Preparation of phosphostatine and phosphoepistatine from L -leucine via high diastereoselective reduction of 3-amino-2-ketophosphonates

The reduction of (3 S )- N,N -dibenzylamino-2-ketophosphonate 5 derived from L -leucine with catecholborane at -20 o C afford the (3 S )- N,N -dibenzylamino-(2 R )-hydroxyphosphonate syn - 6 , whereas the reduction of (3 S )- N -benzylamino-2-ketophosphonate 9 with Zn(BH 4 ) 2 at -78 o C gave the (3 S )- N -benzylamino-(2 S )-hydroxyphosphonate anti - 10 . The reduction in both cases was in good chemical yields and high diastereoselectivity. The hydrolysis and hydrogenolysis of syn - 6 and anti - 10 gave phosphostatine 12 and phosphoepistatine 13 , respectively.


Introduction
(4S)-Amino-(3S)-hydroxy-6-methylheptanoic acid (Statine) 1, a nonproteinogenic amino acid, is a key component of pepstatin, a natural hexapeptide antibiotic isolated by Umezawa and coworkers from various species of actinomices. 1 Additionally, (-)-statine 1 has attracted a lot of interest because of its potential use in the treatment of hypertension, congestive heart failure, malaria and Alzheimer's disease.For these reasons, many synthetic routes toward statine 1 and their analogues have been developed. 2n the other hand, phosphonates and phosphinates functionalized with amino and hydroxy groups have attracted considerably attention in recent years for their role in biologically relevant processes such as inhibition of rennin and HIV protease, human calpain I and their use as haptens in the development of catalytic antibodies. 3In particular, γ-amino-β-hydroxyphosphonic acids 2 and 3 have resulted in unique phosphate mimics with resistance to phosphatase hydrolysis. 4Additionally, the esters of the phosphonic acids 2, 3 and their analogues have been used as inhibitors of D-alanine:D-alanine ligase, 5 and as excellent Leu10-Val11 replacements (LVRs) in the angiotensin II, providing a more potent inhibitory activity for rennin over porcine pepsin and bovine cathepsin D. 6 As a result, numerous synthetic methods for chiral non-racemic β-amino-α-hydroxyphosphonic 2 have been developed. 7However, to the best of our knowledge, only a few synthetic approaches to obtain optically active esters of 3-amino-2hydroxyphosphonic acid 3 have been described in the literature, which involve the reaction of the anion of methylphosphonate with α-aminoaldehydes, 5,6 and the catalytic asymmetric aminohydroxylation of β,γ-unsaturated phosphonates, 7f but in both methodologies the yields and diastereoselectivities remain low.Recently, Yokomatsu et al. 8 described the synthesis of 3amino-2-hydroxyphosphonates with an improved diastereoselectivity via the dihydroxylation of β,γ-unsaturated phosphonates and the subsequent regioselective amination of their cyclic sulfates.

