Water as a promoting media for 1,3-dipolar cycloaddition of phosphorylated azides to internal alkynes

The 1,3-cycloaddition of  -phosphoryl azides to activated internal alkynes such as dimethyl acetylenedicarboxylate and tetramethyl acetylenediphosphonate as well sodium azide to tetramethyl acetylenediphosphonate readily proceeds in water without any co-solvent or additive to afford the corresponding phosphorylated 1,2,3-triazoles in excellent yields. The ester group in carbalkoxy function of these compounds can be easily converted to carbamido group via the reaction with amines in MeOH thereby expanding the range of potentially biologically active heterocyclic aminophosphonates of such type.


Introduction
Among the reactions used nowadays for design of specifically acting low molecular weight molecules for modern drug discovery, so called "click" methodology 1 allowing quick and easy generation of large libraries of compounds became more and more used.The main principles of the above methodology comprise modular and wide in scope reactions giving very high yields and great diversity of products.These reactions of readily available starting substrates should proceed either in the absence of solvent or in a solvent that is benign (preferably water) to favor a reaction with one single reaction product.The most popular among the click reactions are cycloadditions of unsaturated species, especially 1,3-dipolar Huisgen cycloadditions of azides and terminal alkynes to give the corresponding triazoles. 2The discovery of catalytic properties of copper(I) which allows high rate and control of regioselectivity of this cycloaddition in the case of terminal alkynes (therefore referred also as Cu-Alkyne-Azide Cycloaddition or CuAAC reaction) to give 1,4-disubstituted 1,2,3-triazoles stimulated the investigations in this field over the last years. 3Even despite the fact that 1,2,3-triazole structural moiety does not occur in nature, a wide range of compounds containing this functionality exhibit diverse biological activities such as triazole-linked glycoconjugates 4 and glycopeptides 5 , 1,2,3-triazole modified nucleic acids, 6 nucleoside analogues, 7 and a range of anti-HIV 8 , anti-epileptic 9 or antimicrobial 10 substances.
Taking into account that a wide range of organophosphorus compounds exhibit different types of bioactivity, 11 recently we have elaborated the effective syntheses of phosphorylated terminal alkynes 12,13 and azides, 14 which were involved in Cu(I) cycloadditions to afford a range of potential drug-candidates such as 1,2,3-triazole-substituted methylenebisphosphonates (N-BPs) 12 and the related phosphonocarboxylates (N-РCs), 13 including those bearing sugar, adamantane, and azidothimidine biogenic moieties, as well as 4-(1H-1,2,3-triazol-1yl)phosphonates having amino acid residues 14 and perfluorinated groups. 15n continuation of this study, in this paper we report on the reactivity of phosphorylated azides towards internal alkynes providing, in water as a sole reaction medium, the facile synthesis of the new polyfunctionalized phosphorus-substituted 1,2,3-triazoles.

