Pyrans functional group transformations

Numerous preparative investigations have been devoted to the conversions of 4/f-pyrans to various heterocyclic systems. We discuss here only those transformations in which the pyran ring is not conserved. Other cases will be considered as functional-group transformations and discussed in Section V.J. Some conversions of pyrans to heterocyclic compounds have also been mentioned in Sections V.A.3, V.C.3, and V.C.4. 

A number of functional-group transformations have been accomplished in substituted 4//-pyrans. Simple conversions and those leading to heterocyclic-ring closures will be discussed separately. 

Treatment of 6-ethoxy-2-phenyl-3-carboxy-5,6-dihydro-4/7-pyran with hydroxylamine transforms the heterocycle to 2-phenylnicotinate <97JHC93>. However, an isoxazole is obtained if the pyran is substituted with an acyl group rather than the ester functionality. Nitriles continue to be key components in pyridine synthesis. The bicyclic ketal 8 is converted cleanly to the pyridine 9 in the presence of an alkyl nitrile using 5 equiv of TMSOMs and 1 equiv of BFrEt20. This combination, generating the active ingredient boron difluoromethanesulfonate, avoids the byproduct cyclohexanones <97JHC325>. 

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