Communication pubs.acs.org/OPRD
Enantiomerically Pure Dibenzyl Esters of L‑Aspartic and L‑Glutamic Acid Cristiano Bolchi, Ermanno Valoti, Laura Fumagalli, Valentina Straniero, Paola Ruggeri, and Marco Pallavicini* Dipartimento di Scienze Farmaceutiche, Università degli Studi di Milano, via Mangiagalli 25, I-20133, Milan, Italy S Supporting Information *
azetidines (IV);16 (b) of both the enantiomers of trans-2(diphenylmethylethylideneamino)cyclopropanecarboxylic acid (V) to be incorporated in dipeptides;17 (c) of a (2S,4R)-4hydroxyornithine derivative (VI);18 and (d) of (S)-2-aminoadipic acid δ-methyl ester (VII) (Chart 2).19 The large employment of (S)-1 and (R)-1 as synthetic intermediates requires a simple, efficient, and robust method to obtain the diester directly from the amino acid enantiomers. The reaction of L-aspartic acid with excessive benzyl alcohol in the presence of little more than 1 equiv of p-toleuenesulfonic acid to give the p-toluenesulfonate of (S)-1 [(S)-1·TsOH] in high yield has long been known and appreciated for its straightforwardness; to our knowledge, the first reports date back to the 1950s.20−22 Contrary to (S)-1, which is a viscous oil separating, on longstanding, crystalline diketo-piperazine 5 (Chart 1),23 (S)-1· TsOH is a high melting, stable solid. However, no analytical methods have been developed directly to determine the enantiomeric composition of optically active samples of (S)-1 and (R)-1, and even the most recent literature relies on improbable comparisons with rather old, small values of specific rotation ascribed to the pure enantiomer. Which is surprising, considering that (S)- and (R)-1, depending on reaction conditions, easily undergo moderate to extensive racemization, hardly detectable and quantifiable by comparison between optical rotations. Furthermore, the reported esterifications with benzyl alcohol, also the most recent ones, still use long banned or unsuitable solvents. The history of the dibenzyl ester of the other proteinogenic acidic amino acid, that is dibenzyl glutamate (2), parallels that of 1: it is used as a synthetic intermediate in the form of ptoluenesulfonate; it is prepared directly from the amino acid enantiomers and benzyl alcohol in the presence of ptoluenesulfonic acid in banned or unsuitable solvents; its enantiomers undergo racemization in the esterification; no analytical methods have been developed directly to determine the enantiomeric composition of its optically active samples. Here, we report two new scalable procedures to prepare enantiomerically pure (S)-1·TsOH and (S)-2·TsOH directly from L-aspartic and L-glutamic acid, respectively; the procedures are as simple and efficient as those usually employed, but exempt from their major failings. Moreover, we determine the enantiomeric excess of both (S)-1·TsOH and (S)-2·TsOH by chiral HPLC and the type of racemate produced by 1·TsOH in
ABSTRACT: (S)-Dibenzyl aspartate p-toluenesulfonate [(S)-1·TsOH] and (S)-dibenzyl glutamate p-toluenesulfonate [(S)-2·TsOH] were efficiently prepared from the respective L-amino acids and benzyl alcohol with very high yields by using cyclohexane as a water azeotroping solvent instead of benzene, carbon tetrachloride, toluene, or benzyl alcohol itself, as reported in literature methods. Preventively, chiral HPLC methods were developed to determine the enantiomeric excess of the two diesters and DSC analyses were performed on the respective ptoluenesulfonates. With the aid of such investigation tools, we demonstrated that (S)-1·TsOH and (S)-2·TsOH were formed enatiomerically pure in cyclohexane, whereas more or less pronounced racemization occurred both in toluene and in benzyl alcohol. The two one-pot procedures, which did not require crystallization of the product or any other purification step, were accomplished on multigram scale.
