Synthesis of Enantiomerically Pure N-Boc-Protected 1,2,3-Triaminopropylphosphonates and 1,2-Diamino-3-Hydroxypropylphosphonates

All possible isomers of 1,2,3-tri(N-tert-butoxycarbonylamino)propylphosphonate 6 were synthesized from the respective diethyl [N-(1-phenylethyl)]-1-benzylamino-2,3-epiiminopropylphosphonates 5 via opening the aziridine ring with trimethylsilyl azide (TMSN3) followed by hydrogenolysis in the presence of di-tert-butyl dicarbonate (Boc2O). [N-(1-phenylethyl)]-1-benzylamino-2,3-epiiminopropylphosphonates (1R,2R,1′S)-5a and (1S,2S,1′R)-5c were smoothly transformed into diethyl 3-acetoxy-1-benzylamino-2-[N-(1-phenylethyl)amino]propylphosphonates (1R,2R,1′S)-9a and (1S,2S,1′R)-9c, respectively by the opening of the aziridine ring with acetic acid. Transformations of [N-(1-phenylethyl)]-1-benzylamino-2,3-epiiminopropylphosphonates (1S,2R,1′S)-5b and (1R,2S,1′R)-5d into diethyl 3-acetoxy-1-benzylamino-2-[(1-phenylethyl)amino]propylphosphonates (1S,2R,1′S)-9b and (1R,2S,1′R)-9d were accompanied by the formation of ethyl {1-(N-benzylacetamido)-3-hydroxy-2-[(1-phenylethyl)amino]propyl}phosphonate (1S,2R,1′S)-10b and (1R,2S,1′R)-10d and 3-(N-benzylacetamido)-4-[N-(1-phenylethyl)]amino-1,2-oxaphospholane (3S,4R,1′S)-11b and (3R,4S,1′R)-11d as side products. Diethyl (1R,2R)-, (1S,2S)-, (1S,2R)- and (1R,2S)-3-acetoxy-1,2-di(N-tert-butoxycarbonylamino)propylphosphonates 7a–7d were obtained from the respective 3-acetoxy-1-benzylamino-2-[N-(1-phenylethyl)amino]propylphosphonates 9a–9d by hydrogenolysis in the presence of Boc2O.

