Tetrahedron p. 623 - 632 (1974)
Update date:2022-08-05
Topics:
Anghelide
Draghici
Raileanu
New dienamino esters (3b-k) were obtained by addition of enamino esters (1b-g) to methyl and ethyl propiolate (2a-b). Z,E-Configuration and a transoid conformation were assigned on the basis of spectral data which indicate also noncoplanarity of phenyl groups whenever present. The corresponding adducts with acetylenedicarboxylic ester (18 and 19) have a cisoid conformation and it was possible to differentiate between thermodynamically and kinetically controlled products. Deuteration experiments showed the existence of a 1,5-proton transfer while comparative examination of a whole series of NMR spectra furnished evidence for a head to tail attachment. Attempts to trap the intermediate zwitterion 10 resulted in the formation of 15a-b corresponding to a cyclobutene intermediate. The reaction repiesents a new synthesis of the benzene nucleus and a practical method to obtain the methyltrimesic and 2,4,6-biphenyltricarboxylic acids. Additions of enamino esters to the triple bond are best interpreted as occurring through a common key intermediate, a zwitterion of type 5 or 10. The former collapses by proton transfer and the resulting imino-derivative 6 tautomerizes to 3. The latter cyclizes to a non-isolable cyclo-butene 12 which by opening of the ring produces the dipolar species 13 which further reacts with propiolic ester. By cyclization of the dienamino esters 3a-j but not of 18 and 19 in dipolar aprotic solvents at 160-190° the corresponding a-pyridones 4a-t were obtained in good yields.
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Doi:10.1039/P19740001970
(1974)Doi:10.1021/jm00198a024
(1979)Doi:10.1002/ardp.19753080211
(1975)Doi:10.1055/s-1975-23737
(1975)Doi:10.1021/jm00242a015
(1975)Doi:10.1021/ja01548a047
(1958)