Pyridines, acylation from carbonyl compounds

Occasionally, [2.2.3]cyclazines have been obtained from pyridines in reasonable yields. Thus (251) is formed in a reaction of the 2-vinylpyridine (252) with methyl propiolate <71 JCS(C)3296>. Acylative cyclization of pyridine-2-carbaldehyde with a,/3-unsaturated carbonyl compounds provides a convenient route to l-acyl[2.2.3]cyclazines (Scheme 38) <74H(2)585). 

As mentioned above, amines of the thienopyridine series synthesized by the Thorpe reaction always contain an electron-withdrawing substituent (acyl, alkoxy-carbonyl, etc.) at position 2. The presence of the o-aminocarbonyl fragment in these compounds makes these compounds useful as synthons for pyridine ring construction in the Friedlaender synthesis. Generally, condensation occurs in the presence of a basic catalyst. The acid-promoted synthesis can be exemplified by the preparation of tetracyclic structure 59 from pyranothienopyridine 60 (1996RFP1417446). 

Esters of the enolic forms of /3-keto esters and /3-diketones are prepared from the corresponding carbonyl compounds and acyl halides in pyridine solution. In this manner, the enol acetate of benzoylacetone, C,HsCOCH=C(OCOCH,)CH is formed in 70% yield. " ... 

In order to understand the effect of these catalysts, it is important to observe that they do not act themselves as enhancers (no effect on light output is observed, when used alone). An explanation is proposed by considering in more detail the reaction steps from diazaquinone (L) to the luminol peroxide intermediate. This reaction involves nucleophilic attack on one of the diazaquinone carbonyls by hydrogen peroxide monoanion. A hydroperoxide species is formed, which can rearrange to the endoperoxide. Either compound is very unstable and collapses to 3-amino-phthalate in its excited state. Perhaps pyridine acylation catalysts could facilitate hydrogen peroxide attack by converting L into a more reactive intermediate. 

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