Solvent-free stereoselective synthesis of phosphoenolpyruvates and their C-P and N-P analogues

Abstract Phosphoenolpyruvate is widely involved in a variety of biochemical processes. Herein we report a facile synthetic method for phosphoenolpyruvates and their C-P and N-P analogues via the classic Perkow reaction under mild solvent-free conditions. A moderate to excellent E/Z-stereoselectivity was observed giving the E-isomer as the main product, as determined by full NMR spectroscopy analysis. Graphical Abstract


Introduction
Organic phosphorus acids, containing carbon-phosphorus (C-P) bond, [1,2] are unique natural products that have been widely used in medicine and agriculture. [3] These include antibiotics, such as fosfomycin, a phosphonic acid derivative with broad activity, [4] herbicides, such as phosphinothricin tripeptide (PTT) and antimalarial compounds, such as FR-900098 [5] (Scheme 1a). The primary reaction that forms the C-P bond is catalyzed by phosphoenolpyruvate (PEP) mutase (PepM) via the equilibrium between PEP and phosphonopyruvate (PnPy), which has been extensively investigated for decades (Scheme 1b). [6][7][8][9] On the other hand, it is well-known that the ionized phosphonate functional group can effectively mimic phosphate or carboxylate, thus synthesis and metabolites of the C-P analogues of PEP may be interesting and helpful for the understanding of the biological mechanism involving organophosphonic acids biosynthesis and catabolism.
As a kind of special enol phosphates (EPs), phosphoenolpyruvates have been prepared via the Perkow reaction of a-halopyruvate with phosphite. The only one reported synthetic procedure by Ghomri yielded mainly the Z isomers, but suffering from harsh reaction conditions and solvent employment. [10] Moreover, to the best of our knowledge, no synthesis of C-P analogues of PEP has been reported to date. Therefore, a more environmentally benign synthetic route to E-configurated PEPs as well as their C-P and N-P analogues is still much desirable. Herein we report a E-stereoselective and high-yield synthesis of phosphoryl, phosphonyl, phosphinyl, and phosphoramic enol pyruvates under mild solvent-free conditions (Scheme 2).