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INTRODUCTION The enantiomers of dibenzyl aspartate (1) and of its regioselectively monodebenzylated derivative, 4-benzyl aspartate (3), are important intermediates in organic synthesis which have found widespread use in many different fields (Chart 1). Oligopeptide and polypeptide synthesis is the most representative one. Dipeptide PPARγ antagonists,1 α-chymotrypsin inhibitors containing dipeptide chains onto a central resorc[4]arene scaffold,2 dipeptide sweeteners,3 and antidiabetic quasi-dipeptide esters4 are only some of the relatively recent examples. Furthermore, the possibility of selectively removing the 1-benzyl by hydrolysis5,6 or hydrogenolysis7 makes 1 a suitable starting material for the preparation of the sweetener Aspartame7 or of glycopeptide libraries.5 Functional poly(Lamino acid)s are another exciting field due to a wide variety of applications in nano- and biotechnology: poly(L-aspartic acid) derivatives, prepared from the S enantiomer of 1, 3 and the Ncarboxy anhydride 4, derived from 3, are reported, for instance, as drug carriers,8 dendrimers with lyotropic liquid crystal behavior,9 photoconductors,10 inhibitors of drugs toxicity,11 and components of inkjet inks.12 Last but not least, the enantiomers of dibenzyl aspartate (S)-1 and (R)-1 are useful C4 chiral building blocks as exemplified by the synthesis (a) of both the enantiomers of N-silylated benzyl 4-azetidinone-2-carboxylate, used to construct hydroxamic acid containing bicyclic β-lactams (I),13 tricyclic diproline analogues (II),14 trans-2-carboxyazetidine-3-acetic acids (III), 15 2-(3-pyridyloxymethyl)© XXXX American Chemical Society
Received: April 27, 2015
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DOI: 10.1021/acs.oprd.5b00134 Org. Process Res. Dev. XXXX, XXX, XXX−XXX
Organic Process Research & Development
Communication
Chart 1. Aspartic and glutamic acid benzyl esters and derivatives
Chart 2. Dibenzyl aspartate enantiomers as chiral building blocks
a much lower final yield (41%)23 because the recystallization loses a great amount of the crude dibenzyl ester ptoluenesulfonate. Forcedly, our evaluation of these procedures relies on the yields and the melting points of the respective final products rather than on their optical rotations. Indeed, these are very contradictory and too low in absolute value to allow the optical purity to be appreciated. After benzene,1,3,5,9−11,14−16,21,25−28 benzyl alcohol,22,23,29,30 and carbon tetrachloride,24 also toluene4,31 has been used as a reaction solvent azeotropically removing water (bp 85.0 °C), although some authors demonstrated that extensive racemization occurs in refluxing toluene.32 It is evident that the above procedures show a number of negative aspects: (a) solvents such as benzene, the most used one, and carbon tetrachloride cannot be employed on a large scale, while benzyl alcohol and toluene seem unsuitable; (b) a substrate highly susceptible to racemization such as 1 is produced in optically active form under different conditions without an analytical method to monitor its enantiomeric composition in a direct manner; (c) crystallizations of optically active 1·TsOH are carried out with no notion of the nature of its enantiomeric system and consequently of the impact of such a physical process in terms of chemical yield and enantiomeric purity. Therefore, in the quest for a novel, safe, and practical procedure to prepare (S)1·TsOH directly from L-aspartic acid and benzyl alcohol, we first searched for an analytical method to determine the enantiomeric composition of 1 without any derivatization and then we characterized its enantiomeric system. Two samples of (S)-1·
order to gain a deeper insight into the preparation and isolation of a single enantiomer of these two substrates.
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RESULTS AND DISCUSSION The preparation of (S)-1·TsOH from L-aspartic acid, 4 equiv of benzyl alcohol, and little more than 1 equiv of p-toluenesulfonic acid in benzene under reflux was described in 1957.21 According to such a procedure, water is azeotropically removed during the reaction and the crude diester toluenesulfonate is precipitated and then recrystallized with a 80−90% yield and with a 158−160 °C melting point. In the same year, Japanese researchers22 and, six years later, U.S. researchers23 report another preparation of (S)-1·TsOH, in which benzene is abolished and benzyl alcohol acts as a reactant and, in addition, as a solvent and azeotropic water carrier. However, according to this procedure, (S)-1· TsOH is obtained with a sensibly lower melting point (151−2 °C)22 or the crude diester p-toluenesulfonate is described to precipitate with a 74% yield by cooling and then to crystallize from water/ethanol (95/5) to give (S)-1·TsOH with 158 °C melting point, but in only 41% yield.23 In 1962, Mazur describes a similar preparation of (S)-1·TsOH, in which carbon tetrachloride is used to remove water azeotropically instead of benzene or benzyl alcohol.24 The yield is high (>70%) and also the melting point (159−161 °C). Noteworthy for the present discussion is that the higher boiling point of the water/benzyl alcohol azeotrope (99.9 °C) in comparison with water/benzene (69.4 °C) and water/CCl4 (66.8 °C) azeotropes is concomitant with a lower melting point (151 °C) of the final product22 or with B
DOI: 10.1021/acs.oprd.5b00134 Org. Process Res. Dev. XXXX, XXX, XXX−XXX
Organic Process Research & Development
Communication
Figure 1. Chiral HPLC chromatograms of (S)-1 (98.1% ee) and (RS)-1.