Results and Discussion
(3S)-N,N-Dibenzylamino-2-ketophosphonate 5 was synthesized in two steps from L-leucine (Scheme 1).Thus, the first step of the synthesis was the tribenzylation of L-leucine with excess of benzyl bromide and K 2 CO 3 under reflux in a mixture of MeOH:H 2 O, obtaining the corresponding (S)-N,N-dibenzylleucine benzyl ester 4 in 77% yield. 11Then, the resulting benzyl ester 4 was treated with the lithium salt of dimethyl methylphosphonate at -78 oC in THF, to obtain the (3S)-N,N-dibenzylamino-2-ketophosphonate 5 in 98% yield.Having efficiently prepared the 2-ketophosphonate 5, we turned our attention to the diastereoselective reduction of the carbonyl group to obtain the (3S)-N,N-dibenzylamino-2hydroxyphosphonates 6 and 7.The reduction was carried out using NaBH 4 , Zn(BH 4 ) 2 and catecholborane as reducing agents.Conditions, yields and diastereomeric ratio are summarized in Table 1.As shown in Table 1, the reduction of 5 with NaBH 4 at 25 o C in methanol afford the 2hydroxyphosphonates syn-6 and anti-7 with excellent chemical yield and moderate diastereoselectivity, in favor of diastereomer syn-6 (entry 1).Identical results were obtained in the reduction of 5 with Zn(BH 4 ) 2 (entry 2).Remarkably, when the reduction of 5 was carried out with catecholborane, only the diastereomer syn-6 could be detected by both 1 H and 31 P NMR (entry 3).Diastereomeric ratio of the reduction of 5 was determinated by means of 1 H and 31 P NMR.In fact, in 31 P NMR the signal for the diastereomer syn-6 was more shielded (34.72 ppm) than that the diastereomer anti-7 (35.37 ppm).The absolute configuration of the new stereogenic center in syn-6 and anti-7 was assigned by analogy with other (3S)-N,N-dibenzylamino-2hydroxyphosphonates. 10herefore, we propose that the reduction of 5 with catecholborane took place under non-chelation control or Felkin-Ahn model, 13 and that the bulkyness of the N,N-dibenzylamino group is sufficient to simultaneously limit the rotamer populations around the hinge bounds adjacent to the carbonyl group blocking the re face of carbonyl group and, thereby allowing the addition of hydride to take in a diastereoselective manner (Figure 1a).On the other hand, the reduction of 5 with Zn(BH 4 ) 2 seems to proceed in such way that the metal ions do not bind sufficiently strongly to the N,N-dibenzylamino and keto groups to induce chelation controlled reaction (Figure 1b).It was expected that the zinc ions would provide increased conformational control and hence higher diastereoselectivity toward the 2-hydroxyphosphonate anti-7; however, this is not the case, thus, the counterion does not appear to be involved on the induction of diastereoselection.This diastereofacial preference is in agreement with that reported previously for the reduction of 1-aminoalkylchloromethyl ketones, 14 for the reductive amination of α-amino ketones, 15 and reduction of 1-aminoalkylchloromethyl ketimines. 16In order to induce the formation of anti-3-amino-2-hydroxyphosphonate, now we turned to the preparation of (3S)-N-benzylamino-2-ketophosphonate 9 (Scheme 2). 17Thus, the starting (S)-N-benzylleucine methyl ester 8 was prepared by treatment of the corresponding amino methyl ester hydrochloride with K 2 CO 3 and benzyl bromide in acetonitrile at room temperature.Then, the methyl ester 8 was treated with the lithium salt of dimethyl methylphosphonate at -78 o C in THF, to afford the corresponding (3S)-N-benzylamino-2-ketophosphonate 9 (Scheme 2).

Scheme 2
Just as we have previously described, 17 the reduction of 9 using Zn(BH 4 ) 2 at -78 o C in THF afforded the corresponding 3-N-benzylamino-2-hydroxyphosphonates anti-10 and syn-11 in good chemical yield and with high diastereoselectivity, with a predominance of the desired anti product (Scheme 3).Finally, the hydrolysis of 2-hydroxyphosphonates syn-6 and anti-10 with bromotrimethylsilane at room temperature 9,18 afforded the corresponding 2-hydroxyphosphonic acid, that without any further purification was treated with palladium on carbon in methanol under hydrogen gas atmosphere at room temperature, obtaining the phosphostatine 12 and phosphoepistatine 13, in 89% and 93% yield, respectively, (Scheme 4).

Scheme 4
In conclusion, we have found a new methodology for the preparation of phosphostatine and phosphoepistatine diastereomerically pures.Additionally, the conditions described in this paper, make this experimental operation a good and simple method to obtain the 3-amino-2hydroxyphosphonates syn and anti in high diastereoselectivity changing the protective group on the nitrogen atom of L-amino acid.

Experimental Section
General Procedures.Optical rotations were taken on a Perkin-Elmer 241 polarimeter in a 1 dm tube; concentrations are given in g/100 mL.For flash chromatography, silica gel 60 (230-400 mesh ASTM, Merck) was used. 1 H NMR spectra were recorded on a Varian INOVA 400 (400 MHz), 13 C NMR (100 MHz) and 31 P NMR on a Varian Mercury 200 instruments.The spectra were recorded in D 2 O or CDCl 3 solution, using TMS as internal reference.Microanalyses were registered on an Elemental VARIO EL III.MS spectra were recorded on a JEOL JMS-700.
Flasks, stirrings bars, and hypodermic needles used for the generation of organometallic compounds were dried for ca. 12 h at 120 o C and allowed to cool in a desiccator over anhydrous calcium sulfate.Anhydrous solvents (ethers) were obtained by distillation from benzophenone ketyl.
(S)-N,N-Dibenzylleucine benzyl ester 4. 11 A solution of benzyl bromide (13.0 g, 9.1 mL, 76 mmol) and methanol (40 mL) was slowly added to a mixture of L-leucine (2.5 g, 19 mmol), K 2 CO 3 (9.2g, 67 mmol) in methanol-water (5:1, 250 mL).The reaction mixture was refluxed for 14 h.Then, the solvent was evaporated under reduced pressure and water was added to the residue, and the resulting mixture was extracted with ethyl acetate (

Table 1 .
Reduction of 2-ketophosphonate 5 with various reducing agents Determined by 1 H NMR at 400 MHz and 31 P NMR at 200 MHz.