An Aminopyrrolidinyl Phosphonates—A New Class of Antibiotics: Facile Synthesis and Predicted Biological Activity

A novel class of aminopyrrolidinyl phosphonates was synthesized in 74% 80% isolated yield by the addition of three-fold excess of primary amines to diethyl 4-chloro-1-butynylphosphonates. The reaction was carried out at room temperature and in the absence of solvent or catalyst to give solely compounds which showed predicted biological activity based on PASS program. Some of the synthesized derivatives of antibiotics exhibit properties for the treatment of stroke, the treatment of acute neurological disorders, and can also be acetyl esterase inhibitors.

For instance, a pyrrolidinyl phosphonates class (known as antibiotic SF-2312) International Journal of Organic Chemistry is a natural antibiotic which is produced by the actinomycete Micromonospora sp. It has not only showed activity as an enolase inhibitor but also found to be one of the most potent natural inhibitors of glycolysis which in turn inhibit cell proliferation [14] [15] [16] [17] [18]. Later, the synthesis of this antibiotic and its analogues has been described by various researchers [19] [20] [21].
In the same context, pharmacologists always agree that the biological activity of both natural and synthetic compounds is related to the chemical structure [22]. Therefore, there are a large number of available computer programs that can evaluate the probability of an organic compound activity to be a drug [23] [24]. For instance, PASS computer program contains a library with information of about 1 million chemical compounds and more than 10,000 biological activities [25]. An algorithm for the practical use of PASS has been described in detail in several publications [26] [27] [28].
Previously, few pyrrolidinylphosphonates and pyrrolidinylphosphonic acids (1-12, Figure 1) were synthesized for the purpose of testing their biological activity. Generally, they were obtained either from readily available pyrrolidine ring or by multistep cyclization reactions. But there is no one general direct method to produce them and a brief summary for their synthesis is described below: Initially, the antibiotic SF-2312 (1) and its analogue (3) were synthesized by a multistep reaction sequence in which ethyl diethoxyphosphorylacetate was converted to N-benzyloxy-2-(diethoxyphosphoryl)-pent-4-enamide followed by oxidative cleavage and hydrolysis [21]. Later, studies showed that their antibiotic activity is due to the inhibition of a glycolytic enzyme called enolase [14] [15] [16]. The reaction of pentanedial with acetamide and acetyl chloride in the presence of phosphorylating agent afforded the pyrrolidinyldiphosphonic acid (2) [29]. The combination of the Kabachnik-Fields reaction with a subsequent ring closure of 5-chloro-2-pentanone with ammonia and diethyl phosphonate produced (4) [30].
Being encouraged by these results, we determined to study amine addition on a shorter chain alkynylphosphonate that has not been explored before. Accordingly, we prepared 4-chloro-1-butentynylphosphonate 15 in our lab by substituting the hydroxy group in but-3-yne-1-ol using thionyl chloride under reflux.
After isolation of the product by distillation, it was lithiated using n-BuLi, and was reacted with chlorophosphonates.
Herein, we report a very facile method for the synthesis of novel aminopyrrolidinyl phosphonates (17a-h) and their predicted biological activity using the PASS program.
Thus, when three equivalents of i-propylamine were added to (15), the pyrroli-  This process represents a general one-pot method for the synthesis of novel oily amino-pyrrolidinyl phosphonates (17a-h) which have not reported before.
In addition, they are thermally and air-stable compounds at room temperature, and are soluble in most organic solvents. Besides, this cyclization reaction is general for both aliphatic and aromatic primary amines as shown in Table 1. In addition, they are of potent biological activity precisely as antibiotics analogous to compounds 1-12 as shown in Table 2.
Unlike primary amines, when secondary amines were used, no heterocycles were detected and only 2-amino-cyclobutenylphosphonates 16 were obtained.
A suggested mechanism for this reaction can be attributed to initial addition of the amine on the carbon-carbon double bond to give a zwitterionic intermediate followed by proton transfer. Then, another hydroamination reaction took place on the double bond in the presence of excess amine to give the intermediate (20). After, nucleophilic attack of the nitrogen atom on C1 onto the carbon C-Cl by S N 2 fashion, an aminopyrrolidinyl phosphonate (17)    This suggested mechanism is also supported by the fact that when secondary amines were used, no hetero cycles were detected and only 2-amino-cyclobutenylphosphonates 16 were obtained. In addition, this reaction was restricted to 4-chloro-1-butynylphosphonate 15. When longer chloro alkynylphosphonates chains were used no heterocyclic structure was observed apparently due to the proximity of the chloro substituted carbon to the conjugated system of the triple bond and the phosphonate group. After the synthesis of compounds (17a-h), their biological activity was screened utilizing the PASS program and were found to be of predicted pharmacological usefulness as listed in Table 2.

Conclusion
In conclusion, a novel aminopyrrolidinyl phosphonate class of compounds (17a-h) was smoothly obtained by addition of primary amines to 4-chloro-1-butynylphosphonate in the absence of solvent or catalyst and relatively in a satisfactory isolated yield. In addition, manifestations of biological activities of the above compounds were observed using PASS program. This reaction can be of interest for the related chemist's community to apply this reaction to other functional groups instead of phosphonates and for the biologists to test their predicted biological activity.
To diethyl (4-chlorobut-1-yn-1-yl)phosphonate (0.22 g, 1 mmol) was added (0.23 g, 3.5 mmol) of isopropylamine in a 10 mL round bottom flask. After stirring at 25˚C for 12 h. the reaction mixture was washed with 0.1 N NaOH solution and the product was extracted with (2 × 20 mL CH 2 Cl 2 ), dried over MgSO 4 , concentrated using a rotary evaporator and the oily product was separated on a silica gel column and was obtained in 80% isolated yield (10% methanol:90% dichloromethane), which was then analyzed by GC/MS, elemental analysis, and NMR spectroscopy. Identical to procedure 17a except adding benzylamine and was obtained in 78% isolated yield.  Identical to procedure 17a except adding phenylamine and was obtained in 76% isolated yield. 1