Scheme 31 Ring expansion of diazo-functionalized cyclobutenones: 1,7-dipolar cycloaddition to diazepinedione [129]

duction of an azido group was accomplished by an electrophilic substitution reaction using the acetal 173 and trimethylsilyl azide catalyzed with BF3 ■ Et2O. Typically, the 2-phenyl substituted case was examined for thermal decomposition. Thus, heating azide 174 for 30 min in refluxing xylene gave a maleimide derivative 180 after treatment of the primary product with water. This maleimide is likely to be formed from 2-aza-2,4-cyclopentadienone 177, to which there are two possible routes, either via extrusion of nitrogen followed by nitrene-induced ring expansion (175 ^ 176) or via consecutive 4n-8n electrocyclic rearrangements followed by extrusion of nitrogen (178 ^ 179). More importantly, the above experiment indicates that polysub-stituted 2-aza-2,4-cyclopentadienone 177 can survive even at higher temperatures. In fact, without addition of water, it could be isolated as a yellow crystal after concentration of the solution. Whereas the parent 2-aza-2,4-cyclopentadienone is known to be anti-aromatic (life time: ca. 2 s at 30 °C) [131,132], the observed extreme stability of 177 is attributed to dou-

Scheme 32 Ring expansion of azido-functionalized cyclobutenones: formation of stable 2-aza-2,4-cyclopentadienone and its reaction with some nucleophiles (Ohno et al. unpublished data)

ble resonance between the carbonyl group and both ethoxy groups. Despite such thermodynamic stability, 177 is reactive enough to give nitrogen hete-rocycles with some nucleophiles: maleimide acetals 181 (with alcohol), cyclic amidines 182 (with amine), a bicyclic nitrogen-heterocycle 183 (for example, with ethylenediamine), and a tetramic acid analog 184 (for example, with lithium malonate) (Scheme 32).

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