TsOH and (RS)-1·TsOH were prepared in refluxing benzene from L- and rac-aspartic acid respectively according to the procedure reported in ref 27. The two enantiomers were very well separated as free bases by HPLC on a chiral stationary phase (Figure 1). The second step of our investigation was the characterization of the enantiomeric system formed by (S)-1·TsOH and (R)-1· TsOH. The knowledge of its nature, that is the number and the location of the eutectics in the binary melting point diagram at least, is very helpful for forecasting and understanding the results of crystallizing any (S)-/(R)-1·TsOH mixture. The significantly higher melting point of (S)-1·TsOH (159 °C)21,24 in comparison with (RS)-1·TsOH (133 °C)33 suggested that 1·TsOH forms a conglomerate. According to procedures previously accomplished for other enantiomer systems,34 we performed IR and DSC analyses to determine the racemate type. The presumed nature of conglomerate was confirmed by the IR spectra of (RS)-1·TsOH and of (S)-1·TsOH, which were identical, and by the DSC analyses of (RS)-1·TsOH, (S)- 1·TsOH and a number of their mixtures (Figure 2).These analyses allowed us to construct the binary phase diagram for mole fractions of (S)- 1·TsOH ranging from 0.5 to 1 (Figure 3). The DSC melting profiles of differently proportioned (RS)-1·TsOH/(S)-1·TsOH mixtures were characterized by the presence of two peaks, the former at 133 °C, representing the fusion of the racemate, and the latter, representing the fusion of the excess of (S)-1·TsOH at temperatures that increase with such an excess (Figure 2). As can be seen in Figure 3, the experimental values fit well with the theoretical ones (solid curve), calculated on the basis of the melting point of (S)-1·TsOH and of its heat of fusion (83.5 J/g) by the Schröder−van Laar equation.35 The DSC melting profiles clearly indicate that samples of (S)1·TsOH, chemically pure but with a significantly lower melting
Figure 2. Superimposed DSC traces of pure (S)-1·TsOH (dark blue trace), (RS)-1·TsOH (green trace), and mixtures with intermediate compositions (other colors).
point than 158−159 °C, contain a considerable amount of a eutectic, that is, racemate. This is the case for (S)-1·TsOH, prepared in toluene in 92% yield according to a recent patent,4 which melts at 145−147 °C. On the other hand, a high melting point, close to 158−159 °C, resulting from a low-yield recrystallization indicates that (S)-1·TsOH is optically pure but after removing a great quantity of racemate. This is the case for the reported preparations in benzyl alcohol. In contrast, the use of benzene or carbon tetrachloride, which form lower boiling water azeotropes, assures both a high yield and melting point of (S)-1·TsOH. All these data suggest that the reaction temperature would be critical for the enantiomeric purity of (S)-1·TsOH. That is why benzene is preferred to toluene and benzyl alcohol and its use persists despite the high toxicity. However, before developing an alternative procedure to prepare (S)-1·TsOH, we C
DOI: 10.1021/acs.oprd.5b00134 Org. Process Res. Dev. XXXX, XXX, XXX−XXX
Organic Process Research & Development
Communication
point (69.8 °C vs 69.4 °C) and composition (8.5% water vs 8.9%). According to the main current solvent selection guides,36,37 cyclohexane is not a “green solvent”, but it is considered usable on an industrial scale on the basis of overall acceptable health, environment, and safety criteria. The new procedure was developed on a multigram scale by starting from 20 g of L-aspartic acid; 1.2 equiv of TsOH monohydrate and 5 equiv of benzyl alcohol were used. At room temperature, the three components formed a clear solution. Addition of cyclohexane resulted in a second clear upper phase. While mixing the two phases, a white precipitate was formed, which dissolved on gradual heating to give two clear liquid phases (40− 50 °C). These were heated at reflux for 6 h; during this time the temperature of the reaction mixture was maintained at about 80 °C, water was distilled and collected in a Dean−Stark apparatus, and (S)-1·TsOH precipitated as a white solid. The mixture of solid and two liquid phases was cooled to 40 °C. At the end of the reaction, the literature procedures that use other solvents prescribe the mixture filtration or concentration and the crystallization of the resultant solid residue in order to eliminate impurities such as benzyl alcohol and TsOH. However, we wished to avoid both the concentration, because benzyl alcohol is high boiling, and a distinct crystallization step. Therefore, we added isopropanol to the reaction mixture at 40 °C to give a suspension of solid in a unique liquid phase. After stirring for 1 h at room temperature, we isolated pure (S)-1·TsOH as a white crystalline solid by filtration in 94% yield. The DSC curve was completely exempt from the eutectic and identical to that of pure (S)-1·TsOH (see dark blue trace in Figure 2), while the chiral HPLC chromatogram of liberated (S)-1 did not show any trace of (R)-1. The spectral data, melting point (160 °C), and optical rotatory power were in line with maximal chemical and enantiomeric purity. The entire procedure was repeated on the same scale twice, and the results proved its reproducibility. Lastly, we considered the conversion of L-glutamic acid into dibenzyl ester p-toluenesulfonate (S)-2·TsOH by treatment with benzyl alcohol in the presence of p-toluenesulfonic acid. According to the literature, this reaction is accomplished in benzene in the majority of cases.1,21,23,38 Alternatively, benzyl alcohol,22 carbon tetrachloride,39 and toluene4,40,41 are reported to be used as a solvent. Excluding benzene, carbon tetrachloride, and benzyl alcohol, we wished to verify whether toluene can replace such solvents or whether the reaction in toluene is accompanied by racemization, as observed for L-aspartic acid, because of the higher boiling point of the water/toluene
Figure 3. Binary melting-point phase diagram of dibenzyl aspartate ptoluenesulfonate. The solid curve represents the values calculated on the basis of the Schröder−van Laar equation.
wished to prove in an unequivocal manner that aspartic acid dibenzylation occurs with racemization both in toluene and in benzyl alcohol by applying the above-described chiral HPLC and DSC analytical methods. The reaction in toluene was accomplished as that in benzene;27 (S)-1·TsOH was obtained in >90% yield, but with a 134 °C melting point and
