Dinucleoside H-phosphonates

The methodology has been resurrected very recently in the H-phosphonate variant. An attempt by Adamo et al. [326] to use a chlorosulfonated polystyrene resin for activation of the nucleoside H-phosphonate monoesters was not particularly impressive, but the corresponding chlorocarbonylated polystyrene performed well, permitting the synthesis of up to a hexanucleotide. Similar results were reported by Mohe et al. [327] for the dinucleoside H-phosphonate synthesis promoted by a phosphorochloridate supported on polystyrene. 

In related Pd(Xantphos)-catalyzed cross-couplings of benzyl halides, reductive elimination was proposed as the slow step, since the overall rate increased with large bite angle ligands [98], P-C bond formation with isolated diastereomers of dinucleoside H-phosphonates was stereospecific, likely with retention of configuration at phosphorus, as above (Scheme 58). 

Dinucleoside H-phosphonate 1 has been synthesized by Meier by reacting 2, 3 -dideoxythymidine with diisopropyldichlorophosphine in the presence of diiso-propylethylamine, and subsequent hydrolysis [187]. 

Dinucleoside-a-hydroxybenzylphosphonates 2 in up to 90% yield were synthesized by reacting the symmetric dinucleoside H-phosphonates 1 with benzaldehyde derivative in the presence of catalytic amounts of a tertiary base like diisopropylethylamine (DIPEA), or tri-ethylamine (TEA). Dinucleoside-a-hydroxybenzylphosphonates 2 are consideied uncharged prodrugs of 5 -nucleoside H-phosphonates 3 and 5 -nucleoside monophosphates 4. These two compounds are released by controlled hydrolysis via two different pathways the phospho-nate-phosphate rearrangement and the direct cleavage mechanism [188]. 

Dinucleoside H-phosphonates were obtained when deoxyribonucleoside H-phosphonate reacted with 3 -benzoylthymidine in the presence of l,3-dimethyl-2-chloroimida-zolinium chloride-coupling reagent [177]. The P H] NMR spectrum of the reaction mixture showed that the signal of the nucleoside H-phosphonate at 5 = 2.81 ppm completely disappeared and new signals were observed at 5 = 8.21 and 7.20 ppm. The chemical shift suggested that nucleoside H-phosphonate was converted into the corresponding dinucleoside H-phosphonate. 

Dinucleoside H-phosphonates were obtained by reacting, in the presence of triphosgene, H-phosphonate with a second mononucleoside having a free 5 -hydroxyl group [174]. 

Dinucleoside H-phosphonate dimers have been used as intermediates for the synthesis of phosphorothioates (10), phosphoramidates (10), phosphate triesters (10), and methyl phosphonates (15) (Fig. 3). This method has been extended to the synthesis of polynucleoside phosphorothioates (7,12,13) and phosphoramidates (7,10,11,16). Much of this chapter is devoted to the coupling and the oxidation reactions. The coupling step is very sensitive to overactivation by acyl chlorides, and thus, precautions must be taken to avoid the consequent side reactions. The oxidation reaction is important because of the fact that all linkages are oxidized at once, and care must be taken at this step to ensure complete oxidation, yet prevent the alkaline hydrolysis of the backbone. Often, poor quality of synthetic oligodeoxynucleotide is the result of hydrolysis of the backbone or incomplete oxidation, not inadequate chain elongation. 

Seela, F. and Kretschmer, U. (1991) Diastereomerically pure Rp and 5p dinucleoside H-phosphonates. The stereochemical course of their conversion into P-methylphosphonates, phosphorothioates, and [ 0] chiral phosphates J. Org. Chem. 56,3861-3869... 

General method for the synthesis of dinucleoside benzyl-phosphonates of type 21 Pd2(dha)3.(CHCl3) (0.0125 mmol, 13 mg), Xantphos (0.025 mmol, 15 mg), a suitable protected dinucleoside H-phosphonate 21 (0.5 mmol), and tetrahydrofuran (THE) (5 mL) were placed in a two-neck flask equipped with a reflux condenser. The apparatus was filled with nitrogen by applying two cycles of vacuum/N2, and A(lV-diisopropylethylamine (0.6 mmol, 78 mg, 105 mL) and benzyl chloride (0.6 mmol, 76 mg, 69 mL) were added. The mixture was refluxed for 4 hours, concentrated under vacuum, and the product purified by silica gel chromatography using pentane-EtOAc (1 1, v/v) with a gradient of MeOH (2.5-10%). Yield >80%. 

