Benzoxazolium salts

Benzoxazolium iodide, N-methyl-cleavage, 6, 192 Benzoxazolium salts structure, 6, 179 synthesis, 6, 218... 

Although benzoxazolium salts are more acidic than their benzothiazol analogs in position 2, isolation of the corresponding dioxa-diazafulvalenes was unsuccessful (72LA126). Comparable to the 1,3-dithiolium- and 1,3-thiazolium salts, 1,2,4-dithiazolium salts yielded on treatment with triethylamine in acetonitrile at 0°C mixed geometrical isomers of the TTDAF... 

Fully conjugated aromatic compounds of the oxazole series are the oxazoles (13), benzoxazoles (14), oxazolium and benzoxazolium salts, e.g. (15), and the oxazole iV-oxides... 

Quaternary oxazolium and benzoxazolium salts are prepared by alkylation (see Section 4.18.3.1.2.2) they are also produced by the action of acetic anhydride and an inorganic acid (perchloric or fluoroboric) on the appropriate acylamino ketone (equation 98). iV-Aryl derivatives are obtained in this way. 

The two-step conversion of the thiazole and benzothiazole into the corresponding thiazolium salts 144 and 146 followed by ring expansion resulted in AMormyl thiazocine 145 and benzothiazocine 147 (Scheme 16) <1995JOC2597>. The mechanism of this ring expansion is similar to that described for the transformation of the benzoxazolium salt 123 into the iV-formyl oxazocine 124 (Scheme 13, Section 14.06.4.5). 

Salt formation and quatemization of benzoxazoles occurs by analogy to oxazoles. In benzoxa-zoles, the electrophilically accessible atoms C-4 and C-5 of the oxazole ring are blocked. Nitrating acid causes substitution of the benzene ring in the 5- or 6-position. However, nucleophiles attack benzoxazoles, benzoxazolium salts and A-alkylbenzoxazolium salts in the 2-position, e.g. ... 

The transformation of acid chlorides into hydroxymethyl ketones can be achieved in good yield under mild conditions using tris(trimethylsilyloxy)ethylene. a-Hydroxy-carboxylic acids, readily available from carboxylic acids, are decar-bonylated at room temperature on treatment with a benzoxazolium salt and triethylamine to give ketones in good yields [equation (7)]. ... 

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