Substituents ring-fused

Oxazoles give acylamino ketones (158) by acid-catalyzed ring scission, although they are somewhat more stable than furans. The oxazole ring is also moderately stable to alkali (74AHCU7)99) as expected, reaction with hydroxide ions is facilitated by electron-with-drawing substituents and fused benzene rings. 

Primary aromatic amines (e.g., aniline) and secondary aliphatic-aromatic amines (e. g., 7V-methylaniline) usually form triazenes in coupling reactions with benzenedi-azonium salts. If the nucleophilicity of the aryl residue is increased by addition of substituents or fused rings, as in 3-methylaniline and 1- and 2-naphthylamine, aminoazo formation takes place (C-coupling). However, the possibility has also been noted that in aminoazo formation the initial attack of the diazonium ion may still be at the amine N-atom, but the aN-complex might rearrange too rapidly to allow its identification (Beranek and Vecera, 1970). 

The hetero-Diels-Alder reaction of homochiral camphor-derived thiabutadienes has given, for the first time, optically active bornene ring-fused dihydrothiopyrans with high diastereoselectivity (Scheme 53) <2000H(53)1685>. The exo endo- t2X o varied with the nature of substituent, varying from 91 9 (X = CH2, R = R = H) to 1 99 (X = CO,... 

Substituents on fused benzene rings undergo the usual reactions expected in the benzene series. The orientation in the diazo coupling reactions with hydroxy compounds (300) and (301) indicates that there is little bond fixation in dibenzofuran, a distinct contrast to naphthalene. 

Similarly, the large a-CH couplings of pyridine, quinoline, and isoquinoline can be used to locate substituents or fused rings in these heterocycles [73 d, i], N-Oxidation and protonation further enhance Jcu of the a-CH bond in pyridine (Table 4.68) due to the positive charge at nitrogen. To conclude, the sites of oxidation and protonation can be derived from the magnitudes of JC I in pyridines, azines, and their fused derivatives [467]. 

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