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Polar Hetero-Diels-Alder Reactions of 4-Alkenylthiazoles with 1,2,4-Triazoline-3,5-diones: An Experimental and Computational Study
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The hetero-Diels-Alder reactions of 4-alkenylthiazoles with 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD)lead to new heteropolycyclic systems in excellent yields and high levels of stereocontrol through anexclusively suprafacial approach. 4-Alkenylthiazoles with a stereogenic center placed at the alkenylicsubstituent react with PTAD giving the corresponding chiral cycloadducts in moderate diastereomericexcesses. The stereochemical course is dominated by the steric interactions at the two diastereomerictransition states. A computational study of these processes with structurally simpler reagents has beencarried out. A concerted pathway via a highly asynchronous transition state is preferred for 2-unsubstituted4-vinyl and 4-styrylthiazoles. However, two alternative and equally likely pathways, concerted andstepwise, have been found to be followed by 2-methyl- or 2-phenyl-substituted 4-styrylthiazoles. Theconcerted pathway features a highly asynchronous transition state. For the stepwise pathway, the rate-determining step is the first one, as the energy barrier for the second step is virtually nonexistent.

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