 2, entry 15).After derivatives 4a-o were successfully obtained under the catalysis of cinchonine-type phase transfer catalyst 1f, we tried to synthesize the enantiomers of 4a-o by using a cinchonidine-type phase transfer catalyst, O-allyl-N-(9-anthracenemethyl) cinchonidium bromide (1i, Table 3), which is the pseudoenantiomer of 1f and was prepared according to the same procedure used for 1f.To our satisfaction, all the enantiomers of 4a-4o were obtained with good to excellent yields (71% to 99%) and excellent enantioselectivity (93% to 99% ee) (Table 3) under the identical conditions for 4a-4o except that catalyst 1f was replaced with 1i.Similar to the results in Table 2, alkylation of 2 with 2-chloro-6-fluorobenzyl bromide resulted in the highest enantioselectivity under the catalyst of 1h (99% ee; Table 3, entry 5). a Reactions were performed with 2 (0.1 mmol), alkylation reagent (0.5 mmol), 50% KOH (0.5 mmol) and 1h (0.01 mmol) in toluene/CHCl 3 at −40 • C. b Isolated yield.c Enantiomeric excess was determined by HPLC analysis using a chiral column with n-hexane-isopropanol as eluent.
Finally, the absolute configuration of the newly synthesized α-amino acid derivatives 4a-4o and 4a'-4o' was established by comparison of their optical rotation values with those reported in the literature.For example, the (S)-configuration of 4l' was confirmed by comparing its optical rotation value ([α]  cleaved the benzophenone imine and tert-butyl ester with 6 N HCl, protected the amino group with Boc, and then confirmed the (R)-configuration by comparing the HPLC retention time of the N-Boc protected amino acid with the literature value [33].Therefore, (R)-configuration was assigned for 4a-4o, and (S)-configuration for 4a'-4o'.

Application
The asymmetric α-alkylation of glycine Schiff base with substituted benzyl bromides can be applied to the synthesis of new derivatives of CPD-15A5 as allosteric antagonists for the ) was hydrolyzed in refluxing hydrochloric acid to give (S)-3-chlorophenylalanine hydrochloride (5) in 92% yield, and then the amino group in 5 was protected with Fmoc-Cl, affording Fmoc-protected (S)-(3-chloro)phenylalanine (6).In the next step, the Fmoc-protected L-phenylalanine methylamide 7 was obtained by condensation of 6 with methylamine in the presence of O-benzo-triazol-1-yl-N,N,N ,N -tetramethyluronium hexafluorophosphate (HBTU) and hydroxybenzotriazole (HOBt).It should be pointed out that the acidic hydrolysis of 4j' didn't racemize the amino acid, because the ee value of 7 is 96%.After Fmoc deprotection of 7 (piperidine/DMF), the resulting L-phenylalanine methylamide 8 was coupled in the presence of HBTU/HOBt with Fmoc-L-4-carbamoylphenylanine (9) to generate dipeptide 10 in 51% yield.Upon treatment with piperidine in DMF, the Fmoc group in 10 was removed smoothly at room temperature, giving the corresponding amine 11 in 95% yield.In the final step, 11 was reacted with 2-cyclohexyl-2-phenyl acetic acid (12) to afford the desired product 13 in 63% yield.

Instruments and Reagents
Melting points were measured on SGW X-4B melting point apparatus (Shenguang, Shanghai, China). 1 H-NMR spectra were recorded on Avance 300 (300 MHz) and 400 (400 MHz) spectrometers (Bruker, Karlsruhe, Germany).Chemical shifts were reported in ppm from tetramethylsilane with the solvent resonance resulting from incomplete deuterium incorporation as the internal standard (CDCl 3 : δ 7.26 ppm). 13C-NMR spectra were recorded on Bruker Avance 300 (75 MHz) and 400 (100 MHz) spectrometers with complete proton decoupling.Chemical shifts were reported in ppm from tetramethylsilane with the solvent resonance as the internal standard.High-resolution mass spectrometry was performed on a Thermo Orbitrap Elite, instrument (Agilent, Palo Alto, CA, USA).Optical rotations were measured on an Autopol IV (d = 589 nm, Hg lamp, 50 mm cell) instrument (Rudolph, NJ, USA).The enantiomeric excess was determined by a 1260 infinity series HPLC (Agilent, Palo Alto, CA, USA) equipped with Chiralpak OD-H, AD-H and IA columns (4.6 mm × 250 mm, Daicel Chiral Technologies, Shanghai, China).Chemicals and solvents were purchased from Linfeng (Shanghai) and Annaiji (Shanghai) in China, and used as received.Purification of the products was carried out by flash column chromatography using silica gel (Yantai Jiangyou Company, Shandong, China, particle size 0.100-0.075mm).

Conclusions
In summary, we have developed a practical method for synthesizing both enantiomers of unnatural α-amino acid derivatives by asymmetric α-alkylation of N-(dibenzylidene)glycine tert-butyl ester (2) with substituted benzyl bromides and 1-(bromomethyl)naphthalene under the catalysis of O-allyl-N-(9-anthracenmethyl) cinchodium bromide (1f) and O-allyl-N-(9-anthracenmethyl) bromide (1i), respectively.A series of both (R)-and (S)-enantiomers of unnatural α-amino acid derivatives were obtained in good to excellent yields and with excellent enantioselectivity, and the procedure is simple, mild and scalable.Furthermore, the stereochemistry of the products is fully predictable and controlled: the cinchonine-type phase transfer catalyst 1f resulted in all (R)-α-amino acid derivatives, whereas the cinchonidine-type phase transfer catalyst 1i afforded the (S)-α-amino acid derivatives.Both resulting enantiomers of the substituted α-phenylalanine derivatives have been used for synthesizing new allosteric antagonists for β 2 AR.

Supplementary Materials:
The supplementary materials containing NMR spectra and HPLC chromatograms can be accessed online.
Author Contributions: X.C. conceived and designed the experiments; L.J. and S.Z.carried out the synthesis and characterization of all compounds; All authors discussed the contents of the manuscript.

Table 1 .
Optimization of the reaction conditions.

Table 2 . Asymmetric alkylation of 2 with 3a-o under the catalysis of 1f. Entry a R Product Yield b ee c
CHCl 3 at −40 • C. b Isolated yield.c Enantiomeric excess was determined by HPLC analysis using a chiral column with n-hexane-isopropanol as eluent.