Homocycle

The first three of a series of papers by Ridd and co-workers on Inductive and Field effects in Aromatic Substitution have appeared. Results of studies of the nitration of 4-phenylp5nidine and of 4-benzylpyridine in aqueous sulphuric acid were reported and use of the usual criteria (para 8.2) showed that in each case the conjugate acid was the species undergoing nitration. The values of where fm refers to the corresponding homocyclic compound (biphenyl or diphenylmethane) when plotted against r, the distance between the... 

Reclosure to form a new heterocyclic or homocyclic ring can occur in azolium ions carrying suitable substituents these reactions are considered under the appropriate substituents. 

Having its pyrazolic 4-position substituted, electrophilic attack on indazoles takes place in the 3-position and in the homocycle (the 5- and 7-positions). The condensation of a benzene ring results in a decrease of the aromaticity of the pyrazole moiety, as in naphthalene compared to benzene, and therefore basic ring cleavage is easier in indazoles than in pyrazoles (Section 4.04.2.1.7(v)). 

The basicities of indazole (1.31), 1-methyl (0.42) and 2-methyl (2.02) derivatives and of eight other substituted indazoles have been measured (67BSF261 The effect of substituents in the 3-position is similar in pyrazoles and indazoles with Api values as follows Me, 0.80 and 0.86 Cl, -3.01 and -2.98 Br, -2.85 and -2.82, respectively. A nitro group in the homocycle has an expected base-weakening effect of -2 pK units, whether it is at the 5- or the 6-position. 

In the section dealing with electrophilic attack at carbon some results on indazole homocyclic reactivity were presented nitration at position 5 (Section 4.04.2.1.4(ii)), sulfon-ation at position 7 (Section 4.04.2.1.4(iii)) and bromination at positions 5 and 7 (Section 4.04.2.1.4(v)). The orientation depends on the nature (cationic, neutral or anionic) of the indazole. Protonation, for instance, deactivates the heterocycle and directs the attack towards the fused benzene ring. A careful study of the nitration of indazoles at positions 2, 3, 5 or 7 has been published by Habraken (7UOC3084) who described the synthesis of several dinitroindazoles (5,7 5,6 3,5 3,6 3,4 3,7). The kinetics of the nitration of indazole to form the 5-nitro derivative have been determined (72JCS(P2)632). The rate profile at acidities below 90% sulfuric acid shows that the reaction involves the conjugate acid of indazole. 

It would be anticipated that 1,2-benzisoxazoles undergo electrophilic substitution in the homocyclic ring, and this has been borne out by experiment. The absence or presence of substituents in the 3-position has no influence on the course of the reaction. In qualitative terms it would also be expected that substitution would occur in the 5- or 7-position (see Section 4.16.2.1.1) but most electrophilic substitutions take place at the 5-position. 

As would be anticipated, amino groups in the homocyclic ring of 1,2-benzisoxazoles behave as normal aromatic amines, forming mono- and bis-acyl derivatives, etc. (67AHC(8)277,p. 296). In th e isomeric 2,1-benzisoxazoles the 3-amino compound exists as such and not in the tautomeric 3-imino form (65CB1562). Amino groups in 3-phenyl substituents behave as normal aromatic amines (67AHC(8)277,p. 331). 

Nitro groups in the homocyclic ring of 1,2-benzisoxazoles have been reduced to the corresponding amino groups without opening of the isoxazole nucleus. SnCU/HCl and Adam s catalyst/H2 have both proved effective in this respect. Similar behavior was observed for nitro groups in the isomeric 2,1-benzisoxazoles (67AHC(8)277,pp.295,33l). 

Conversion of 2-cyano-5,5-dimethyl-3-ethoxy-2-cyclohexene (601) into (602) by hydroxylamine has been accomplished (73MI41604). Reduction of the homocyclic ring in the 3-substituted 2,1-benzisoxazole occurred with H2/Pd-C to give the tetrahydro analogs (equation 65) (74JHC395). 

Rochelmeyer, after oxidation experiments on acetylsolanidine with chromic acid, concluded that the heterocyclic residue could not be attached to the homocyclic portion of the molecule at one point only as in formula (II). The first positive evidence as to the nature of the heterocyclic residue was provided by Prelog and Szpilfogel, ° > who investigated... 

Conjugated dienamines were found to give predominantly double four-membered-ring adducts as well as a small amount of the six-membered-ring adduct (466,468). This important result indicates preferred attack at the terminal carbon of the dienamine system (in contrast to alkylation, for instance) in the generation of an initial zwitterionic intermediate. Addition of sulfonyl chloride and triethylamine to a homocyclic dienamine gave only the bridged product (446). 

