Potent Hydrophylic Dienophiles. Synthesis and Aqueous Stability of Several 4-Aryl- and Sulfonated 4-Aryl-1,2,4-triazoline-3,5-diones and Their Immobilization on Silica Gel

Article abstract of DOI:10.1021/jo00164a005

The purpose of this investigation is the development of a series of sulfonated 4-aryl-1,2,4-triazoline-3,5-diones (TADs) useful as potent dienophiles for Diels-Alder reactions in aqueous solution and capable of providing a TAD moiety immobilized on an insoluble support.TADs 4, 5, 23, 24, and 29 were all prepared by oxidation of the corresponding urazoles with N2O4.The urazole precursors were prepared by chlorosulfonation of the appropriate 4-arylurazole, followed in some cases by hydrolysis and neutralization.While TAD sulfonic acids 5 and 29 were not stable toward isolation, the presence of the bulky isopropyl groups in 23 and 24 rendered these TADs isolable in pure form and sufficiently stable in water to allow Diels-Alder reactions to compete successfully with attack on the TAD moiety by the solvent (see following paper).Urazolesulfonyl chlorides 2, 18, and 19 reacted with aminopropylsilylated silica gel 31 to give the corresponding immobilized sulfonamides, which were readily oxidized to TAD silica gels 33 (red) and 34 (purple).TAD acid 23 and 31 gave silica gel 35 in which the TAD moiety was attached to the gel via an ionic bond. 1,3-Dienes were selectively and quantitatively removed from solution by these silica gels and could be recovered quantitatively therefrom.