Oxazolium salts intramolecular

The photocycloaddition of aliphatic and aromatic aldehydes with 2,4,5-trimethyloxazole (131) gave bicyclic oxetanes 132 in almost quantitative yields hydrolitic cleavage led selectively to erytro a-amino-P-hydroxy methyl ketones 133 <00CC589>. The oxazolium salt 134 was converted to the azomethine ylide 136 via electrocyclic ring opening of the oxazoline 135. Intramolecular cycloaddition afforded 137 in 66% overall yield which was transformed into the aziridinomitosene derivative 138 . 

A novel oxazole to imidazole transformation occurs when the oxazole (168) reacts with the imidoyl chloride (169) in the presence of phosphoryl chloride. The process probably involves an initial quaternization of the oxazole with subsequent ring opening of the oxazolium salt to form a ketimine (170). Intramolecular nucleophilic attack then leads to the imidazole product (Scheme 96). 

In later work, Vedejs and Piotrowski" expanded the model study to prepare an advanced tricyclic intermediate containing the complete carbon skeleton of 1109. In this case, the authors generated an intramolecular oxazolium salt to achieve their goal. Thus a mixture of 4-chlorobutyronitrile and diazo ketone 1116 was added to... 

Wenkert and co-workers prepared a series of 5-alkenyl-substituted oxazolium salts and investigated their intramolecular Diels-Alder reactions as a means to construct alkaloids (Scheme 1.427). The starting oxazoles 1662a-c, 1663, and 1664 were prepared from an acyclic methyl ester and lithiomethyl isocyanide. Methylation of 1662a-c, 1663, and 1664 with methyl trillate then afforded the corresponding iV-methyloxazolium salt quantitatively. The oxazolium salts 1665, 1666, and 1667 were stable for up to 3 h at 90°C. 

An indoloquinone was synthesized by Vedejs and Monahan in a reaction sequence that featured an intramolecular 1,3-DCA of azomethine ylide 9, which was generated in situ from oxazolium salt 8 (Scheme 3, equation 1)... 

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