Chromone ring recyclization

The pyran ring of chromones (425) is cleaved by reaction with primary or secondary amines to yield enamines (426) which may be recyclized by treatment with acid (73MI22305, 72CHE(8)416, 65JCS3610). 

The rearrangements of phenols which are accompanied by hydroxy group transpositions are called the Wessely-Moser reaction (equations 50 and 51). In essence, these rearrangements are recyclizations of flavonoides 114 via the ring-opened form 115 to give the novel structures 116. Compounds that can participate in these rearrangements are flavones (114, R = H, R = Aryl), flavonoles (114, R = OH, R = Aryl), isoflavones (114, R = Aryl, R = H), chromones (114, R = H, R = Alkyl), chromonoles (114, R = OH, R = Alkyl), xanthones (114, R R = benzo) as well as benzopyrylium salts (e.g. see Reference 95). 

Chromones behave similarly towards both hydroxylamine and hydrazines. Ring opening via C-2 followed by recyclization giving azoles is of preparative interest. This is demonstrated by the reaction of chromone with phenylhydrazine, which leads either to the phenylhydrazone 5 or (via the enehydrazine 4) to the pyrazole 6 ... 

The reversible ring-opening of the chromone system giving o-hydroxyphenyl-1,3-diketones 2 (see above) can also occur by acid catalysis. If a fiirther OH function is present in position 5, it also takes part in the recyclization. Chromones and especially flavones with an unsymmetrically substituted benzene ring undergo isomerization by this route when treated with strong acids (Wesseley-Moser rearrangement) e.g. 7 8 ... 

Chromone derivatives and their analogues 704 and 705 were used for synthesis of 4-fluoroalkylpyrimidines. In both cases, reaction with NCN binucleophiles was accompanied with recyclization of the y-(thia)pyrone ring to give (2-(thio) hydroxyphenyl)-substituted pyrimidines or their analogues (Table 37). 

---