Hetero ring-forming

The hetero ring in 4-(l -hydroxyalkylidene)-5-oxazolones is cleaved by alcoholic HCl to form alkyl a-acylaminoacylacetates, which cyclize to oxazole-4-carboxylates [Eq. (25)]. This rearrangement occurs directly in alkali, and a carbon-14 tracer study has substantiated a mechanism involving ring opening followed by the alternative ring closure. ... 

Permanganate oxidation of isoquinoline carrying benzo methoxy groups, or a 5-iodo-or 5-carboxy group, gives hetero ring fission whereas when a 5-amino group is present the pyridine-3,4-dicarboxylic acid is formed. 

The amino structure for 9-aminoacridine (77) has been confirmed by IR studies (65JCS5230), and NMR spectral data in DMSO are consistent with the diamino structures (78) and (79) (64JCS1423). Amino groups on fused benzene rings and (3 to nitrogen in hetero rings exist in the amino form, and the reactions of these compounds are similar to those of aniline and the naphthylamines. Aminopyridine JV-oxides exist very predominantly in the amino form (57JCS4375). 

The temperature of decomposition depends also on steric factors The spiro compound 196 c with the five-membered dithiaphospha ring is much more stable than the spiro compound 196 d with the six-membered hetero ring. Only in the latter compound can a stable 1.2-dithia-cycloalkane be formed by radical cleavage Once again the preparative behaviour is in accord with the results of mass spectroscopy (p. 112). 

Methylpyridinium ions (181) react reversibly with hydroxide to form a small proportion of the pseudo-base (182). The term pseudo is used to designate bases that react with acids measurably slowly, not instantaneously as for normal acid-base reactions. Fused benzene rings reduce the loss of resonance energy when the hetero ring loses its aromaticity and hence pseudo-bases are formed somewhat more readily by 1-methylquinolinium, 2-methylisoquinolinium and 10-methylphenan-thridinium, and much more readily by 10-methylacridinium ions. Pseudo-bases carrying the hydroxy group in the a-position are usually formed preferentially, but acridinium ions react at the y-position. 

If the hetero ring is in the form of a pyridone, pyrone or A-oxide, or contains a strongly electron-donating substituent (OR or NR2), electrophilic substitution into the hetero ring can compete with substitution into a fused benzene ring. In some such compounds, substitution occurs entirely in the... 

Ozonolysis of benzo[6]thiophene results in rupture of the hetero ring the aldehydes (154) and (155) are formed (74TL4069). 

The reaction of N-containing nucleophiles such as hydroxylamine, hydrazine and its derivatives, with pyran-4-ones and their benzo derivatives has been studied intensively. The early workers assumed that oximes and hydrazones were formed in these reactions, but it was later realized that under normal conditions the pyrans rarely yield the expected derivatives because of the susceptibility of the hetero ring to the nucleophile. This is especially true when the pH of the medium is above 7 those reactions which proceed under acidic conditions usually lead to the expected derivative (77T3183). 

Cleavage of the hetero ring of 3-nitrobenzo[A]thiophene is the first step in a synthesis of ( )-2-aryl-l-[2-(methylsul-fonyl)phenyl]-l-nitroethenes (Scheme 176). The sulfonyl-stabilized anion derived from these alkenes by metallation adds in Michael fashion to the nitrovinyl unit to form 4-nitrothiochroman 1,1-dioxides as an inseparable mixture of two diastereomers. The overall process corresponds to expansion of a thiophene ring to a thiopyran unit <2004T4967>. 

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