4-Pyrones, 2,6-disubstituted, from

Pyridones from 1,3,5 triketones, 46,60 Pyrolysis, diketene to ketene, 46, SO a-PYRONE, 46,101 in Diels Alder reactions, 46, 104 4-Pyrones, 2,6 disubstituted, from 1,3,5 pentanetnones by acid cycliza tion, 46, 61... 

Very recently, the concise synthesis of a range of disubstituted 2-pyrones 96 from (thiophenyl)acetic acids and readily available trifluoromethyl enones via an isothio-nrea mediated one-pot Michael addition/lactonization/thiol elimination sequence has been demonstrated. Derivatization of these reactive pyrones to generate additional high-value products was next investigated and compounds 96a-c were prepared in good yields [39b] (Scheme 31). 

Attempts to substitute the nucleus of 4H-thiopyran-4-one (3) are of interest whereas bromine reacts on y-pyrone (1), to give 3-mono-substituted and 3,5-disubstituted bromopyrones, no bromination of the nucleus occurs with 4//-thiopyran-4-one (3) under a variety of conditions.80 An unstable complex is formed, from which 3 may be regenerated (but see Ref. 90). On nitration of 3 only the pyrylium salt (48, X = N03) is obtained.90 The corresponding hydrochloride (48, X = C1) is also stable and can be sublimed.9... 

The reduction of 3,4-dichlorocyclobutene (222) in the presence of metal carbonyls has been utilized to prepare the parent complex [223, MLn = Cr(CO)4, Mo(CO)3, W(CO)3, Fe(CO)3, Ru(CO)3 orCo2(CO)6] (equation 32) .Morerecently, reaction ofNi(CO)4 with 3,4-dihalocyclobutenes (X = Br or I) or with 222 in the presence of AICI3 produced the corresponding (cyclobutadiene)nickel dihalides . Methodology for the preparation of 1,2- or 1,3-disubstituted (cyclobutadiene)Fe(CO)3 complexes from 1,2- or 1,3-disubsli-tuted-3,4-dibromocyclobutenes has been presented - In turn, the substituted dibromo-cyclobutenes are prepared from squaric esters. The reaction of cz5-3,4-carbonyldioxycy-clobutene and substituted variants with l c2(CO)9 orNa2Fe(CO)4 also produces (cyclobu-tadiene)Fe(CO)3 complexes . Photolysis of a-pyrone generates 3-oxo-2-oxabicyclo [2.2.0]hex-5-ene (224) which undergoes photolysis with a variety of metal carbonyls to afford the parent cyclobutadiene complex 223 [MLn = CpV(CO)2, Fe(CO)3, CoCp. or RhCp] (equation 33) 2 0. 

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