4-pyrones, Nazarov cyclization

Trimethylsilyl triflate (5 equiv.) also converts 3-ethoxycarbonyltetrahydro-y-pyrones (3) into cyclopentenone esters (4) by ring opening followed by Nazarov cyclization. 

SCHEME 19.36. Photochemical Nazarov cyclization of a 4-pyrone in the presence of add. 

A synthesis of cross-conjugated 2-cyclopenten-l-ones from dialkenyl ketones is readily induced by TMSOTf (eq 73). A strong fluorine-directing effect has been observed for such Nazarov-type cycUzation, as mixtures of products have been observed for nonfluorinated dialkenyl ketones. The addition of 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) as a cosolvent dramatically accelerates the cyclization. Other acids such as BF3-OEt2, FeCls, polyphosphoric acid, or TiOH are less effective while neither TMSI nor TMSOMe promote this cyclization at all in CH2CI2. 3-Ethoxycarbonyltetrahydro-)/-pyrones also undergo such Nazarov-type cyclization. ... 

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