Alkyl halogens

Reactive alkyl halogen compounds can be used to modify specifically hydroxyl groups in carbohydrates, polymers, and other molecules. Chloro- or bromo-derivatives of short alkyl chains 

Modification of hydroxyl groups with such compounds can be done in 3-10 M NaOH by reacting from 25°C to 40°C for 1.5-4 hours. 

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Pyrimidine, 5-acetyI-2,4-dimethyI-synthesis, 3, 125 Pyrimidine, acylamino-deaeylation, 3, 85 Pyrimidine, alkoxy-hydrolysis, 3, 91 Primary Synthesis, 3, 134 synthesis, 3, 132, 134 Pyrimidine, 2-alkoxy-aminolysis, 3, 92 rearrangement, 3, 92, 135 synthesis, 3, 134 transalkoxylation, 3, 92 Pyrimidine, 4-alkoxy-aminolysis, 3, 92 rearrangement, 3, 92, 135 synthesis, 3, 134 transalkoxylation, 3, 92 Pyrimidine, 6-alkoxy-aminolysis, 3, 92 rearrangement, 3, 92, 135 synthesis, 3, 134 transalkoxylation, 3, 92 Pyrimidine, alkyl-halogenation, 3, 76 nitration, 3, 77 oxidation, 3, 76 synthesis, 3, 124... 

Thiophenes substituted with groups such as alkyl, halogens, OCH3, and SCH3 show small but characteristic differences between 2- and 3-substituted compounds. In these cases, however, it is the 2-isomer which shows the less complex spectrum. Thus, 2-substituted alkylthio-phenes and halothiophenes show a single band with greater extinction than the 3-isomers whose spectra exhibit two peaks in a broadened absorption band. These differences are also present in the spectra of 2,5- and 3,4-dihalosubstituted compounds. In 2-substituted thiophenes, the intensity of the band varies inversely as the electronega-... 

R, R, R = H, lower alkyl, halogen, CF3, aryl, alkylai-yl, amino, alkylamino, AcNH, CN X,... 

Another approach uses reactive alkyl halogen compounds containing a terminal carboxylate group on the other end to form spacer arms off the dextran polymer from each available hydroxyl. In this manner, Brunswick et al. (1988) used chloroacetic acid to modify the hydroxyl groups to form the carboxymethyl derivative. The carboxylates then were aminated with ethylene diamine to create an amine-terminal derivative (Inman, 1985). Finally, the amines were modified with iodoacetate to form a sulfhydryl-reactive polymer (Figure 25.14). 

Usually, the formation of a new chiral centre involves the conversion of a prochiral sp carbon atom into one with sp hybridisation, the methods most generally used being the aldol and related condensations, pericyclic reactions (especially the Diels-Alder reaction), epoxidation, cyclopropanation and additions to double bonds (hydrogenation and hydroboration). Another possibility is the conversion of a prochiral sp carbon atom into a chiral centre, as for instance in the a-substitution (alkylation, halogenation, etc.) of a ketone. 

The utilization of a-amino acids and their derived 6-araino alcohols in asymmetric synthesis has been extensive. A number of procedures have been reported for the reduction of a variety of amino acid derivatives however, the direct reduction of a-am1no acids with borane has proven to be exceptionally convenient for laboratory-scale reactions. These reductions characteristically proceed in high yield with no perceptible racemization. The resulting p-amino alcohols can, in turn, be transformed into oxazolidinones, which have proven to be versatile chiral auxiliaries. Besides the highly diastereoselective aldol addition reactions, enolates of N-acyl oxazolidinones have been used in conjunction with asymmetric alkylations, halogenations, hydroxylations, acylations, and azide transfer processes, all of which proceed with excellent levels of stereoselectivity. 

Figure 9.12 Plot of log Ki0Q versus log Kiow for a alkylated and ha-logenated (R, = alkyl, halogen) phenylureas (R2 = R3 = H A, halogen, see margin below), phe-nyl-methylureas (R2 = CH3, R3 = H, d), and phenyl-dimethylureas (R2 = R3 = CH3, ). The slope and intercept of the linear regression using all the data is given in Table 9.2 (Eq. 9-26i) each subset of ureas would yield a tighter correlation if considered alone (e.g., Eq. 9-26j).

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