Results and Discussion
Electron-withdrawing groups on acetylenes are well-known to increase the cycloaddition reaction rate, however, under classical conditions the reactions with azides proceed not too fast even for activated internal alkynes.For example, 1,3-dipolar cycloaddition of organic azides to 1-trimethylsilyl carbalkoxy-acetylenes was completed over 14 h in refluxing toluene, 16 in the case of dimethyl acetylenedicarboxylate (DMAD), the reaction required prolonged heating of the components (e.g., 90 o C for 6 h 17 or 80 o C, 19 h 18 ) in the absence of the solvent.The cycloadditions of internal alkynes can be promoted by microwave irradiation 19 or application of copper(I) or ruthenium(II) catalysts. 20Besides MW or application of catalysts, the reaction rate may be optimized by addition of water and application of water-alcohol media (usually tBuOH:H2O/4:1) as often used for CuAAC reactions.
Despite the strong acceleration effect of water for many organic reactions, 21 where low miscibility or solubility of organic compounds is not detrimental and in some cases facilitates the isolation of products, 22 not too many examples of alkyne-azide cycloadditions have been performed in pure water as a reaction medium 21e using more often terminal 23 than internal 23a,24 alkynes.For alkynes of both types the reactions were carried out in the presence of different catalysts and only a few publications reported on cycloaddition without any additives (in three cases of four DMAD was used as an alkyne).1b,23a,25 Note that in these cases the additional functionalities in the starting azide did not tolerate the aqueous reactions due to partial hydrolysis. 26Apparently, the warnings relating to the possible instability of organophosphorus compounds in water resulted in its limited application in organophosphorus synthesis (the known examples comprise the Wittig, Kabachnik-Fields, aza-Michael reactions and synthesis of functionalized stabilized phosphorus ylides via the three-component reaction of triphenylphosphine, dialkyl acetylenedicarboxylates and various nucleophiles). 27e have found that cycloaddition of -phosphorylalkyl azides 1a,b to DMAD 2a proceeded rapidly in water as a sole solvent without a catalyst to afford 4-phosphorylalkyl-1,2,3-triazoles 3a,b in excellent yields and did not affect the ester groups at the phosphorus atom.The reactions were completed in 36 h at room temperature or 1.5-2 h at 60 o C. Note that phosphorylated azides 1a,b and final products 3a,b are soluble in water in contrast to DMAD and formation of homogeneous mixture visually indicates the completion of the reaction.Similarly, phosphorylated azides react readily with tetramethoxy acetylenediphosphonate 2b.As the latter is soluble in water, in this case the homogeneous reaction was completed over shorter period of time.The products 3a-c were isolated by extraction with DCM followed by chromatography via short plug of silica gel.
Scheme 1. Reaction of phosphorylated azides with internal alkynes in water.
Application of Cu(I) salts (CuI or CuBr) as catalysts does not influence significantly the reaction rate but required additional work-up to remove the copper impurities from the final products.Moreover, in tBuOH:H2O mixed solvent the reaction in which Cu(I) catalyst was generated in situ from CuSO4 and sodium ascorbate, was accompanied (50 o C) by partial DMAD polymerization and required application of DMAD excess for quantitative conversion of the starting azide.
Addition of sodium azide to acetylenediphosphonate 2b proceeded in water even more faster giving, in 0.5 h at room temperature, the corresponding sodium salt 4 in quantitative yield which may be easily isolated by simple removal of water.
In contrast, addition of sodium azide to DMAD 28 or to the related bis(diphenyl(thio)phosphoryl)acetylene 29 performed in MeOH and DMF required additional work-up procedures.The reaction in water is exothermic and reasonable dilution of the reaction mixture (see experimental) and maintenance of the temperature below 20 o C are needed to avoid formation of side products.Further acidification of reaction mixture is accompanied by prototropic schift to yield the 2H-1,2,3-triazole-diphosphonate 5 which structure was unambiguously confirmed by X-ray analysis of a single crystal.In crystalline form the molecule of 5 (Figure 1) occupied a special position around a C2 axis passing through the N1 atom.Similar to 4,5-bis(diphenylthiophosphinoyl)-1,2,3-triazole, 29b the molecule of 5 was present in a 2H tautomeric form.The C1-P1 bond (1.7859(18) Å) was by 0.04 Å shorter than that in the above mentioned analog with diphenylthiophosphoryl groups 4,5-(1.826Å), indicating less steric crowding.The crystal of 5 consists of infinite chains of bifurcate N-H…O hydrogen bonds (N…O 2.877(2) Å, N-H…O 135.5(6)°,O…H…O 88.9(13)°) (Figure 2) along the crystallographic axis c formed by the phosphoryl oxygen atoms and the hydrogen at the nitrogen atom in the 2-position of the heterocycle.In addition, to expand the range of such potentially biologically active substances, triazoles 3a,b bearing methoxycarbonyl groups were transformed into the corresponding amides 6a-c via the reaction with primary amines in MeOH.Reaction proceeded at room temperature and completed over 12, 14 and 24 hours in the case of pyrrolidinethylamine, 2-pyridinethylamine and butylamine, respectively.To obtain bicyclic product 7 the reaction with ethylenediamine was performed in very dilute solution over 7 days.It should be noted that the attempt to synthesize compound 6a via one-pot three-component reaction of DMAD, phosphorylated azide and butylamine in water was unsuccessful.Moreover, the treatment of the aqueos reaction mixture containing preformed triazole 3b with butylamine was accompanied by partial hydrolysis of carbomethoxy groups to afford the target bisamide 6a and the corresponding bis(butylammonium) salt in approximately 1:1 ratio.Application of secondary amines in this reaction resulted in considerable decrease of the reaction rate and change of the reaction course.Thus, the reaction of 3b and Et2NH in a month led to diethylammonium salt 8 rather than in the desired amide.
The final 1,2,3-triazoles 3a-c were isolated as yellowish oils in 87-95% yields by extraction with DCM followed by flash-chromatography on silica gel.The amides 6a-c, 7 were isolated with high purity after removing of volatiles from the reaction mixtures followed by drying in vacuo.In the 31  5 ppm for the phosphonate groups attached to the triazole ring and at ~25 ppm for the phosphonate moiety attached to the ring through an alkylene chain.Taking into account that the signal of phosphonate groups in the starting substrate 2b is observed at ca. -8 ppm, the singlets at 4 and 7.5 pm were assigned to phosphonate groups in 5-and 4-positions of the heterocycle in 3с.The general pattern of the 1 H and 13 C NMR spectra fit well to the depicted structures.In the IR spectra of the compounds the characteristic bands of phophоryl and carbonyl groups were observed at 1266-1280 and 1735 cm -1 in the case of esters 3a-c and at 1245-1253 and ~1676 cm -1 for the amide derivatives 6a-c,7.

