Phosphate Protection

DBU, CH3CN, 140 s. The 2-(4-nitrophenyl)ethyl (Npe) phosphate protective group and the 2-(4-nitrophenyl)ethoxycarbonyl (Npeoc) group are stable to these conditions, but the cyanoethyl group is not. 

Phosphate protection may begin at the stage of phosphoryl chloride (phosphorus oxychloride). A protective group may be introduced by reaction of this... 

All the approaches for deblocking protective groups described earlier in this book have found application in the removal of protective groups from phosphorus derivatives. Because phosphate protection and deprotection are commonly associated with compounds that contain acid-sensitive sites (e.g., glycosidic linkages and DMTr-O- groups of nucleotides), the most widely used protective groups on phosphorus are those that are deblocked by base. 

The following sections primarily describe many of the methods used for the cleavage of some of the more common phosphate protective groups. Since most of these groups are introduced by either the phosphate or phosphite method, little information is included here about their formation. The cited references generally describe the means that were used to introduce the protective group. In some cases, methods of formation are described, but this is done only when alternative methods to the phosphate or phosphite procedure were used. 

These derivatives, used for 5 -phosphate protection, are prepared by using the DCC coupling protocol and are cleaved with 2 N NaOH at The protected phosphates can be purified using benzoylated DEAE-Cellulose. 

These groups, along with a number of other trialkylsilylethyl derivatives, were examined for protection of phosphorothioates. Only the phenyl-substituted silyl derivative was useful, because simple trialkylsilyl derivatives were prone to acid-catalyzed thiono-thiolo rearrangement. Other trialkylsilylethyl derivatives also suffer from inherent instability upon storage,but the trimethylsilylethyl group has been used successfully in the synthesis of the very sensitive agrocin 84 and for intemucleotide phosphate protection with the phosphoramidite approach. 

This group, used for 5 -phosphate protection, has hydrophobicity similar to that of the dimethoxytrityl group and thus was expected to assist in reverse-phase HPLC purification of product from failure sequences in oligonucleotide synthesis. The group is cleaved with Bu4N F in DMSO at 70°. ... 

The 4 -pyridylethyl group was found to be more effective for intemucleotide phosphate protection than the 2 -pyridylethyl group, because its cleavage proceeded with greater efficiency. It is cleaved in a two-step process acylation with PhOCOCl increases the acidity of the benzylic protons, facilitating E-2 elimination by ammonia." ... 

This easily prepared lipophilic 5 -phosphate protective group is cleaved by NCS oxidation (dioxane, triethylammonium hydrogen carbonate, 2 h, rt) followed by ammonia-induced j3-elimination. ... 

Halogen-substituted phenols were originally introduced for phosphate protection to minimize intemucleotide bond cleavage during deprotection. 

The lipophilicity of this phosphate protective group helps in the chromatographic purification of oligonucleotides. It is removed by the oximate method. ... 

A polymeric version of this group has been developed for terminal phosphate protection in ribooligonucleotide synthesis. ... 

This highly lipophilic group is cleaved with isoamyl nitrite in Pyr/AcOH. The use of a lipophilic 5 -phosphate protective group aids in reverse-phase HPLC purification of oligonucleotides. 

This lipophilic group, developed for 5 -phosphate protection in oligonucleotide synthesis, is removed with 80% AcOH in 1 h. The related trityloxyethylamino group has been used in a similar capacity for phosphate protection and is also cleaved with 80% AcOH. ... 

Morpholine has been used for 5 -phosphate protection in oligonucleotide synthesis and can be cleaved with 0.01 N HCl without significant depurination of bases having free exocyclic amino functions. 

Goetzl, E.J., Kong, Y. and Mei, B., 1999, Lysophosphatidic acid and sphingosine 1-phosphate protection ofT cells from apoptosis in association with suppression of Bax, J. Immunol. 162 2049-2056. 

Sphingosine 1 -phosphate protects human umbilical vein endothelial cells from semm-depiived apoptosis by nitric oxide production, J. Biol. Chem. 276 10627-10633. 

Example 40 mononucleoside phosphotriester derivatives with 5-acyl-2-thioethyl bioreversible phosphate-protecting groups have been synthesized by Imbach and his associates [74a,b] in their research on intracellular delivery of 3 -azido-3 -dideoxythymidine-5 -mono-phosphate. 

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