Ortho attack

Using a strong acid, 1,1,3,3-tetraphenylallene afforded 1,1,3-triphenylindene by an electrophilic ortho attack of the intermediate allyl cation [7]. 

Regioselectivity in Friedel-Crafts acylations can be quite sensitive to the reaction solvent and other procedural variables.45 In general, para attack predominates for alkylbenzenes.46 The percentage of ortho attack increases with the electrophilicity of the acylium ion, and as much as 50% ortho product is observed with the formylium and... 

Irradiation of thiones with 589-nm light in the presence of electron-deficient olefins produced 99 and 100. By-product 89 is the result of an ortho attack on the benzene ring. 

The obvious way to obtain this information is to carry out reactions with various Z groups and to analyze the products for percent ortho, meta, and para isomers, as has so often been done for electrophilic substitution. However, this procedure is much less accurate in the case of free-radical substitutions because of the many side reactions. It may be, for example, that in a given case the ortho position is more reactive than the para, but the intermediate from the para attack may go on to product while that from ortho attack gives a side reaction. In such a case, analysis of the three products does not give a true picture of which position... 

The combination of a metal phenoxide and a Lewis acid facilitates alkylation of the phenolic moiety by 1,1,3,3-tetramethoxypropane and promotes specific ortho attack. Spontaneous cyclization leads to a mixture of two stereoisomers of 2,4-diethoxychroman (81JHC1325). 

The nature of the substituents and, especially, the connection between simple benzylated isoquinolines and the nature of the cyclized products of ortho attacks, are the heart and substance of this review book. 

The "alphabet" used in this structural index is totally indifferent to the capricious and arbitrary rules laid down by the Chemical Abstracts. Quite simply, it is based on the location of the substituents and their identity in the nuclear isoquinoline skeleton before it is distorted by a hypothetical "ortho attack." The definition of this "atomic" alphabet is the substance of this introduction. The nature and variety of this "ortho attack" is addressed here as well. 

This family is classified in this collection as an ortho-attack on a 1,1-disubstituted tetrahydroisoquinoline where there is a methyl, or some other group (an ortho (2,1-XX) attack). 

This family is viewed as an ortho-attack on the 8-position of the isoquinoline ring. This produces a four-ring system known as an aporphine. 

The prefix "seco" is an unusual term occasionally encountered in the literature of natural products. Just as the term "ortho-attack" indicates the generation of a new ring, the term "seco" indicates the destruction of a ring. A secoisoquinoline is formed from a 1-substituted tetrahydroisoquinoline by the loss of the 1,2-bond. Transferring a hydrogen atom from the a-carbon to the nitrogen, and reshuffling the electrons, results in the formation of a new double bond. 

With the simpler 1-benzyl derivatives (those which have not undergone any ortho-attack), the removal of the 1,2-bond usually produces a 2-styryl substituted phenethylamine. Again, this would be located in the entry that described the parent isoquinoline. 

In general a phenol will undergo direct carboxylation of the nucleus when the dry sodium salt is heated under pressure with carbon dioxide (the Kolbe-Schmidt reaction). Addition of the weakly electrophilic carbon dioxide is promoted by electron release from the oxyanionic site. With phenol itself the ultimate product is salicylic acid (o-hydroxybenzoic acid) predominantly ortho attack may be attributable to stabilisation of the transition state through chelation. 

These results seem to indicate that there are two pathways leading to the nitro products for the 2- and 4-phenylpyrimidines. Nitration of the free base, or of the protonated heterocycle, leads to meta nitration. However, if an N-nitro intermediate is formed, which can undergo intramolecular nitration in a solvent cage, then ortho attack results (Fig. 12). 

Consider the nitration of toluene (Following fig.). The amount of meta substitution is very small as expected and there is a preference for the ortho and para products. The formation of more ortho substitution compared to para substitution is due to the fact that there are two ortho sites on the molecule to one para site and so there is double the chance of ortho attack to para attack. Based on pure statistics it would be expected that the ratio of ortho to para attack to be 2 1. In fact, the ratio is closer to 1.5 1. In other words, there is less ortho substitution than expected. This is because the ortho sites are immediately next door to the methyl substituents and the size of the substituent tends to inference with ortho attack— a steric effect. The significance of the steric effect will vary according... 

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