Acetylation preferential

Any of the customary reducing agents suitable for converting aryl nitro groups to arylamines (Fe, HC1 Sn, HC1 H2, Ni) may be used. Acetylation of p-aminophenol may be carried out with acetyl chloride or acetic anhydride. The amino group of p-aminophenol is more nucleophilic than the hydroxyl group and is acetylated preferentially. 

When the triazole contains both an aliphatic and an aromatic amino group, the former is acetylated preferentially. Thus 4-amino-5-aminomethyl-3-benzyltriazole, stirred with 1 Eq of acetic anhydride in pyridine, gave 4-amino-5-acetamidomethyl-3-benzyltriazole (20 C, 15 hr, 90%). The 1- and 2-methyl analogs were similarly made, but in aqueous pyridine, whereas the use of dry pyridine and 4 Eq of acetic anhydride acetylated both amino groups (75%). Acetic formic anhydride was used similarly to obtain either mono- or di-formylated products, as required [73JCS(P1)1634]. 

The partial synthesis of vinblastine itself makes use of 20-acetoxycatharanthine (223), prepared from catharanthine as described earlier. The modified Polonovski reaction in the presence of vindoline gave an intermediate immonium ion, which was reduced to 20 -acetylvinblastine (Scheme 37). Mild alkaline hydrolysis afforded deacetylvinblastine, the secondary hydroxy-group of which could be re-acetylated preferentially, with formation of vinblastine (271). ... 

Methyl 4,6-O-methylene-o-D-mannopyranoside was the only product isolated from the LiBr-catalised transacetalation of the unprotected methyl glycoside with dimethojqmiethane. Cyclopentylidene derivatives of pentoses have been prepared in moderate yields by treatment of the free sugars with cyclopentanone in the presence of copper (II) sulphate and sulphuric acid. D-Xylose formed the diacetal (11) (also used in Scheme 3 below), whereas from D-ribose the 2,3-monoacetal (12) was obtained. A novel, selective synthesis of (5)-configurated 4,6-pyruvate acetals of methyl D-hexopyranosides is illustrated in Scheme 1. It relies on transacetalation from the dimethyl acetal of 3,4-dimethoxybenzophenone to give, after acetylation, preferentially the intermediate (13) with an axial aryl substituent which, on oxidation, suffers rapid degradation to a carboxylic acid group. ... 

Free hydroxyl groups on the terminal nucleosides in deoxyribopolynucleotides can be acetylated preferentially and selectively, without affecting the amino groups in the pyrimidine and purine rings, with acetic anhydride in aq. medium while a pH of 7 is maintained by addition of 4 N NaOH. E. s. A, Stuart and H. G. Khorana, Am. Soc. 85, 2346 (1963). 

Crystallization Method. Such methods as mechanical separation, preferential crystallisation, and substitution crystallisation procedures are included in this category. The preferential crystallisation method is the most popular. The general procedure is to inoculate a saturated solution of the racemic mixture with a seed of the desired enantiomer. Resolutions by this method have been reported for histidine (43), glutamic acid (44), DOPA (45), threonine (46), A/-acetyl phenylalanine (47), and others. In the case of glutamic acid, the method had been used for industrial manufacture (48). 

Nitration of 4-(2-thienyl)- (301) and 4-(3-thienyl)-pyrazoles (302) mainly occurs on the thiophene ring, but when acetyl nitrate is used as the nitration agent small quantities of products nitrated on the pyrazole ring are isolated (position of the nitro group uncertain) (80CS( 15)102). Pyrazol-l -ylpyridines (303) undergo electrophilic reactions (bromination, chlorination and nitration) preferentially in the pyrazole ring. Thus, the nitration of (303 R = R = = H) either with a mixture of nitric acid and sulfuric acid at 10-15 °C or with... 

In the case of indazoles the reaction of indazole, 5-nitroindazole or 6-nitroindazole with glycosyl halides and mercury(n) cyanide gives exclusively 2-glycosylindazoles (670), (673) and (675) (7QJHC1435). Similarly, the reaction of 1-trimethylsilyl derivatives of indazole, 3-cyanoindazole, 4-nitroindazole, 5-nitroindazole and 6-nitroindazole with 2,3,5-tri-O-acetyl-D-ribofuranosyl bromide gives only, or preferentially, the 2-ribofuranosyl derivatives (670)-(674) 7QJHC117, 70JHC1329). 

The acid-catalyzed additions of bromide and chloride ion to thiiranes occurs readily, with halide preferentially but not exclusively attacking the most substituted carbon atom of the thiirane. The reaction of 1-substituted thiiranes with acetyl chloride shows a slight preference for halide attack at the less substituted carbon atom (80MI50601). For further discussion of electrophilic catalysis of halide ion attack see Section 5.06.3.3.2. The reaction of halogens with thiiranes involves electrophilic attack on sulfur (Section 5.06.3.3.6) followed by nucleophilic attack of halide ion on carbon. 

Lead tetraacetate fragmentation has not been applied to the 20-hydroxy-18, 20-cyclo steroids. However, preferential cleavage of the 17,20-bond would be expected, as was observed in the chromic acid oxidation of a saturated 20-hydroxy-18,20-cyclo steroid in hot acetic acid which affords the 18-acetyl-17-ketone in 50-60% yield. 

Acylation of l,2,3,4-tetrahydro-j8-carboline derivatives takes place preferentially at the pyr-N. Thus l,2,3,4-tetrahydro-j8-carboline yields a 2-formyl and a 2-acetyl derivative, which give 2-methyl- ... 

Due to their thermal instability, this method cannot be applied to the preparation of benzo-thiepins. Although the ft-oxo sulfoxide moiety in precursors such as 5-methoxy-4-phenyl-l-benzothiepin-3(2/7)-one 1-oxide makes them candidates for a Pummerer reaction, treatment with acetic anhydride and triethylamine at - 30 C results in preferential enol acetylation to afford the corresponding 1-benzothiepin 1-oxide.14... 

In addition protein domains have been identified which bind to modified histone tails. The so-called bromodomains bind to acetylated histone tail, but have little or no affinity to unmodified tails. Further known binding domains include chromodomains and SANT domains which possess preferential binding to methylated and unmodified tails. 

The Pummerer reaction346 of conformationally rigid 4-aryl-substituted thiane oxides with acetic anhydride was either stereoselective or stereospecific, and the rearrangement is mainly intermolecular, while the rate-determining step appears to be the E2 1,2-elimination of acetic acid from the acetoxysulfonium intermediates formed in the initial acetylation of the sulfoxide. The thermodynamically controlled product is the axial acetoxy isomer, while the kinetically controlled product is the equatorial isomer that is preferentially formed due to the facile access of the acetate to the equatorial position347. The overall mechanism is illustrated in equation 129. 

The aldehyde group of laevulinic aldehyde reacted preferentially with the ester phosphorane (38), while the 2-acetyl groups of the benzofurans (39) were selectively methylenated with methylenetriphenylphosphorane. The aldehyde group was protected as the dimethylacetal in the synthesis of the steroidal a-methylene-aldehyde (40). 

As substrates, pyrrolotetrazoles 12 and 13 have been used in a variety of electrophilic substitutions. It has been observed that with the exception of bromination, monosubstitution (acetylation, benzoylation, carbamoylation, formylation, azo coupling, nitrosation, and reaction with dimethyl acetylenedicarboxylate (DMAD)) occurs preferentially at C-5, if the 5- and 7-positions are both available. Upon bromination, double substitution occurs at C-5 and C-7 with the same substrates. It has further been observed that substitution at C-7 occurs only if C-5 is occupied <2001J(P1)729>. 

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