The 1-phenylsulfonylallenyl terminus in intramolecular Diels-Alder reactions: Tether length and reactivity

Methods are described for the synthesis of 1-phenylsulfonylalka-1,2,(n + 4) ,(n + 6)-tetraenes 5 (n = 2 - 4), and preliminary findings on their thermal ly-induced intramolecular cycloaddition behaviour are reported. The reactio n of 5 (n = 4) is slow, and engages the unactivated pi-bond, whereas those of 5 (n = 2 & 3) proceed readily and exo-selectively with dienophilic parti cipation of the activated terminal pi-bond, to give the expected [4 + 2] cy cloadducts. The reactivity of 5 (n = 2) is particularly noteworthy since cy cloaddition proceeds under exceptionally mild conditions.