Pyridinium enolates

Pyridinium enol betaines (see p 284) react with cyclopropenones as electrophilic C3-building blocks [13] to yield 3,4,6-trisubstituted 2//-pyran-2-ones 35 ... 

First a Michael addition of the pyridinium ylide to cyclopropenone occurs. This is followed by a reorganization of the intermediate 33 with pyridine elimination giving the keto ketene 34, which by ring closure yields the 2//-pyran-2-one system. Sulfonium and phosphonium ylides can be used instead of pyridinium enol betaines. 

When chiral enolates are allowed to react with W-fluoro-2,4,6-trimethyl-pyridinium triflate (K, Table 3b), moderate diastereomeric excesses are achieved [79] (equation 44)... 

The addition (150-157) of Grignard reagents, alkoxides, hydroxide, sulfides, cyanide, and enolate anions to pyridinium and isoquinolinium salts again provides a variety of cyclic enamines of potential synthetic use. 

The intramolecular cyclization of enolate of l-tryptophyl-3-((3-ketobutyl) pyridinium bromide (160) afforded enamine 161, which undergoes stereoselective acid cyclization with cone. HCl to give the pentacyclic ketone 162 (Catalytic hydrogenation of 162 led to (d,l)-pseudoyohimbone (163) (76JA3645). Again, H3-H15 were found to have the tmns configuration in 162. 

Pyridinium p-toluenesulfonylmethylide 91 has been used as a formyl anion equivalent for conjugate addition to N-substituted maleimides to give the enol ethers 92, which were readily deprotected to give the aldehydes 93 (80TL705). 

Hydroxypyridine (86, R = H) and its derivatives also belong to the class of heterocyclic enols. In benzene and dioxane, 3-hydroxy-pyridine occurs as the neutral molecule (and not as a betaine).Its reaction with diazomethane, in heterogeneous media, gives a mixture of 3-methoxypyridine (86, R = Me) (10%) and l-methyl-3-hydroxy-pyridinium betaine (87) (30%If tert-butanol is used as a... 

Tetraphenyl-3//-azepine (2) is formed by the action of sodium hydride on 1-benzyl-2,4,6-triphenylpyridinium tetrafluoroborate (1) in refluxing toluene.37 The 3//-azepine. which arises by attack of the carbanion, generated at the benzylic carbon, at the 2-position of the pyridine ring, is also formed, unexpectedly, in the reaction of the pyridinium tetrafluoroborate with the enolate of ethyl 2-methyl-3-oxobutanoate. 

The ring expansion reaction of diaryl cyclopropenones and cyclopropene thiones occurring with pyridinium, sulfonium, and phosphonium enolate betaine 427268-270) is related to 1,3-dipolar cycloadditions. This process results in formation of 2-pyrones 428 by loss of pyridine (or sulfide or phosphine) and insertion of the remaining fragment C=C-0 to the C1(2)/C3 bond of the cyclopropenone ... 

Allyl pyridinium betaines 441 isoelectronic with enol betaines 427 likewise reacted with diphenyl cyclopropenone by elimination of pyridine272,213 The product formation, different in aprotic and protic media (phenol 443 in aprotic solvent, A3,5-hexadienoic esters 445 in alcohol solvent), suggested that the diene... 

Likewise, pyridinium and sulfonium enolate betaines 427 react with 4,4-diacyl triafulvenes to give ring expansion to the six-membered ring of 2-diacylmethylene pyrane 560269) ... 

In the hands of Fozard and Jones (65JOC1523) bromination of pyridinium salt 303 (Scheme 69, upper part) yielded bromides 304a and b. Acetylation of ketone 303 and monobromide 304a led to enol esters 305 and 306, respectively. 

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