[figure: see text] This paper demonstrates that both 1,2,2,6,6-pentamethylpiperidine (PMP) and 1,4-dioxane can act as hydride donors in palladium-catalyzed polyene cyclizations of 2 and 3. Studies using PMP-d3 and dioxane-d8 either incorporate a deuterium atom into the monosubstituted product or completely inhibit the hydride transfer so that the second ring closure occurs in high yield. Dabco is the best substitute for PMP.
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Lau et al. (2001) studied this question.
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