Krohnke

Krohnke observed that phenacylpyridinium betaines could be compared to 3-diketones based on their structure and reactivity, in particular, their ability to undergo Michael additions. Since 3-dicarbonyls are important components in the Hantzsch pyridine synthesis, application of these 3-dicarbonyl surrogates in a synthetic route to pyridine was investigated. Krohnke found that glacial acetic acid and ammonium acetate were the ideal conditions to promote the desired Michael addition. For example, N-phenacylpyridinium bromide 50 cleanly participates in a Michael addition with benzalacetophenone 51 to afford 2,4,6-triphenylpyridine 52 in 90% yield. 

A route to pyridines which involves an isolated 1,5-dicarbonyl compound, has been reported. Aldol reaction of enone 57 with methylketone 58 generated 1,5-diketone 59. When this was submitted to the reaction conditions for a Krohnke reaction, thiopyridine 60 was isolated. 

F. Krohnke [Ann. Chem. Liebigs 600, 211 (1956)1 assumes that A -(2,6-dichloro-benzyOquinolinium hydroxide is also completely dissociated in ions. 

A convenient method leading to pyrans (38) consists in the nucleophilic addition of R anions to 2,6-disubstituted pyrjdium salts, in which the y-position (secondary carbonium ion) is more reactive than the a-positions (tertiary carbonium ions), in opposition to the reactivity of 2,4,6-trisubstituted pyrylium salts.Krohnke and Dickore as well as Dimroth and WolH showed that 2,6-diphenyl-pyrylium salts add the anions R of nitromethane, 1,3-diketones, malonodinitrile, ethyl cyanoacetate, and benzoylacetonitrile. Similar reactions are known in the flavylium series. -Nonactivated R ... 

The formation of quaternary salts from benzyl halides and related compounds occurs readily and has been known for many years. More recently, Krohnke and co-workers, who have studied the reactions of many heterocyclic quaternary salts, reported the formation of 5 from pyridine and benzylidene dibromide on heating the reactants together for 1 hr at 100°. The salt is sufficiently stable to be recrystallized from methanol containing a trace of hydrogen bromide. Isoquinoline gives a similar salt. 

To the best of our knowledge, the hrst paper which mentioned an A-(l-haloalkyl)pyridinium compound appeared 66 years ago in the Chemische Berichte (Krohnke 33CB1386). Tlie author described the reaction of phenacyl pyridinium derivatives 1 with bromine in acetic acid to give the halides 2 (36CB2006 37CB864). Tire addition of bromine to the double bonds of A-vinylpyridinium salts 3 and 4 giving the adducts 5 and 6 has also been reported (51CB399) (Scheme 1). 

A different type of denitrocyclization reaction reported by Krohnke et al. involving C-nucleophiles is represented by treatment of some A-(2,4,6-trini-trobenzyl)pyridinium salts, e.g. 168, with sodium hydroxide providing high... 

The light sensitivity of nitrones in solution was observed by Kamlet and Kaplan, " who postulated the formation of oxaziranes. Krohnke formulated the rearrangement caused by light on a nitrone by using an oxazirane as an intermediate. Splitter and Calvin successfully isolated the rearranged products (9, 11, 12) and identified them as known oxaziranes [Eq. (6)]. 

Enkelmann V, Morra BS, Krohnke C, Wegner G, Heinze J (1982) Chem. Phys. 66 303... 

Another reagent that convert benzylic halides to aldehydes is pyridine followed by /7-nitrosodimethylaniline and then water, called the Krohnke reaction. Primary halides and tosylates have been oxidized to aldehydes by trimethylamine N-oxide, and by pyridine N-oxide with microwave irradiation. ... 

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