- Intramolecular inverse electron-demand [4+2] cycloadditions of ynamidyl-tethered pyrimidines: Comparative studies in trifluorotoluene and sulfolane
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Three representative 6,7-dihydro-5H-cyclopenta[b]pyridin-4-amines were synthesized using an intramolecular inverse electron demand hetero–Diels–Alder/retro–Diels–Alder sequence between pyrimidines (acting as azadienes) and ynamides (acting as dienophiles). Two solvents of this reaction, sulfolane and trifluorotoluene, were compared at 210 °C and the former consistently led to higher yields. In addition, these studies confirmed the importance of the steric bulk of the C5-position of the pyrimidinyl cycloaddition precursor.
- Donnard, Morgan,Duret, Guillaume,Bisseret, Philippe,Blanchard, Nicolas
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- INTRAMOLECULAR DIELS-ALDER REACTIONS OF 2-(ALKYNYL)PYRIMIDINES AND 2-(ALKYNYL)PYRIDINES
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Pyrimidines 3,7 and 13 carrying an ο-alkynyl side-chain -CR2(CH2)nCH2CCH (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.
- Frissen, A. E.,Marcelis, A. T. M.,Geurtsen, G.,Bie, D. A. de,Plas, H. C. van der
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p. 5151 - 5162
(2007/10/02)
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