Conclusions
The phosphorylated azides react with symmetric internal alkynes, both phosphorylated and nonphosphorylated, and in this case the highest reaction rate was observed in water without any cosolvent or additive.This approach is especially advantageous from green chemistry point of view.Furthermore, the alkoxycarbonyl substituted derivatives could be further transformed to corresponding amides thereby expanding the range of potentially biologically active heterocyclic aminophosphonates of such type.

Experimental Section
General.NMR spectra were recorded with a Bruker AMX-300 spectrometer using residual proton signals of deuterated solvent as an internal standard ( 1 H, 13 C) and H3PO4 ( 31 P) as an external standard.The 13 C NMR spectra were registered using the JMODECHO mode; the signals for the C atom bearing odd and even numbers of protons have opposite polarities.IR spectra were recorded on a Fourier-spectrometer "Magna-IR750" (Nicolet), resolution 2 cm -1 , 128 scans.Melting points were determined with MPA 120 EZ-Melt Automated Melting Point Apparatus and were uncorrected.Dimethyl acetylenedicarboxylate, amines and sodium azide were purchased from Aldrich and used without further purification.

Figure 1 .
Figure 1.The general view of 5 in crystal.Atoms are represented by thermal ellipsoids.Labels with suffix "A" denote atoms generated by an (0.5-x,0.5-y,z) symmetry operation.
P NMR spectra 1,2,3-triazole-4,5-dicarboxylates 3a,b demonstrated singlets at ca. 25 and 30 ppm, respectively, which were slightly upfield shifted comparing with the signals for the starting azides.Vice versa, the signals of the amide derivatives 6a-c and 7 are slightly downfield shifted relative the resonances of their carbomethoxy-substituted precursors.Nevertheless, the disposition of the signals has the general pattern: in the case of 1,2,3-triazoleethylphosphonates 3a, 6b,c, and 7 the signals were observed at ca 25-26 ppm and at ca 30 ppm for the related propylphosphonates 3b,6a.The signals of phosphorus atoms in the molecule of trisphosphorylated 1,2,3-triazole 3c were observed at ca. 4 and 7.