Ylids pyridinium

The use of pyridinium ylids in the synthesis of carbo- and hetero-cycles has been reviewed (157 references)," with a particular focus on nucleophilic addition-eliminations n = 2,3, 6). 

Loss of HCN from the intermediate adduct failed to occur, though the related pyridinium ylid 211a underwent addition with loss of HCN to give an aromatic system (213 X = C(CN)2).296 A compound analogous to 218 was also isolated during the preparation of 2 1 5.294,295a... 

The substrate in this class of reaction68"80 (Scheme 6) is a pyridinium ylid, which undergoes a 1,3-cycloaddition with an acetylene. Sasaki and... 

In a few cases 1 2 molar adducts such as 75 are formed.278 If the pyridine can form an ylid such as 76, then an alternative cyclization to an indolizine (78) takes place to some extent.278 Many pyridinium ylids corresponding to 76, but not possessing an ionizable proton at the 2- or... 

Indolizines by a Consecutive 3CR of Acid Chlorides, Alkynes, and Pyridinium Ylids... 

Dichloro(l, 3-propanediyl)platinum and its bis(pyridine) derivative have been studied by a number of authors. Dichloro(l,3-propanediyl)platinum, and the corresponding substituted 1,3-propanediyl platinum compounds release the parent cyclopropane on treatment with potassium cyanide, potassium iodide, a tertiary phosphine, carbon monoxide, and other ligands.2,6 Reduction by means of hydrogen or lithium aluminum hydride yields chiefly isomeric substituted propanes. Dichlorobis(pyridine)(l,3-propanediyl)platinum in refluxing benzene yields a pyridinium ylid complex, - (CH3CH2CHNC5Hs)-PtpyCla. 

Dipolar cycloadducts 176 are available from the reactions of cyclopropenes with azomethine ylids, pyridinium ylids aryl nitrile oxides " ", alkyl nitrile... 

Thermolysis of sulphonyl azides in the presence of pyridine results in the formation of pyridinium ylids It is not known whether the... 

Substrate 3-Substitution product 7 Pyridinium ylid 7. Benzene.sul-phonamide %... 

Boekelheide and Fedoruk applied this reaction with success to the synthesis of a pyrrolo[l,2-a]imidazole (214) [Eq. (24)1 using the same conditions that had been used for the corresponding pyridinium ylid. Excess of ethyl propiolate was thought to act as the dehydrogenating agent. Pyrrolo[l,2-a]benzimidazoles (e.g., 215) were prepared in the... 

Pyridinium ylids can be stabilized if the anionic portion of the ylid is delocalized 42 77>. 

Much of the work on pyridinium ylids has been carried out by Krohnke and his papers should be consulted for background information on this subject 9M1). A general discussion of pyridinium ylids is also presented by Johnson 77>. Alkylation and acylation reactions are quite common with this ylid and it has been shown that C-alkylation followed by reductive cleavage of the pyridine residue is a useful method for the synthesis of alkylated ketones and esters 66>. Another interesting elimination occurs if the ylid is treated with nitrosobenzene 89). 

A rather interesting synthesis of a pyridinium ylid was reported by Phillips and Rails 113>. The reaction of pyridine with bromoacetic acid in the presence of benzaldehyde at 120 °C in nitrobenzene solvent gives l-(2-hydroxy-2-phenylethyl)pyridinium bromide in 75% yield. The mechanism suggested for the reaction involves the formation of the the pyridinium betaine which decarboxylates to the ylid. Subsequent reaction of the ylid with benzaldehyde and protonation gives the final product. 

Methoxy-l-athoxyi-ai bonylimin[Pg.585]

Pyridinium-Ylid R3 R2 Ri rSrR CO-OC2HS. . . -lH-l,2-diazepin Ausbeute 1% d.Th.] Literatur... 

The formation of a highly reactive nitrene can be used for immobilization of a polymer. The in-situ crosslinking of polymers is induced by exposure to UV light (350 nm) of polymers prepared by free-radical polymerization of the desired monomer to which a pyridinium ylid-type monomer is added in small amounts. Depending on the final product requirements, more than one monomer can be incorporated. This flexibility allows one to tailor a process in a very simple manner. The immobilization of reactive polymer on various surfaces produces a template from which numbers of separation can be performed ion exchange, reverse phase, affinity, or chiral chromatography. The support can also be used in product preparation such as DNA/peptide synthesis. 

Reactions with pyridinium ylids Decarboxylative condensation... 

Another useful method involves the intermediacy of a pyridinium ylid as a 1,3-dipole in a cycloadditionJ ... 

N-Alkoxycarbonylimino-pyridinium ylids Alkoxylureas Aminals, N,N -dihydroxy-Azocarboxylic acid esters 1,1-Azohydroperoxides O-Carbamylhydroxyl-amines (15)... 

Alkoxyazoxy compds. N-Alkoxycarbonylimino-pyridinium ylids N -Alkoxycarboxylic acid hydrazides... 

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