Preparation selective removal

In some instances, the resist polymer can be prepared in a single step by direct polymerization of the protected monomer(s) (37,88), entirely avoiding the intermediate PHOST. HOST-containing resist polymers have also been prepared by free-radical copolymerization of a latent HOST and a stable, acid-labile monomer, eg, the copolymerization of acetoxystyrene with tert-huty acrylate, followed by selective removal of the acetoxy group (89) (Fig. 30). 

Aiyl esters, prepared from the phenol and an acid chloride or anhydride in the presence of base, are readily cleaved by saponification. In general they are more readily cleaved than the related esters of alcohols, thus allowing selective removal of phenolic esters. 9-Fluorenecarboxylates and 9-xanthenecarboxylates are also cleaved by photolysis. To permit selective removal, a number of carbonate esters have been investigated aryl benzyl carbonates can be cleaved by hydrogenolysis aryl 2,2,2-trichloroethyl carbonates, by Zn/THF-H20. 

However, treatment of cortisone 3,20-bissemicarbazone with acetic anhydride and pyridine removes the 20-semicarbazone group preferentially. Selective removal of a protecting group can be also achieved by a selective reaction to give a new intermediate which can be converted into the desired product ketone. Thus progesterone 20-monoenol acetate (42) is prepared from the 3,20-bisenol acetate (40) via selective electrophilic attack of iodine at C-6 followed by reductive dehalogenation of (41). ... 

Unsubstituted 20-ketones undergo exchange dioxolanation nearly with the same ease as saturated 3-ketones although preferential ketalization at C-3 can be achieved under these conditions. " 20,20-Cycloethylenedioxy derivatives are readily prepared by acid-catalyzed reaction with ethylene glycol. The presence of a 12-ketone inhibits formation of 20-ketals. Selective removal of 20-ketals in the presence of a 3-ketal is effected with boron trifluoride at room temperature. Hemithioketals and thioketals " are obtained by conventional procedures. However, the 20-thioketal does not form under mild conditions (dilution technique). ... 

The tetrahydropyranyl ether, prepared from a phenol and dihydropyran (HCl/EtOAc, 25°, 24 h) is cleaved by aqueous oxalic acid (MeOH, 50-90°, 1-2 h). Tonsil, Mexican Bentonite earth, HSZ Zeolite, and H3[PW,204o] have also been used for the tetrahydropyranylation of phenols. The use of [Ru(ACN)3(triphos)](OTf)2 in acetone selectively removes the THP group from a phenol in the presence of an alkyl THP group. Ketals of acetophenones are also cleaved. ... 

Primary 1,2-diamines can be obtained by hydrogenolysis of the benzylic N-auxiliary bond over Pd/C, e.g., the preparation of 249d. The cleavage is not selective in the presence of other benzylic bonds in the molecule, e.g., aryl substituents at Cl - C2. However, in the presence of methoxyphenyl or dimethoxyphenyl C substituents, the N substituents can be selectively removed, as exemplified by the preparation of the primary 1,2-diamine... 

Since sulphones 204 are easily available compounds one would expect that they could be used as starting materials for the preparation of sulphoxides via the selective removal of one oxygen atom from the sulphonyl group (equation 112). Up to now, there is only one example reported of a direct reduction of a sulphone to a sulphoxide. The bicyclic dideuterio sulphone 205 after 24 h treatment with three-fold excess of diisobutyl aluminium hydride in boiling dichloromethane gave the corresponding sulphoxide 206 in 36% yield (equation 113). A two-step procedure for the selective reduction of sulphones to sulphoxides, which involves an initial reaction of sulphone 204 with aryldiazonium tetrafluoroborate 207 to form aryloxysulphoxonium salt 208 and its subsequent reduction (equation 114), was alluded to by Shimagaki and coworkers and... 

The most important example of this category is Raney nickel, which is extensively used in hydrogenation reactions in fine chemistry. The catalyst has been named after Murray Raney who invented this catalyst in 1924. It is prepared by the reaction of a powdered nickel-aluminium alloy with aqueous sodium hydroxide to selectively remove a large fraction of the aluminium component (.see Figure 3.12). The product consists of porous nickel with a high... 

Protected primary allylic amines are prepared from allyl halides and di-r-butyl imidodicarbonate 179 in the presence of lithium iodide, followed by the selective removal of one of the protecting groups by means of trifluoroacetic acid, e.g. equation 66187. 

Since Ugi is now an emeritus and he and his co-workers cannot continue their experimental studies, we propose that the analog l-amino-5-desoxy-5-thio-D-xylopyranose 61b should be prepared and be used as a reagent of U-4CRs. It has a good chance to form stereoselectively high yields of products whose auxiliary group can be selectively removed. 

The degree of polymerization (DP) of pulp treated with a cellulase-free xylanase was found to increase, apparently due to the selective removal of xylan, which has a lower DP (47). TTius, treatment of pulps with xylanases has been shown to increase their viscosities (27, 48, 49). However, even low cellulase activities in the enzyme preparations result in decreased viscosity. 

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