Basic properties ring substitution

The electrophilic properties of substituted 6//-l,3-thiazine-6-ones (208) in solution also shows reactivity at C-2 in acidic conditions and at C-6 in basic conditions. The regioselective reaction of 4-ethoxycarbonyl-2-phenyl-6//-l,3-thiazine-6-one with dimethylamine leads, after ring opening and reclosure, to two diasteriomeric 5-dimethylcarboxamido-4-ethoxy-carbonyl-2-phenyl-A2-thiazolines 209 and 210, whose structures were confirmed by X-ray diffraction studies on 210 (Scheme 84) (88BSF897). Com-... 

Furthermore, the strongly metallic character of selenium weakens the C-Se bond and thus favors reactions involving opening of the ring. The basicity of the three heterocycles is approximately in the same order, the nitrogen atom of selenazole and thiazole possessing much the same properties as the heteroatom of pyridine. Of the two carbon atoms ortho to nitrogen, that is, the 2-carbon and the 4-carbon, only the one in the 2-position is fairly active as a result of its interaction with selenium or sulfur. The 4- and 5-positions of thiazole and selenazole are more susceptible to electrophilic substitution than the 3- and 5-positions of pyridine. This is particularly true of the 5-position of selenazole. Thus it can be said that the 2- and 5-positions of the selenazoles and thiazoles... 

Because acid-base properties on the inside of molecules will be discussed, those macrocycles (and related compounds) which possess functional groups hanging on the outside of the ring, e.g. lariat ethers or substituted polyazamacrocyles (Kaden, 1984 Gokel, 1991, 1992), will be mentioned only briefly. From the large group of azamacrocycles only a small fraction will be discussed in detail because the basic functionality rarely is located in a defined i/j-position. 

Besides the applications of the electrophilicity index mentioned in the review article [40], following recent applications and developments have been observed, including relationship between basicity and nucleophilicity [64], 3D-quantitative structure activity analysis [65], Quantitative Structure-Toxicity Relationship (QSTR) [66], redox potential [67,68], Woodward-Hoffmann rules [69], Michael-type reactions [70], Sn2 reactions [71], multiphilic descriptions [72], etc. Molecular systems include silylenes [73], heterocyclohexanones [74], pyrido-di-indoles [65], bipyridine [75], aromatic and heterocyclic sulfonamides [76], substituted nitrenes and phosphi-nidenes [77], first-row transition metal ions [67], triruthenium ring core structures [78], benzhydryl derivatives [79], multivalent superatoms [80], nitrobenzodifuroxan [70], dialkylpyridinium ions [81], dioxins [82], arsenosugars and thioarsenicals [83], dynamic properties of clusters and nanostructures [84], porphyrin compounds [85-87], and so on. 

The literature of the xanthenes provides many examples of compounds with various substituted aromatic nuclei at C-9, and other examples with different substituents on the xanthene ring. The basic skeleton, however, has not been modified in recent years. It is apparent that the crowded aromatic subunit will slow reactions involving the formation of intermediates involving reaction at C-9 and may retard the subsequent reactions of the intermediates as well. This observation predicts slower rates of electron transfer, for example, and subsequent photoreduction than would be expected for unsubstituted xanthenes. Our interest was to synthesize a new series of dyes without substituents at that position and to investigate the effect of C-9 substitution on photochemical and photophysical properties. 

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