Combination of these various methods has led to a vast number of derivatives in which H is progressively replaced by one or more monofunctional group such as F, Cl, Br, I, CN, R, Ar, OR, SH, SR, NH2, NR2, etc. The cyclic silanes SisHm and Si6Hi2 were prepared in the late 1970s via (SiPh) which were themselves the first known homocyclic silane derivatives (F. S. Kipping, 1921) ... 

Another fertile area of current interest is the synthesis of stable homocyclic polysilane derivatives.Typical examples are cyclo-(SiMe2)7, > (cyclo-Si5Me9)-(SiMe2) -(cyclo-SisMeg), n = 2-5, and several new permethy-lated polycylic silanes such as the colourless crystalline compounds bicyclo[3.2.1]-Si8Mei4 (mp 245°), bicyclo[3.3.1]-Si9Mei6 (mp >330°) and... 

We now consider other homocyclic polymorphs of sulfur containing 6 - 20 S atoms per ring. A rhombohedral form, t-sulfur, was first prepared by M. R. Engel in 1891 by the reaction of concentrated HCl on a saturated solution of thiosulfate HS2O3 at 0°. It was shown to be... 

Ether groups in the benzene ring of quinazoline behave as in ethers of homocyclic aromatic compounds, e.g., they can be demethylated with anhydrous aluminum chloride. Allyl ethers also undergo a Claisen rearrangement/ ... 

Amides, prepared by condensation of tryptamine or substituted tryptamines with a large number of aliphatic, homocyclic, aromatic,and heterocyclic acids, have been used in the reaction. In few cases only did ring closure fail. ... 

The application of the Hammett equation to bicyclic aromatic compounds of the quinoline and isoquinoline type may be envisaged in several ways. In quinoline, e.g., the homocyclic ring A in 9) may be... 

Both of these approaches have been attempted, and both are substantially equivalent for heterocyclic (e.g. quinoline and isoquinoline) and homocyclic (naphthalene) systems. Consequently, in the subsequent discussion it is fruitful to include the available work on naphthalene derivatives. In the case of the fused six-membered rings, Eq. (3) is not applied because it does not permit treatment of the 5- and 8-positions, and the available series as a whole are too short to make this treatment useful. 

Some investigations have been inspired by another special circumstance concerning the structure of the fundamental heteroaromatic rings like the parent aromatic homocyclic hydrocarbons, these structures are readily amenable to theoretical treatment by the approximation methods of quantum mechanics. Quantitative studies are clearly desirable in this connection for a reliable test of the theory and, indeed, they have been utilized to this end. ... 

The above examples indicate a general behavior which roughly parallels that found in homocyclic aromatic systems. The less usual order, I >Br >C1, found for the non-activated halogenofurans is reminiscent of the similar order I > Br > Cl > F, observed by Tronov and Kruger for the halogenobenzenes. Again, it would be of interest to establish the position of fluorine in the order of reactivity of the halogenofurans. 

There has been relatively little work on the mechanism of arylation of heterocyclic compounds, but the arylation of homocyclic compounds has been studied in great detail. Those mechanistic aspects of the reactions with heterocycles which have been examined parallel closely the corresponding aspects of the reactions of homocycles, and there is... 

Szwarc and his co-workers have measured the methyl affinities (that is, reactivities toward methyl radicals compared with that of benzene) of a number of heterocyclic compounds. These, together with the methyl affinities of some homocyclic compounds, are set out in Table IX. 

The mechanism of hydroxylation of heterocyclic compound has not been studied, but the mechanistic aspects of hydroxylation of homocyclic compounds may again serve as a basis for discussion. 

Homo-brenzcatechln, n. homopyrocatechol. -campher, m. homocamphor. -campher-saure, /. homocamphoric acid, homo-chrom, a. of uniform color, homochro-mous (Dyeing) monogenetic. -cyklisch a. homocyclic. -edrisch, a. (Cryst.) homo-hedral. 

Cyclic Hydrocarbons. These are structures in which the carbon atoms form a ring instead of an open chain. They are also called carbocyclic or homocyclic compounds. They are divided into two classes alicyclic (or cycloaliphatic) and aromatic compounds. 

Unlike carbocyclic (or homocyclic) hydrocarbons which have been discussed previously, heterocyclic compounds contain in their rings other atoms, such as nitrogen, oxygen, or sulfur, in addition to carbon. Some examples are shown below ... 

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