Krohnke aldehyde synthesis

The classic Krohnke aldehyde synthesis results from the displacement of pyridinium salts aromatic nitroso compounds to give nitrones which are hydrolyzed to aldehydes.Phenacyl bromide reacts with pyridine and then nitrosobenzene to give phenylglyoxal in 76% yield after acid hy lysis. The pyridinium salts in these reactions must be activated in some way toward displacement to effect efficient conversions. 

Krohnke aldehyde synthesis. Transformation of benzyl halides into aldehydes via their pyri-dinium salts which, on treatment with p-nitrosodi-methylaniline, give nitrones. Hydrolysis of the nitrones yields aldehydes. 

Via intermediates Krohnke aldehyde synthesis from chlorides via nitrones... 

Krohnke aldehyde synthesis s. 11, 230 Cyclohexanones from nitrocyclohexenes... 

New syntheses of 6-chloropterin (266) and 6-formylpterin (269) have been elaborated. The chloro-compound is available in one step by the action of acetyl chloride in trifluoroacetic acid on pterin 8-oxide (265), whereas the aldehyde is the product of the multi-stage process outlined in Scheme 62, the first stage of which is the production of the pyrazine-aldehyde (268) by a Krohnke aldehyde synthesis on the chloromethyl derivative (267). The chloropterin condenses easily with arylthiols and alkyl mercaptans, but not with alkylamines. 

Grosche et al. [33] established the classic Krohnke pyridine synthesis on the solid phase (Fig. 6.22 (A, B a, c)). The polymer-bound enones used in this reaction were obtained from polymer-bound acetophenones as well as polymer-bound aldehydes, thus providing enones with different substitution patterns (Fig. 6.19 (B and C)). 

All these reactions can be effected conveniently under mild conditions and at low temperatures. Thus the procedure is especially advantageous for the preparation of, / -unsaturated aldehydes and substituted glyoxals. A review of the reaction has been published by Krohnke, where he gives also experimental details.545 The Krohnke process also enabled Ried and Bender546 to succed in the first synthesis of a heterocyclic dialdehyde (2,3-thionaphthenedicarbaldehyde). 

Dyestuff intermediate, vulcanising accelerator. Reagent used in Krohnke synthesis of aldehydes from alkyl halides. Used as 0.025% soln. in EtOH for photometric detn. of Pd, Pt, Rh, Ru, Os, Ir. Green plates (Et20). Sol. EtOH, Et20 si. sol. H2O. Mp 92.5-93.5° (85-87°). p. 9.71 (25°). Steam-volatile. 

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