Substituents s. a. Position

Substituents s. a. Position shift —, angular, stereospecific introduction 14, 754 Substitution (s. a. Addition, substituting. Replacement)... 

The position of the BII maximum, in flavones, is affected by the introduction of additional substituent(s) at position(s) 6 and/or 8 which, in general, produce a bathochromic shift placing this maximum above 270 nm (Voirin, 1983). This is the case of xanthomicrol (5,4-dihydroxy-6,7,8-trimethoxyflavone) (Figure 7.4). 

Substituents in the pyrrole ring affected drastically the ratio of pyrrole-1- to pyrrole-2-carbodithioate isomers. With unsubstituted pyrrole, only the 1-isomer 135 (R = R = R = H, R = Et) was formed (63%) and practically no 2-isomer was detected. When just one methyl group was introduced into pyrrole s a-position, the pyrrole-2-carbo-dithioates 134 became the only product (46%) (Equation (38)). Any combinations of alkyl substituents on the pyrrole ring selectively gave pyrrole-2-carbodithioates 134 in a yield of up to 71%. With an aryl substituent at the a-position, along with the major pyrrole-2-carbodithio-ates 134 (44-59%), the 1-isomers 135 were also formed in 24—33% yields (Equation (38)). 

Nucleophilic aromatic substitution occurs only if the aromatic ring has an electron-withdrawing substituent in a position ortho or para to the halogen. The more such substituents there are, the faster the reaction goes. As shown in Figure 16.19, only ortho and para electron-withdrawing substituents can stabilize the anion intermediate through resonance a meta substituent offers no such resonance stabilization. Thu.s, p-chloronitroben-zene and o-chloronitrobenzene react with hydroxide ion at ISO C to yield substitution products, but m-chloronitrobenzene is inert to OH". 

When common names are used, the Greek letters a, p, y, S, and so forth, are often added as a prefix to locate substituents. The a-position in a carboxylic acid is next to the carboxyl group an a-substituent in a common name is equivalent to a 2-substituent in an lUPAC name. GABA is an inhibitory neurotransmitter in the central nervous system of humans. Alanine is one of the 20 protein-derived amino acids. 

The contribution of the mesomeric field effect can be estimated by combining equations (4.134) and (4.140). The charge produced at position j by our substituent S at position i is, as we have seen, given by - charge exerts a field effect given by equation (4.134). The total mesomeric field effect... 

The most studied approaches to ring closure reactions is the S j l substitution of aromatic substrates that have an appropriate substituent in a position ortho to the leaving group. An important example of this approach is the synthesis of substituted indoles by the photoinduced substitution reaction of o-haloanilines (bromo, iodo) with carbanions from ketones, followed by spontaneous ring closure in the reaction media ... 

Both compounds react by an S l mechanism and their relative rates reflect their acti vation energies for carbocation formation Because the allylic chloride is more reactive we reason that it ionizes more rapidly because it forms a more stable carbocation Struc turally the two carbocations differ m that the allylic carbocation has a vinyl substituent on Its positively charged carbon m place of one of the methyl groups of tert butyl cation... 

Tetiasubstituted anthiaquinones give a slightly reddish blue tint to greenish blue color depending on the substituents and their positions, eg, 1,4,5,S-tetraaminoanthraquinone is blue green. 

Reactions with strongly basic nucleophiles such as potassium amide in liquid ammonia may prove much more complex than direct substitution. 2-Chloro-4,6,7-triphenylpteridine reacts under these conditions via an S ANRORC mechanism to form 2-amino-4,6,7-triphenylpteridine and the dechlorinated analogue (78TL2021). The attack of the nucleophile exclusively at C-4 is thereby in good accord with the general observation that the presence of a chloro substituent on a carbon position adjacent to a ring nitrogen activates the position meta to the chlorine atom for amide attack. 

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