Several 2-and 5-propynyloxymethylpyrimidines were synthesized and their mtramolecular Diels-Alder reaction was studied. The roducts of the reaction were 5,7-dihydrofuro[3,4+Jpyridines and 1,3-dihydrofuro 3,4-cJpyridines, respectively. P Introduction of one or two alkyl (aryl) groups at the a or y position of the side-cham of the 5-propyn phenomenon is h loxymethylpyrimidines results in an increased reaction rate. This ~scussed in terms of relative rotamer population. Substitution at the alkyne group or in the pyrimidine ring retards the rate of the cycloaddition reaction. Recently, there has been a growing interest in inverse electron demand Diels-Alder reactions of six-membered heterocycles like tetrazines and triazines with appropriately substituted alkenes or alkynesl. Several less x-electron deficient diazinesl-3 and even pyridines*$ have also been shown to undergo this type of reaction, especially when the intramolecular version of this reaction is employed. In previous studies we observed that pyrimidines containing an -X-CH2-CHz-C&H (X = 0, S, NAc, CI-Iz) group at position 2 (or 5) required reaction temperatures of 180 -2OO'C to give these intramolecular Diels-Alder reactions 3. Surprisingly, when X = C(CN)z the cycloadditions could be performed at much lower temperatures (about 13O'Y.Y. This rate enhan-X = 0, S, NAc, CCNh, CH2
cement is not uncommon in intramolecular reactions and can be ascribed to two factor&. First, the Thorpe-Ingold (scissoring) effect, caused by the two cyano groups, decreases the angle at Ca between diene and dienophile, thus enabling a closer approach between the diene and dienophile and thereby increasing the reaction rate 7. Secondly, it has been recently recognized