Aryl ring

This is the branch-poiat differentiatiag phenylalanine (25, R = H) from tyrosiae (25, R = OH). Both phenylalanine and tyrosiae contain an aryl ring, a three-carbon side chain (a Cg—Cg fragment), and a nitrogen. Decarboxylation yields a two-carbon side chain (a Cg—Cg fragment), eg, 2-phenethylamine (59, R = H) from phenylalanine and tyramine (59, R = OH) from tyrosiae, although it is not certain that ia all cases decarboxylation must precede use ia alkaloid constmction. 

Mass spectral fragmentation patterns of alkyl and phenyl hydantoins have been investigated by means of labeling techniques (28—30), and similar studies have also been carried out for thiohydantoins (31,32). In all cases, breakdown of the hydantoin ring occurs by a-ftssion at C-4 with concomitant loss of carbon monoxide and an isocyanate molecule. In the case of aryl derivatives, the ease of formation of Ar—NCO is related to the electronic properties of the aryl ring substituents (33). Mass spectrometry has been used for identification of the phenylthiohydantoin derivatives formed from amino acids during peptide sequence determination by the Edman method (34). 

Boron Bromide. Approximately 30% of BBr produced in the United States is consumed in the manufacture of proprietory pharmaceuticals (qv) (7). BBr is used in the manufacture of isotopicaHy enriched crystalline boron, as a Etiedel-Crafts catalyst in various polymerization, alkylation, and acylation reactions, and in semiconductor doping and etching. Examples of use of BBr as a catalyst include copolymerization of butadiene with olefins (112) polymerization of ethylene and propylene (113), and A/-vinylcarbazole (114) in hydroboration reactions and in tritium labeling of steroids and aryl rings (5). 

It has been shown that aromatic rings of B-ttiaryl-A/-ttiaryl substituted rings are orientated perpendicular to the plane of the bota2iae ring (114,115). If the aryl rings are substituted it is possible to obtain mixtures of cis and trans isomers (atropisomerism) ia which the aryl substituents ate on the same or opposite sides of the plane of the bora2iae ring, respectively (115). 

Ring closures which depend on the conversion of the heteroatom into an electrophile are mostly associated with the formation of thiophene, selenophene and tellurophene rings and some illustrative examples are shown in Scheme 17. The last example which concerns the conversion of reaction with isocyanides is of particular interest since it appears to entail the attack of an electrophilic nitrogen species on the aryl ring. 

Simple aziridines are optically transparent in the UV region of the electromagnetic spectrum (B-69MI50401). In more highly substituted aziridines, such as the 2-aroyl-3-arylaziridines (9), there is an interaction between the carbonyl and aryl ring substituents... 

Indazole-3-carboxylic acid, 1-aryl-ring cleavage, 5, 246... 

The pATr+ values allow for a comparison of the stability of carbocations. The carbocations that can be studied in this way are all relatively stable carbocations. The data in Table 5.1 reveal that electron-releasing substituents on the aryl rings stabilize the carbocation (more positive pA r+) whereas electron-withdrawing groups such as nitro are destabilizing. This is what would be expected from the electron-deficient nature of the carbocation. 

The first reaction can be conducted using various derivatives of methylenemalonic ester, such as malononitriles 7, malonamides 8, P-keto-esters 9 or Meldrum s acid 10. Substitutions of the aryl ring (including fused rings) and within the aryl ring are well tolerated for this reaction. 

Condensation of Af-aryliminochlorides with malonic ester followed by thermal cyclization, as initially reported by Just, was found to be a general method for the preparation of 2, 3, 4-substituted quinolines. Various substituents on the aryl ring of the iminochloride proved uneventful, even though the conditions required to generate the iminochloride utilized PCI5. 

Likewise, diamides, e. g. 90, formed by bis(2-azidobenzoylation) of diamines, suffer ring expansion of both aryl rings to yield the bis(aminocarbonyl) derivatives, e. g. 91. 

For the approach of N2 in the xy-plane of the aryl ring, i. e., side-on addition, o-electron withdrawal by substituents will increase the ai-a interaction. A positive constant pF is therefore expected for addition to 4-substituted aryl cations. 7i-Elec-tron donation by substituents will increase the b2-b2 interaction, for which a negative value of the constant pR is plausible. For the approach of N2 along the z-axis, i.e., end-on addition, the ai-ai interaction should be at least as strong as for the side-on reaction as substantial positive value of pF is therefore expected. The b2-b2 interactions should approximatively cancel. Interactions between the two occupied orbitals should lead to a slight destabilization, but interaction with n should reduce the importance of the b2-b2 interaction relative to the ai-ai interaction, i.e., Pr < Pf -... 

In Gegenwart von Acetanhydrid werden Stilbene bzw. Zimtsaure-Derivate reduktiv in a-Stel lung zum Aryl-Ring acctyliert. Man elektrolysiert in DMF als Losungsmittel an Quecksilber-Kathoden in Gegenwart von Aluminiumoxid ... 

The earlier work on acidolysis of the aryl-tin bond is reviewed in reference (97). Attachment of the proton to the aryl ring is rate-determining, and the Hammett p-factor for the reaction has been shown to... 

Gassman, P.G. Fentiman Jr., A.F. J. Am. Chem. Soc., 1969, 91, 1545 1970, 92, 2549. For a discussion of the use of the tool of increasing electron demand to probe neighboring-group activity by double bonds, sigma bonds, and aryl rings, see Lambert, J.B. Mark,... 

Perhaps it is not unexpected and surprising that there is no /3-aryl participation in the solvolysis of triarylvinyl substrates, as the intermediate vinyl cation is especially stable because of charge delocalization into the a-aryl ring and has no need for extra stabilization. 

Substituted 4-aryl-l,3-dioxolanes are ring opened by TiCU and at low temperatures subsequently recyclise ortho to the activating group on the aryl ring to give 4-hydroxyisochromans. A further stereoselective isomerisation to henzofurans occurs at higher temperatures <96JCS(P1)2241>. 

One additional point should be discussed here, concerning the substantial emphasis that has been placed on the differences between alkyl and aryl isocyanides. It has been suggested, primarily on the basis of infrared evidence, that aryl isocyanides are better 7r-acceptors than alkyl isocyanides (90). Qualitatively this difference is easily rationalized. One can see that delocalization of charge into 7r -orbitals on an aryl ring in aryl isocyanide-metal complexes should be possible, whereas no such possibility exists for alkyl isQcyanide-metal complexes this means that aryl isocyanides should be better ir-acceptors. Of course, the simple qualitative model gives one no measure of the relative importance of this effect. 

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