52
S. Lopes et al. / Journal of Molecular Structure 919 (2009) 47–53
In order to give further insight into the unusual thermolysis
process of the studied 2-benzoyl-2H-azirine, we now also exam-
ined the possibility of occurrence of initial formation of the isox-
azole, followed by isomerization to the oxazole ring, since the
thermal rearrangement of 4-acylisoxazoles to 4-acyloxazoles is
known [15]. In reference [15] the synthesis of the oxazoles from
the corresponding isoxazoles was rationalized considering ring
contraction reaction to give a 2H-azirine intermediate, followed
by a ring expansion process via C2AC3 bond cleavage and cycli-
zation. Therefore, the thermolysis of 2-benzoyl-2-chloro-2H-azi-
rine-3-carboxylate 1b could also involve the formation of
isoxazole 3b followed by the rearrangement to oxazole 15. How-
ever, this possibility was rolled out, since upon heating at reflux
for 5 h a solution of isoxazole 3b in toluene no reaction was
observed.
4. Conclusion
It is clear from the analysis of the matrix-isolated 4-chloro-5-
phenyl-1,3-oxazole-2-carboxylate 15 and methyl 4-chloro-5-
phenylisoxazole-3-carboxylate 3b FTIR spectra that this technique
allows to distinguish easily these isomeric heterocycles. Therefore,
it has been demonstrated that methyl 2-benzoyl-2-halo-2H-azi-
rine-3-carboxylates 1 undergo thermal ring expansion to give 4-
halo-5-phenyl-1,3-oxazole-2-carboxylates (15 and 18) in high
yield. These 1,3-oxazoles can also be obtained in high yield from
haloazidoalkenes 4 (Scheme 5).
Acknowledgment
S. Lopes and C. M. Nunes acknowledge FCT for the Ph.D. Grants
(SFRH/BD/29698/2006 and SFRH/BD/28844/2006).
References
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Fig. 6. Infrared spectra of MCPIC (3b) (1145–530 cmÀ1 region): (a) spectrum of
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X
Toluene
Toluene
Δ, 5h
N
Δ, 7h
Ph
CO2Me
96-97%
95-98%
O
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15 X = Cl
18 X = Br
N
N3
X
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MeO2C
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4a X = Br
4b X = Cl
1a X = Br
1b X = Cl
Scheme 5.
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comparison with the theoretically predicted simulated spectrum,
which nicely fits the observed spectrum (see Figs. 5 and 6). This
spectrum of MCPIC is significantly distinct from that of the product
resulting from the thermolysis of 1b (which, as already shown, cor-
responds to the 1,3-oxazole 15).