Results and discussion
We initially selected diethyl 2-chloro-3-oxosuccinate 1a and trimethyl phosphite (MeO) 3 P 2a as the test reaction to investigate and optimize the parameters of the Perkow reaction shown in Table 1.
Solvent screening experiments revealed that solvent-free condition gave the highest yield (94%) at 25 C as expected ( Table 1, entry 9), which agrees with the results described in our previous report, that high regioselective O-phosphorylation of a-chloroketones can be performed by solvent-free Perkow reaction. [11] The PEP product 3aa could be obtained neatly with high conversion (88-94%) at different temperatures, but declined yields were found under lower or higher temperature respectively (Table 1, Entries 10 and 13). Thus, room temperature was selected as the most suitable condition for the next preparations. No side-products from the competitive Arbuzov reaction were detected, probably because of the much stronger Perkow reactivity of a-chloropyruvates containing much stronger electron withdrawing C ¼ O groups than normal a-chloroketones.
Furthermore, the ratio of E/Z-isomers determined by NMR spectroscopy was found to be obviously affected by the solvents used (Table 1, Entry 1-9). The best E/Z-selectivity of 3.7:1 could be obtained in hexane but giving lower conversion (Table 1, Entry 5). The reaction temperature seemed not to have significant effects on the stereoselectivity. We thus employed the neat condition in the following preparations even though with some loss of E/Zstereoselectivity.
With the optimized conditions in hand, the applicability of the Perkow reaction to different a-chloropyruvates (1a-f) and various P(III)-reagents (2a-h) was investigated (Table  2). Pleasingly, our synthetic procedure for the Perkow reaction exhibited extreme tolerance in the preparation of structurally different PEPs. Various types of P(III)-reagents possessing P-O, P-C, and/or P-N bonds and a-chloro-pyruvates with different substituents all gave the corresponding EPs compounds in high yields. Good results were also obtained even with a second substituent at the a-chloro carbon atom. Unexpectedly, compared with the a-alkoxycarbonyl substrates 1a and 1e, the a-aryl a-chloropyruvates 1 b, 1d, and 1f always gave much higher E-selectivity, especially those with -CH 3 or -CF 3 group at the para-phenyl position, affording E-isomers stereospecifically such as 3dc, 3fa, and 3fb. This may be explained by the mechanism discussed below ( Figure 2).
However, most of the E/Z-mixed PEP products tend to display complicated NMR spectra and thus it is difficult to assign the peaks, although some have been analyzed by computational chemistry in the literature. [10] Therefore, in order to confirm the structure and E/Z-composition of PEP products, we isolated both isomers of compound 3ae. In the 1 H NMR spectra of E/Z-3ae (Figure 1a, left), two typical signals of vinyl hydrogen atoms (6.30 and 6.56 ppm in 1:2.3 ratio) at low field could be clearly identified, though with a small difference in the chemical shift. At high field the 1 H NMR signals of the methyl substituents were also separated into two groups in the same 1:2.3 ratio representing two triplets and a quartet formed by two overlapping triplets, respectively (Figure 1a, right). Moreover, the integral ratio of two 31 P nuclei in 31 P NMR (Figure 1c') agreed well with the results in the 1 H NMR spectra.
The 1 H, 31 P-HMBC spectra (Figure 1d) of the correlation of 31 P nucleus (d P ¼ 34.4 ppm) with the vinyl proton (d H ¼ 6.56 ppm) clearly show the product with the E-configuration as the major product formed. Furthermore, according to the NOESY spectra (Figure 1e), the methylene -OCH 2 protons showed no correlation with the phenyl protons in the E-isomer of 3ae. Thus, the composition of the isolated product of 3ae should be a mixture of E/Z-isomers in ratio of 2.3:1.
Based on the above NMR analysis method, all the E/Zratios of the products are given in Table 2. Different E/Zstereoselectivity from 1:1 up to 100:0 was observed in the isolated products. Thus, the E-dominating stereoselectivity under this solvent-free conditions shows the inverse preference as compared to the reaction results in refluxing toluene reported by Ghomri. [10] A general synthetic procedure for phosphoenolpyruvates and their analogues can be described as following: a mixture of the a-chloropyruvate (1 mmol) and the P(III)-reagent  (1 mmol) was stirred at room temperature for 1-2 h and course of the reaction was monitored by 31 P NMR. After the reaction was completed, column chromatography (n-hexane: ethyl acetate ¼ 2:1 to 1:1) separation gave the target product. The isolation of E-and Z-isomers was performed by preparative liquid chromatography on an Agilent 1260 Infinity II instrument using acetonitrile and water (3:2) as the mobile phase. To be mentioned, the E/Z-ratio in the isolated products is somewhat higher than that in the crude mixture.
To demonstrate the practical utility, the reaction of diethyl 2-chloro-3-oxosuccinate 1a and triethyl phosphite (EtO) 3 P 2b was carried on a gram-scale providing the desired product 3ab in 85% yield (Scheme 3).
With respect to the generation of the E/Z-stereoselectivity in our reaction, a reasonable explanation based on the Cram's rule is that in Perkow reaction, the P(III)-reagents should generally prefer attacking the carbonyl atom from the less sterically hindered side in case of ketones with asymmetric carbon atoms in the a-position, thus finally affording the E-products after a known rearrangement process ( Figure 2). [12] Conclusion In summary, we have developed a stereoselective synthesis of phosphoenolpyruvate and their novel analogues with P-O, P-C, and/or P-N bonds via the Perkow reaction. In total 25 PEP-type compounds were thus conveniently prepared in high yields under mild solvent-free conditions, starting from various kinds of P(III)-reagents and a-chloropyruvates. A moderate to excellent (up to 100:0) E/Z-stereoselectivity was Table 2. Synthesis of phosphoenolpyruvates and the related analogues a . a Reaction conditions: 1a-f (1 mmol), 2a-h (1 mmol), 25 C, neat, 1-2 h. Isolated products. acquired giving the E-isomers as the main products on the basis of full NMR spectroscopy identification.

Disclosure Statement
There are no conflicts to declare.