A facile synthesis of alpha,beta-unsaturated carboxylic acids from gem-dibromomethylarenes is described. gem-Dibromomethylarenes are employed for the first time in the Knoevenagel-Doebner reaction as aldehyde equivalents for the efficient synthesis of alpha,beta-unsaturated carboxylic acids.
An improved synthesis of exo-heterocyclic
dienes like N-substituted 4,5-dimethylene
2-oxazolidinones
1, by a one-step method from diacetyl and isocyanates, was
described. This highly convergent
synthetic strategy has been successfully used for the preparation of
novel (Z)-5-ethylidene-4-methylene analogs 4 in fair yields. Both dienes
1 and 4 undergo efficient addition of
symmetric
dienophiles in thermal Diels−Alder reactions, inasmuch as they react
stereo- and regioselectively
with the unsymmetric olefins methyl vinyl ketone (MVK) and methyl
propiolate. This regioselectivity was greatly improved by using Lewis acid catalysts
(TiCl4, AlCl3). The nitrogen atom of
the
2-oxazolidinone ring seems to control the orientation of the dienophile
approach. These results
have been rationalized in terms of the frontier molecular orbital
theory by ab initio calculations.
For dienes 4, addition of MVK gave the endo
isomer as the major product. Dimerization of
dienes
4 was also highly regio-, chemo-, and stereoselective,
giving only isomer 17. This reaction
furnished
a second product, which corresponded to dienes 18 obtained
by the [1,5] sigmatropic rearrangement
of 4. The structure of the dienes and main products was
established by NMR experiments and
X-ray diffraction analysis.
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