Remarkably stable mesoionic oxazolones possessing an oxazolo[3,2-a]quinolinium structure (
5a–b),
8,
16,
17) were obtained by the double cyclisation of phenylglycine-
o-carboxylic acids (
3a–c) in refluxing acetic anhydride or in benzoic anhydride at 140°, The
0-Ac group was eliminated to give the corresponding lactones (
6a–b) or replaced by
O-Ts (
7).
IR stretching vibrations of the
endo-carbonyl were in the range 1710–1768 cm
?1, while
v1-CH exhibited unusually high values (3159–3194 cm
-1). 1-Acyl derivatives could be obtained only with TFAA (
23–24), although easy deuteration of the same position took place in the presence of traces of trifluoroacetic acid. Hydrolysis of
5b led to the α-quinolone-N-acetic acid
27a. In the case of
5a, hydrolysis was accompanied by self-acylation of the nucleophilic site at C-4 with formation of a dimeric acid
28a. The presence of an additional Me group in phenylalanine-
o-carboxylic acid (
36) activates the corresponding mesoionic oxazolone
37 so that 1-acylation becomes possible with formation of the fused oxazole
38 by the Dakin-West reaction. Temperature dependent magnetic non-equivalence of methylene protons has been observed in acids
3b-d and f and also in the 7-membered anhydride
48b.
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