Pyrans => conjugated ketones

Similarly, 1,5-diketones give pyrans. Conjugated 1,4-diketones, such as 1,4-diphenylbut-2-en-l,4-dione is converted to 2,5-diphenylfuran with formic acid, 5% Pd/C, PEG-200, and a sulfuric acid catalyst with microwave irradiation. Formic acid reacts with alcohols to give orthoformates. Note that alkynyl ketones are converted to furans with palladium (II) acetate. ... 

Mono-enolisation of a 1,5-diketone, then the formation of a cyclic hemiacetal, and its dehydration, produces 4//-pyrans, which require only hydride abstraction to arrive at the pyrylium oxidation level. The diketones are often prepared in situ by the reaction of an aldehyde with two moles of a ketone (compare Hantzsch synthesis, 8.14.1.2) or of a ketone with a previously prepared conjugated ketone - a chalcone in the case of aromatic ketones/aldehydes. It is the excess chalcone that serves as the hydride acceptor in this approach. 

Enamine (235) obtained from cyclic ketones and the acetal of /V-methyl-2-pyrrolidone gave a fused 2-pyrone [83IJC(B)1083]. 2//-Chromenes were obtained from of 3,5-dichlorosalicylaldehyde and enamines (94RRC183) (Scheme 42). The pyran ring is formed by a reaction of aminals of conjugated w-dimethylaminoaldehydes with cyclic /1-dicarbonyl compounds (94IZV285) (Scheme 43). 

Absorption at 240-250 nm by 4//-pyrans has been noted (61CB1784), although this has been disputed and in general 4//-pyrans are characterized by a weak shoulder at ca. 225 nm (69JOC3169). Introduction of an ethoxycarbonyl or acetyl group at C-3 causes a shift of the maximum to 270 and 284 nm, respectively, which are further shifted to 285 and 296 nm by a second of these substituents. These maxima are considerably different from those of 0-alkoxy a,/3-unsaturated esters and ketones indicative of additional conjugation with the second double bond of the pyran. 

Both simple and fused poly substituted pyran-2-ones can be obtained from the reaction between substituted allenyl esters and ketones activated by an electron-withdrawing group. A base-catalysed nucleophilic conjugate addition affords a homoallylic ester and lactonisation completes the one-pot sequence (Scheme 19) <06S2731>. 

The conjugate addition of dithiols to hexa-l,4-diyn-3-ones yields p,P -bis-l,3-dithiane ketones, masked 1,3,5-triketones, which have been converted into dihydro- and tetrahydro-pyrans <03OLl 147>. 

Destannylative reactions. l-Trimethylstannyl-2,4-pentadiene reacts with aldehydes and ketones in a -y-selective manner, whereas conjugated dienes are generated from 3-(tributylstannyl)propenyl pyrans. ... 

The Mn02 mediated oxidation of 2.7 provided the desired 2,6-c/j-tetrahydro-pyran 2.8a (dr = 7 1, entry 1), but the reaction also produced ketone 2.8b (2.8a 2.8b = 3 1) as a result of a slower oxa-conjugate addition due to the absence... 

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