We noticed that isomeric aziridinephosphonates 5 could also serve as convenient substrates for synthesis of the protected 1,2-diamino-3-hydroxypropylphosphonates 7 when an appropriate oxygen nucleophile is applied to open the aziridine ring (Scheme 1).
When phosphonate (1R,2R,1′S)-5a was treated with glacial acetic acid at room temperature for 24 h opening of the aziridine ring at C3 was observed. Phosphonate (1R,2R,1′S)-9a was isolated in 59% yield after the chromatographic purification. However, irradiation of the reaction mixture in a microwave oven allowed us to shorten the reaction time to 1.5 h. Thus, phosphonate (1R,2R,1′S)-9a was obtained in 61% yield after purification on a silica gel column. Again, regioselectivity of the aziridine ring opening at C3 was evident from 1 H and 13 C NMR spectral data of 3-acetoxyphosphonates 9, with chemical shifts of H-C3 and the appearance of a two-bond H-C3-H coupling. Careful analysis of crude reaction mixtures showed that a minor regioisomer (opening at C2) was not formed. Hydrogenolytic removal of N- (1-phenylethyl) and N-benzyl groups in (1R,2R,1′S)-9a performed in the presence of Boc2O gave O-acetyl-N,N-diBoc-protected phosphonate (1R,2R)-7a in 74% yield after purification by column chromatography (Scheme 5).
In the attempts to optimize reaction conditions for the reaction of phosphonate (1S,2R,1′S)-5b with acetic acid, several parameters were changed including the reaction time, temperature, solvent/neat and the application of microwave irradiation.
We noticed that isomeric aziridinephosphonates 5 could also serve as convenient substrates for synthesis of the protected 1,2-diamino-3-hydroxypropylphosphonates 7 when an appropriate oxygen nucleophile is applied to open the aziridine ring (Scheme 1).
When phosphonate (1R,2R,1′S)-5a was treated with glacial acetic acid at room temperature for 24 h opening of the aziridine ring at C3 was observed. Phosphonate (1R,2R,1′S)-9a was isolated in 59% yield after the chromatographic purification. However, irradiation of the reaction mixture in a microwave oven allowed us to shorten the reaction time to 1.5 h. Thus, phosphonate (1R,2R,1′S)-9a was obtained in 61% yield after purification on a silica gel column. Again, regioselectivity of the aziridine ring opening at C3 was evident from 1 H and 13 C NMR spectral data of 3-acetoxyphosphonates 9, with chemical shifts of H-C3 and the appearance of a two-bond H-C3-H coupling. Careful analysis of crude reaction mixtures showed that a minor regioisomer (opening at C2) was not formed. Hydrogenolytic removal of N- (1-phenylethyl) and N-benzyl groups in (1R,2R,1′S)-9a performed in the presence of Boc2O gave O-acetyl-N,N-diBoc-protected phosphonate (1R,2R)-7a in 74% yield after purification by column chromatography (Scheme 5). In the attempts to optimize reaction conditions for the reaction of phosphonate (1S,2R,1′S)-5b with acetic acid, several parameters were changed including the reaction time, temperature, solvent/neat and the application of microwave irradiation.
In the attempts to optimize reaction conditions for the reaction of phosphonate (1S,2R,1 S)-5b with acetic acid, several parameters were changed including the reaction time, temperature, solvent/neat and the application of microwave irradiation.
Molecules 2019, 24, x FOR PEER REVIEW 6 of 15 contaminated with the starting materials and other organophosphorus compounds. Undoubtedly, in the presence of acetic acid, protonation of the ring nitrogen atom occurred and formed aziridinium ions A and B (Scheme 10). If we assume that these ions are additionally stabilized by hydrogen bonding with the BnNH-C1 nitrogen atoms, a spatial disposition of the diethoxyphosphoryl group would have been responsible for the observed differences in reactivity of diastereoisomeric pairs. Thus, the approach of the acetate anion to the less substituted carbon atom (C3) from the opposite side of the protonated nitrogen atom in A is sterically facilitated while in B the sterically bulky diethoxyphosphoryl group retards the attack of the nucleophile. Similar observations were made several years ago on diastereoisomeric 2,3-epiimino- 1-hydroxypropylphosphonates [27]. To rationalize the problem of the oxygen to nitrogen transfer of the acetyl group observed during the acetolysis of phosphonate (1S,2R,1′S)-5b, possible transition states were discussed. First, 3-acetoxypropylphosphonate (1R,2R,1′S)-9a should be transformed into a chair-like (six membered) conformer C which is energetically unfavoured since the bulky diethoxyphosphoryl group is positioned axially and thus the O→N acetyl transfer was not noticed under the reaction conditions applied (Scheme 11). On the other hand, in an analogous chair-like conformer of 3-acetoxypropylphosphonate (1S,2R,1′S)-9b (D) the diethoxyphosphoryl group is located in the equatorial position minimizing steric interactions. Under these circumstances the acetyl transfer provides (1S,2R,1′S)-E as a primary product (not isolated). The 3-hydroxypropylphosphonate readily underwent cyclization to form substituted 1,2-oxaphospholane (3S,4R,1′S)-F (also not isolated). However, their prior formation was proved by isolation of monophosphonate ester (1S,2R,1′S)-10b [by the ring opening of (3S,4R,1′S)-F] and (3S,4R,1′S)-11b. To rationalize the problem of the oxygen to nitrogen transfer of the acetyl group observed during the acetolysis of phosphonate (1S,2R,1 S)-5b, possible transition states were discussed. First, 3-acetoxypropylphosphonate (1R,2R,1 S)-9a should be transformed into a chair-like (six membered) conformer C which is energetically unfavoured since the bulky diethoxyphosphoryl group is positioned axially and thus the O→N acetyl transfer was not noticed under the reaction conditions applied (Scheme 11). On the other hand, in an analogous chair-like conformer of 3-acetoxypropylphosphonate (1S,2R,1 S)-9b (D) the diethoxyphosphoryl group is located in the equatorial position minimizing steric interactions. Under these circumstances the acetyl transfer provides (1S,2R,1 S)-E as a primary product (not isolated). The 3-hydroxypropylphosphonate readily underwent cyclization to form substituted 1,2-oxaphospholane (3S,4R,1 S)-F (also not isolated). However, their prior formation was proved by isolation of monophosphonate ester (1S,2R,1 S)-10b [by the ring opening of (3S,4R,1 S)-F] and (3S,4R,1 S)-11b.

