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Pericyclic Transformations at the Periphery of Chromen‐4‐one (=4 H ‐1‐Benzopyran‐4‐one): An Unusual Preference for a 1,5‐Shift of Allylic Moieties over the Ene Reaction
Author(s) -
Singh Gurmit,
Singh Gurpinder,
Ishar Mohan Paul S.
Publication year - 2003
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.200390008
Subject(s) - chemistry , allylic rearrangement , intramolecular force , stereochemistry , ene reaction , medicinal chemistry , catalysis , organic chemistry
Quite unlike the reported facile ene reactions on the periphery of many related heterocyclic systems, similarly disposed moieties on the periphery of the chromen‐4‐one (=4 H ‐1‐benzopyran‐4‐one) system fail to undergo an ene reaction and display a rather unusual preference for an overall [1,5] shift of the allylic C‐atom. Thus, heating xylene solutions of 2‐( N ‐allylanilino)‐, 2‐( N ‐crotylanilino)‐, and 2‐( N ‐cinnamylamino)‐substituted ( E )‐(oxochromenyl)propenoates 9a – c and 2‐[allyl(benzyl)amino]‐, 2‐[benzyl(crotyl)amino]‐, and 2‐[benzyl(cinnamyl)amino]‐substituted ( E )‐(oxochromenyl)propenoates 16a – c in a sealed tube at 220–230° leads to a [1,5] shift of the allylic moieties (allyl, crotyl, cinnamyl), which is followed by intramolecular cyclization involving the N‐atom and the ester function, to give the 3‐allyl‐3‐crotyl‐, and 3‐cinnamyl‐substituted‐1‐phenyl‐ or 1‐benzyl‐2 H ‐[1]benzopyrano[2,3‐ b ]pyridine‐2,5(1 H )‐diones 10a – c and 17a – c . The anticipated carbonyl–ene reaction in the 2‐( N ‐allylanilino)‐, 2‐( N ‐crotylanilino)‐, 2‐( N ‐cinnamylanilino)‐, 2‐[allyl(benzyl)amino]‐, 2‐[benzyl(crotyl)amino]‐, and 2‐[benzyl(cinnamyl)amino]‐substituted 4‐oxochromene‐3‐carboxaldehydes 8a – c and 15a – c is also not observed, and these molecules remain untransformed under identical conditions. No [1,5] shifts of benzyl, phenyl, or methyl groups are observed, even in the absence of allylic moieties, though facile [1,5]‐H shift occurs in 2‐(benzylamino)‐ and 2‐(phenylamino)‐substituted ( E )‐(oxochromenyl)propenoates 23a , b , which is followed by a similar intramolecular cyclization leading to the 2 H ‐[1]benzopyrano[2,3‐ b ]pyridine‐2,5(1 H )‐diones 24a , b .