3-Component reaction pyran ring

In a one-pot three-component reaction, aromatic aldehydes, malononitrile and 1,3-dicarbonyl compounds react to form 2-amino-5-carboxy-4-aryl-47/-pyran-3-carbonitriles 87. The reaction proceeds by an initial Knoevenagel condensation of malononitrile with the aromatic aldehyde to afford the 2-benzylidenemalononitrile intermediate 88. Michael addition of the activated methylene group forms the 1,5-dicarbonyl equivalent 89, which upon ring closure affords 477-pyrans (Scheme 29) <2004SL871, 1999H(51)1101 >. 

The cycloaddition reaction of o-thioquinones with acyclic 1,3-dienes is a finely tuned process that can involve either reagent behaving as the diene component. It has now been established that formation of a 2-spiro-linked thiopyran by a [2-1-4] reaction is kinetically controlled and that the [4-1-2] alternative leads to the thermodynamic product, a 1,4-benzoxathiin (Scheme 31). Cyclic dienes yield only the oxathiin <03T5523>. Incorporation of the thionocarbonyl diene unit into fiiran and pyran rings and reaction with carbohydrate glycols leads to tricyclic oxathianes, desulfurisation of which yields 2-deoxydisaccharides <03T4249>. 

Pyran ring by 3-component reaction from / -dioxo compds., aldehydes... 

Pyrano[2,3-b]pyiidines 26, 244 Pyran ring by 3-component reaction 26, 812 4H-Pyran -... 

A number of mefhods for fhe synthesis of fused pyrans were developed using multicomponent reactions in PEG. Thus, the annulation of a pyran ring using the three-component reaction of 1/5-cyclohexanediones, arylaldehydes, and malononitrile was found to proceed in the presence of barium(II) triflate xmder mild conditions in aqueous PEG 400 providing tetrahydrochromenes 105 (Scheme 58) [89]. The recyclability of the catalyst was studied using the... 

Glycerol was found to be a very efficient solvent for another catalyst-free three-component reaction furnishing fused pyrans 105, 106, and 161-164 (Scheme 97) [141]. The 2-aminopyran ring was annulated to a variety of scaffolds including 1,3-cyclohexanones, a- and 3-naphthols, 4-hydroxy-6-methyl-2-pyrone, and barbituric acid using the reaction with aromatic aldehydes and methylene active nitriles. It was reported that glycerol recovered from the model reaction (105, Ar = Ph, R = CN, = Me) was reused 20 times in this reaction without... 

By connecting the alcohol and allylsilane moieties into a single molecule, the above Sakurai reaction becomes a two-component system (instead of three components as in the previous scheme). This enables the formation of the pyrane ring in a single step, as illustrated in studies directed... 

As in the synthesis of other bipyridines, several routes to 4,4 -bipyridine have been devised where one of the pyridine rings is built up from simpler components. For example, a dimer of acrolein reacts with ammonia and methanol in the presence of boron phosphate catalyst at 350°C to give a mixture of products including 4,4 -bipyridine (3.4% yield), and in a reaction akin to ones referred to with other bipyridines, 4-vinylpyridine reacts with substituted oxazoles in the presence of acid to give substituted 4,4 -bipyridines. ° ° Condensation of isonicotinaldehyde with acetaldehyde and ammonia at high temperatures in the presence of a catalyst also affords some 4,4 -bipyridine, and related processes give similar results,whereas pyran derivatives can be converted to 4,4 -bipyridine (56% conversion), for example, by reaction with ammonia and air at 350°C with a nickel-alumina catalyst. Likewise, 2,6-diphenyl-4-(4-pyridyl)pyrylium salts afford 2,6-... 

A three-component one-pot coupling of 2,6-dimethylphenyl isocyanide, methyl 2,4-dioxopentanoate and alkynoic esters leads to the isatin-like compounds (or fused pyrans) 20 (Scheme 5). The reaction proceeds by initial formation of the zwitterionic intermediate 21, which is protonated by methyl 2,4-dioxopentanoate. Attack of the newly generated nucleophile then forms the intermediate 22. The pyrrolidinone ring is formed upon loss of methanol and electrocyclization then occurs to afford the desired 2//-pyrans (Scheme 5) <2005S1049>. 

A review of the use of electrocyclic reactions in synthesis has been presented" and of asymmetric electrocyclic reactions that result in diastereo- or enantio-selectivity selection as a consequence of the influence of a chiral component, be it substrate or catalyst, on the electrocyclic bond-forming process." Computational studies of 0 electrocyclic ring-opening reactions of 2-pyrone and 6-fluoro-2-pyrone are pseudoper-icyclic 6-fluoro-2-pyranol are a borderline case and 2-pyranol, pyran, and 6-fluoro pyran reactions are pericyclic in character." ... 

Reduced Pyrans.— This widely occurring ring system attracts a great deal of synthetic attention, and the Diels-Alder reaction is particularly suited to the preparation of compounds in this class bearing a variety of substituents. The use of arylmethylenemalonaldehydes can lead to a variety of heterocycles since each such compound can behave as a 4tc or a 27t component. Olefins react to... 

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