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Pyridines from unsaturated 1,5-dicarbonyl

Our own group is also involved in the development of domino multicomponent reactions for the synthesis of heterocycles of both pharmacologic and synthetic interest [156]. In particular, we recently reported a totally regioselective and metal-free Michael addition-initiated three-component substrate directed route to polysubstituted pyridines from 1,3-dicarbonyls. Thus, the direct condensation of 1,3-diketones, (3-ketoesters, or p-ketoamides with a,p-unsaturated aldehydes or ketones with a synthetic equivalent of ammonia, under heterogeneous catalysis by 4 A molecular sieves, provided the desired heterocycles after in situ oxidation (Scheme 56) [157]. A mechanistic study demonstrated that the first step of the sequence was a molecular sieves-promoted Michael addition between the 1,3-dicarbonyl and the cx,p-unsaturated carbonyl compound. The corresponding 1,5-dicarbonyl adduct then reacts with the ammonia source leading to a DHP derivative, which is spontaneously converted to the aromatized product. [Pg.262]

Preparation of Pyridines from Enaminones and 1,3-Dicarbonyl Compounds OR u.p-Unsaturated Carbonyl Compounds... [Pg.270]

This reaction was first reported by Krohnke et al. in 1961. It is the synthesis of 2,4,6-trisubstituted pyridine derivatives involving the formation of pyridinium ylide from pyridine and a-bromoketone, which undergoes the 1,4-Michael addition to an a, -unsaturaled compound to form 1,5-dicarbonyl compounds and cyclizes with ammonium acetate. Therefore, it is generally known as the Krohnke pyridine synthesis or Krohnke reaction. In this reaction, the intermediate 1,5-dicarbonyl compounds do not need to be isolated from reaction mixture. Because three different substituents can be introduced into pyridine ring, this reaction becomes the ideal model for combinatorial synthesis, and a library pool containing pyridine from 9 to over 200 has been generated by this reaction. [Pg.1695]

A great variety of methods is available for the ring synthesis of pyridines the most obvious approach is to construct a 1,5-dicarbonyl compound, preferably also having further unsaturation, and allow it to react with ammonia, when loss of two mole equivalents of water produces the pyridine. 1,4-Dihydropyridines, which can easily be dehydrogenated to the fully aromatic system, result from the interaction of saturated... [Pg.123]

It has also been found that the reaction of a-keto-ketene dithioacetals (8) with the carbanions derived from methyl ketones (9) gives unsaturated 1,5-dicarbonyl compounds (10). The latter compounds can be ring-closed to pyridines (11) by treatment with ammonium acetate. ... [Pg.290]

The Hantzsch synthesis of pyridines features 1,4-dihydropyridines in the first step. A variant of the method uses hydroxylamine (Table 17-5), which can be regarded as an oxidized version of ammonia. With this reagent, pyridines are formed directly from 1,5-dicarbonyl compounds, in turn readily made by Michael additions of enolates to a,j8-unsaturated aldehydes and ketones (Section 18-11). Formulate the mechanisms of the two steps in the pyridine synthesis shown. [Pg.1138]


See other pages where Pyridines from unsaturated 1,5-dicarbonyl is mentioned: [Pg.31]    [Pg.397]    [Pg.290]    [Pg.220]    [Pg.397]    [Pg.743]    [Pg.743]    [Pg.194]    [Pg.365]    [Pg.365]    [Pg.25]    [Pg.323]    [Pg.292]    [Pg.743]    [Pg.454]    [Pg.365]    [Pg.171]   


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1,4-Dicarbonyl - from

Unsaturated dicarbonyl

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