2-cyanoethyl protecting group

The phosphitylation procedure activated by tetrazole led to the phosphite structure (step a) which was effectively oxidized by TBHP to yield the corresponding phosphate (step b). Finally all 2-cyanoethyl protecting group were removed by the action of DBU in the presence of the silylating reagent bis(trimethylsilyl)acetamide BSA (step c). The latter is indispensable to secure total deprotection. 

Example 30 Bialy and Waldman in their synthesis of protein phosphates 2A inhibitor (4S, 5S, 6S, lOS, US, 12S) cytostatin have found that application of the fluorenylmethyl protecting group (Fm) allowed successful formation of the phosphate moiety [62]. Initial experiments with the methoxybenzyl group, successfully employed in the synthesis of fostriecin [63] failed. The 2-cyanoethyl protecting group was also not suitable because that one 2-cya-noethyl group could be removed without destroying the whole molecule (Examples 25 and 26). 

Removal of the 2-cyanoethyl protecting group by mild alkali yielded... 

In the iV,iV -dicyclohexylcarbodiimide-alkyl phosphate reaction described here, 2-cyanoethyl phosphate (64) offers an additional advantage, as the 2-cyanoethyl protecting group is readily eliminated by mild, alkaline hydrolysis, as shown in the conversion of (77) (see Scheme 3, R == nucleoside residue) into... 

Propose a mechanism to account for cleavage of the /3-cyanoethyl protecting group from the phosphate groups on treatment with aqueous ammonia. (Acrylonitrile, H2C=CHCN, is a by-product.) What kind of reaction is occurring ... 

Removal of the jS-cyanoethyl protecting group occurs according to the following equation. Show a mechanism for this reaction. What is the role of the cyano group in this reaction ... 

Formulate mechanisms for all the hydrolysis reactions that effect deprotection of the solid-phase supported dinucleotide in the scheme at the bottom of p. 1209. [Caution The cyanoethyl protecting group is not removed by nucleophilic hydrolysis of the phosphate ester, because such a process would cleave all P-0 bonds indiscriminately. Can you imagine another way Hint Like other carboxylic acid derivatives (Section 20-1), the a hydrogen in nitriles is acidic.]... 

A widely used protecting agent is 2-cyanoethanol (- 3-hydroxypropanonitrile, hy-dracrylonitrile), which is condensed with phosphates with the aid of DCC. The 2-cyanoethyl (Ce) group is quantitatively removed as acrylonitrile by treatment with weak bases (H.G. Kho-rana, 1965). 

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. 

Na, ammonia. These conditions also remove cyanoethyl- and benzyl-protective groups. Phosphorothioates are similarly deprotected. 

Example 19 Crich and Dudkin have used phosphoroamidite containing two different protecting groups 0-benzyl-0-2-cyanoethyl-iV,Ar-diisopropyl-phosphoroamidite [48]. This phosphitylating reagent was prepared in excellent yield from 2-cyanoethyl NyN diisopropylchlorophosphoroamidite, which was immediately used for coupling with an appropriate alcohol in the presence of tetrazole and oxidized without delay with TBHP. This kind of phosphorylation procedure was used in the synthesis of 4,8,12,16,20-pen-tamethyl-pentacosylphosphoryl / -o-mannopyranoside, an unusual / -man-nosyl phosphoisoprenoid from Mycobacterium avium. 

The dimethoxytrityl ester protecting group is now removed by treatment with mild acid (CCI3CO2H), which is insufficiently reactive to hydrolyse the amide protection of bases, or the cyanoethyl protection of the phosphate. The coupling cycle can now be repeated using a phosphoramidite derivative of the next appropriate nucleoside. The sequences will be continued as necessary until the desired oligonucleotide is obtained. 

It then remains to remove protecting groups and release the product from the support. All of these tasks, except for the removal of the dimethoxytrityl group, are achieved by use of a single deprotection reagent, aqueous base (ammonia). The cyanoethyl groups are lost from the phosphates by base-catalysed elimination, and amide protection of the bases is removed by base-catalysed hydrolysis. The latter process also achieves hydrolysis of the succinate ester link to the support. 

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