General
1 H NMR spectra were taken in chloroform-d (CDCl3), benzene-d6 (C6D6) or deuterium oxide (D2O) on a Bruker Avance III (600 MHz). For spectra recorded in CDCl3 and C6D6 tetramethylsilane (TMS) was used as an internal standard; chemical shifts δ are given in ppm with respect to TMS and coupling constants J in Hz. Chemical shifts in D2O are referenced to the residual solvent peak, δ = 4.80 ppm. 13 C NMR and 31 P NMR spectra were recorded in a 1 H-decoupled mode for CDCl3, C6D6, and D2O solutions on the Bruker Avance III (600 MHz) spectrometer at 151 and 243 MHz, respectively. IR spectral data were measured on a Bruker Alpha-T FT-IR spectrometer. Melting points were determined on a Boetius apparatus and are uncorrected. Elemental analyses were performed by the Microanalytical Laboratory of the Faculty of Pharmacy (Medical University of Lodz) on a Perkin Elmer PE 2400 CHNS analyzer and their results were found to be in good agreement (±0.3%) with the calculated values. Polarimetric measurements were conducted on an Optical Activity PolAAr 3001 apparatus. Microwave irradiation experiments were carried out in a Plazmatronika RM 800 microwave reactor. The reaction was carried out in a 50 mL glass vial.
The following adsorbents were used: column chromatography, Merck silica gel 60 (70-230 mesh), analytical thin layer chromatography (TLC), Merck TLC plastic sheets silica gel 60 F254. TLC plates were developed in dichloromethane-ethyl acetate and dichlorometane-methanol solvent systems. Visualization of spots was effected with iodine vapors. All solvents were purified by methods described in the literature.

General
1 H NMR spectra were taken in chloroform-d (CDCl 3 ), benzene-d6 (C 6 D 6 ) or deuterium oxide (D 2 O) on a Bruker Avance III (600 MHz). For spectra recorded in CDCl 3 and C 6 D 6 tetramethylsilane (TMS) was used as an internal standard; chemical shifts δ are given in ppm with respect to TMS and coupling constants J in Hz. Chemical shifts in D 2 O are referenced to the residual solvent peak, δ = 4.80 ppm. 13 C NMR and 31 P NMR spectra were recorded in a 1 H-decoupled mode for CDCl 3 , C 6 D 6, and D 2 O solutions on the Bruker Avance III (600 MHz) spectrometer at 151 and 243 MHz, respectively. IR spectral data were measured on a Bruker Alpha-T FT-IR spectrometer. Melting points were determined on a Boetius apparatus and are uncorrected. Elemental analyses were performed by the Microanalytical Laboratory of the Faculty of Pharmacy (Medical University of Lodz) on a Perkin Elmer PE 2400 CHNS analyzer and their results were found to be in good agreement (±0.3%) with the calculated values. Polarimetric measurements were conducted on an Optical Activity PolAAr 3001 apparatus. Microwave irradiation experiments were carried out in a Plazmatronika RM 800 microwave reactor. The reaction was carried out in a 50 mL glass vial.
The following adsorbents were used: column chromatography, Merck silica gel 60 (70-230 mesh), analytical thin layer chromatography (TLC), Merck TLC plastic sheets silica gel 60 F 254 . TLC plates were developed in dichloromethane-ethyl acetate and dichlorometane-methanol solvent systems. Visualization of spots was effected with iodine vapors. All solvents were purified by methods described in the literature.
No special protection measures were taken when working with liquid (oily) aziridinephosphonates even under solvent free conditions.

Reaction of
3.4.2. Reaction of Aziridinephosphonates 5 with AcOH in the Microwave Reactor (General Procedure, Method B)