Methodologies to create functional and topological diversity in libraries of nucleoside phosphoramidates have been improved by the use of parallel solid-phase synthesis. A representative 600-member library of dinucleoside phosphoramidates (16) and tri-nucleoside phosphoramidates (17) was synthesised from CGP-support-bound di- and tri-nucleoside H-phosphonates (5 -DMT protected) and selected amines. The library was screened for antiviral activity against HBV and some of its members showed potent activity. ... 

The aroylphosphonic function was utilized elegantly for the protection of the 0=P—H moiety in the synthesis of dinucleoside hydrogen phosphonates. Although dinucleosidyl... 

The use of the Michaelis-Arbuzov reaction has been made in the synthesis of dinucleoside-3 -5-phosphorothiolates using disulphides (73) and nucleoside 5 -dialkylphosphite derivatives. Particularly efficient reactions were achieved using a 5 -Wj(trimethylsilyl)phosphite (74) which afforded the phosphorothiolate directly after aqueous work up. Derivative (74) was obtained by treating the corresponding nucleoside H-phosphonate with chlorotrimethylsilane in pyridine in the presence of triethylamine. 

The cyclic dinucleotide phosphorothioate (79) has been prepared" stereoselectively and in good yield from the 3 -hydrogen phosphonate of the dinucleoside phosphorothioate (80) by standard activation with pivaloyl chloride, followed by sulphurisation with elemental sulphur. Interestingly the second stereocentre is formed with complete Rp stereoselectivity. This is attributed to the reduced conformational freedom associated with the formation of the cycle, since thiooxidation of H-phosphonate intermediates is known to be stereoselective. ... 

The transesterification of aryl nncleoside 3 -H-phosphonate with alcohols or with suitably protected nucleosides under mild conditions furnished the corresponding ester or dinucleoside (3 -5 ) H-phosphonate diesters [182],... 

Dinucleoside phosphorothioates are chiral analogues of phosphodiesters in which one of the nonbridging oxygens has been replaced by sulfur. These compounds are important research tools in stereochemical investigations, in mechanistic studies of various enzymatic reactions, and other biochemical stndies [210-212]. There have been only a few methods for the preparation of ribonncleoside phosphorothioates [213-216]. The H-phosphonate approach is the best method for the preparation of phosphorothioate diesters [217-219]. 

A crucial point in the synthesis of optically pure dinucleoside phosphorothioates is the separation of the precursors of the final products that differ only in their configuration around phosphorus. Column chromatography on silica gel is an efficient and convenient way to separate the diastereomers of H-phosphonate diesters. The results obtained reveal that the sulfurization of suitable protected diribonucleoside H-phosphonates with elemental sulfur is a stereospecific reaction, which is most likely proceeding with the retention of the configuration at the phosphorus center. 

SCHEME 47.44. Synthesis of dinucleoside 4-pyridylphosphonates via stereospeciflc addition-elimination of the H-phosphonate diester to A -tritylpyridinium salt. 

However, conversion of the dinucleoside //-phosphonate diester 10 to the phosphorothioate analogue 11 effected by a 0.02 M solution of 2 in 2% aqueous pyridine is relatively slow it reportedly takes 3 h for a complete reaction. While a solution of 2 in 2% aqueous acetonitrile containing triethylamine can completely transform 10 into 11 within 30 s, the use of this sulfurization mixture is incompatible with automated solid-phase oligonucleotide synthesis given the rapid formation of a yellow precipitate caused by triethylamine. In the absence of triethylamine, 10 is not sulfurized under these conditions. Nonetheless, 10 is completely converted to 11 within 20 min when a 0.02 M solution of 1 in 2% aqueous pyridine is used for the sulfurization reaction. Thus, 3i/-l,2-benzodithiol-3-one in aqueous pyridine is compatible with automated solid-phase synthesis of both DNA and RNA oligonucleoside phosphorodithioates or phosphorothioates from appropriate //-phosphonate derivatives. ... 

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