For cleavage

Pyndinium p-toluenesuHonate (PPTS) reagent 1 for protection of alcotwls as the tetrahydropyranyt ethers as well as for cleavage of ethers (in warm ethanol)... 

PROTECTION FOR THE HYDROXYL GROUP, INCLUDING 1.2- AND 1,3-DIOLS Half-lives for cleavage of 5 -protected thymidine in 80% AcOH at 15 ... 

The triethylsilyl ether is approximately 10-100 times more stable than the TMS ether and thus shows a greater stability to many reagents. Although TMS ethers can be cleaved in the presence of TES ethers, steric factors will play an important role in determining selectivity. The TES ether can be cleaved in the presence of a /-butyldimethylsilyl ether using 2% HE in acetonitrile. In general, methods used to cleave the TBDMS ether are effective for cleavage of the TES ether. 

Cyclobutyl Carbamate (Chart 8). The half-life for cleavage in neat CF3CO2H is > 300 min. 

The bulky triphenylmethyl group has been used to protect a variety of amines such as amino acids, penicillins, and cephalosporins. Esters of N-trityl a-amino acids are shielded from hydrolysis and require forcing conditions for cleavage. The a-proton s also shielded from deprotonation, which means that esters elsewhere in the molecule can be selectively deprotonated. 

Some of the methods used to cleave the benzyl group should also be effective for cleavage of the MPM group. ... 

Exonucleases. Like the endonucleases they are restriction enzymes which act at the 3 or 5 ends of linear DNA by hydrolysing off the nucleotides. Although they are highly specific for hydrolysing nucleotides at the 3 or 5 ends of linear DNA, the number of nucleotides cleaved are time dependent and usually have to be estimated from the time allocated for cleavage. Commercially available exonucleases are used without further purification. 

The ionization and direct displacement mechanisms can be viewed as the extremes of a mechanistic continuum. At the 8 1 extreme, there is no covalent interaction between the reactant and the nucleophile in the transition state for cleavage of the bond to the leaving group. At the 8 2 extreme, the bond formation to the nucleophile is concerted with the bondbreaking step. In between these two limiting cases lies the borderline area, in which the degree of covalent interaction between the nucleophile and the reactant is intermediate between the two limiting cases. The concept of ion pairs is important in the consideration of... 

FIGURE 5.18 Methods for cleavage of disulfide bonds in proteins, (a) Oxidative cleavage by reaction with performic acid, (b) Reductive cleavage with snlfliydryl compounds. Disulfide bridges can be broken by reduction of the S—S link with snlfliydryl agents such as 2-mercaptoethanol or dithiothreitol. Because reaction between the newly reduced —SH groups to re-establish disulfide bonds is a likelihood, S—S reduction must be followed by —SH modification (1) alkylation with iodoac-etate (ICH,COOH) or (2) modification with 3-bromopropylamine (Br— (CH,)3—NH,). 

Pancreatic RNa.se is an enzyme specific for cleavage where a pyrimidine ba.se lies to die 3 -side of die pho.sphodie.ster it acts endo. The products are oligonucleotides widi pyrimidine-3 -P04 ends ... 

Half-lives for Cleavage of 5 -Protected Thymidine in 80% AcOH at 15" ... 

Mercaptopyridine A-oxide, CH2CI2. A thousand fold excess of this reagent is required to achieve good yields for cleavage in solid-phase peptide synthesis. 

A -Dicyclopropylmethylamide (Dcpm-NRCO-) (C3H5)2CH-NRCO-Half-lives for Cleavage of CH3CONHR in NeatTFA at rt... 

Ceric ammonium nitrate (CAN), CH3CN, H2O, it, 12 h, 96% yield. Benzylamides are not cleaved under these conditions. Some of the methods used to cleave the benzyl group should also be effective for cleavage of the PMB group. Ceric ammonium nitrate is also used to cleave the PMB group from a sulfonamide nitrogen. ... 

AgN02, MeOH, H2O, 24°, cool to 0°, add KCN, then HCl, 96% yield." The reduced electron density of the propargylic alkyne directs the electrophilic silver to the other alkyne and activates it for cleavage. 

Electrolysis at a Hg cathode, -0.5 to -0.6 V, LiC104, CH3CN, Pyr. The trichloroethyl ester, which requires a greater reduction potential for cleavage, is retained under these conditions." ... 

Lead tetraacetate and periodic acid complement one another in their applicability as reagents for glycol cleavage. The water sensitive lead tetraacetate is used in organic solvents, while periodic acid can be used for cleavage of water-soluble diols in aqueous medium. 

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 ... 

Potassium ferf-butoxide was purchased from Columbia Organic Chemicals Company, Columbia, South Carolina. The molar ratio of potassium ferf-butoxide to water is critical, and the amounts specified represent optimum quantities for cleavage of 0.10 mole of ketone.2,3... 

An interesting kinetic study has been made of the cleavage of aryltrimethyl-stannanes by aqueous methanolic alkali at 50°C695. This reaction is base-catalysed as shown by the following rates (105Arx) for cleavage of the 3-trifluoro-methylphenyl compound in methanol (3 vol.) at the concentrations (in parentheses) of sodium hydroxide (2 vol.) 148 (3.805 M), 173.5 (5.33 M), 207 (7.71 M). The mechanism approximates to that given in equilibria (270) and (271)... 

See Wallace, O.B. Springer, D.M. Tetrahedron Lett., 1998, 39, 2693 for cleavage of thiol esters to thiols with NaSMe in methanol and Choi, J. Yoon, N.M. Synth. Commun., 1995, 25, 2655 for borohydride exchange resin-Pd(OAc)2 reductive cleavage of thiol esters to thiols. 

Certain other sulfur-containing reagents also cleave methyl and other ethers see Hanes-sian, S. Guindon, Y. Tetrahedron Lett., 1980, 21, 2305 Williard, P.G. Fryhle, C.B. Tetrahedron Lett., 1980, 21, 3731 Node, M. Nishide, K. Fuji, K. Fujita, E. J. Org. Chem., 1980, 45, 4275. For cleavage with selenium-containing reagents, see Evers, M. Christiaens, L. Tetrahedron Lett., 1983, 24, 377. For a review of the cleavage of aryl alkyl ethers, see Tiecco, M. Synthesis, 1988, 749. 

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