Tetrahedron p. 5151 - 5162 (1989)
Update date:2022-08-02
Topics:
Frissen, A. E.
Marcelis, A. T. M.
Geurtsen, G.
Bie, D. A. de
Plas, H. C. van der
Pyrimidines 3,7 and 13 carrying an ο-alkynyl side-chain -CR2(CH2)nCH2C<*>CH (R=H, CN; n=1,2) at the 2-position undergo intramolecular inverse electron demand Diels-Alder reactions across the C-2 and C-5 positions.Loss of hydrogen cyanide, caused by a retro-Diels-Alder reaction, from the intermediate cycloadducts leads to annelated pyridines 5,9 and 15, respectively.Similarly, from the nitropyridines 16 the 2,3-dihydronitro-1H-indenes 18 are obtained.The influence of electronic and steric effects on the rate of cycloaddition is discussed.Gem-disubstitution on the chain connecting the reaction centers leads to a considerable rate enhancement for compounds 3 vs 13.Compounds 7, having an extra methylene group in the tether between diene and dienophile, react much slower than compounds 3 due to decreased entropic assistance.
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