Intramolecular Diels-Alder reactions of 2-(alkynyl)pyrimidines and 2-(alkynyl)pyridines

Pyrimidines 3, 7 and 13 carrying an ω-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.