Benzo-annellation, influence

This generalization will be qualified if the extension of conjugation includes heteroatoms. A pyrido-annelation, for example, should stabilize, overall, more than a benzo-annelation, but its influence on particular orbital energies will depend on the nodal properties of the MOs concerned in relation to the heteroatom. An orbital with a nonzero coefficient at the heteroatom will be stabilized, relative to the carbocyclic analog when the heteroatom is more electronegative than carbon and the substitution is isoelectronic i.e., the heteroatom supplies the same number of electrons to the system as the atom it replaces (see Section II,B,I). 

The search for benzo[c]pyrylium systems and the elaboration of synthetic approaches to its development could lead to the discovery of novel regularities that might be applied in syntheses of different types of heteroaromatic and aromatic compounds. On the other hand, this route could give a possible systematic study of the influence of benzo[c]annelation on reactions of the pyrylium cation. 

The goal of the present review is to demonstrate the wide potential for using newly developed methods to synthesize 2-benzopyrylium salts and to discuss in detail the numerous and unusual transformations of this class of oxygen cations in comparison with monocyclic pyrylium salts, i.e., the study of influence of benzo[c]annelation. 

The l-methylbenzo 7>]furo-, benzo[ ]thieno-, and benzo[ ]selenolo[3,2- ]pyrroles (75), (76), and (77) were prepared <83JHC49> and the influence of annelation and of the heteroatom upon reactivity was tested toward acetylation and lithiation <83JHC6l>. The l-methylbenzo[ ]furo[3,2- ]pyrrole (75) was acetylated at C-2. 

The l-methylbenzo[A]thieno[2,3-/)]pyrrole (149) and benzo[6]selenolo[2,3-6]pyrrole (150) were prepared (83JHC49) and the influence of annelation and of the heteroatom upon the reactivity towards acetylation and lithiation was investigated <83JHC6l>. The sulfur and selenium analogues with [h,2-b (75)-(77) and [2,3-b (149) and (150) annelation were acetylated at C-2 and C-3. No reaction at all was observed with butyllithium except in the case of 1 -methylbenzo[6]selenolo[2,3-/ ]pyrrole (150), where the opening of the selenophene nucleus, after car-boxylation and the action of diazomethane, gave l-methyl-2-methoxycarbonyl-3-(2 -methyl-selenophenyl)pyrrole (151) and the positional isomer (152). 

Reports of [4 + 2] cycloadditions of cyclobutenes bearing electron-withdrawing groups on the double bond have appeared. Predominantly exo products are formed from (709 X = CN, CO2R, or COCl) and cyclopentadiene, whereas (709 X = COjH) gives mainly (88%) the cndo-isomer. This reversal of stereochemical course is rationalized by the existence of an intramolecular H-bond and its influence on the non-bonded interactions in the transition state. A new synthesis of benzo-cyclobutenes employs the condensation of (709 X = COjMe) with butadiene and subsequent oxidative decarboxylation and dehydrogenation. This sequence has also been applied to the synthesis of the annelated benzocyclobutene (710). " ... 

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