Controlling Regiochemistry in the Gold-Catalyzed Synthesis of Unsaturated Spiroketals
摘要:
A novel gold-catalyzed synthesis of unsaturated spiroketals that addresses regioselectivity issues commonly reported in metal-catalyzed spiroketalization of alkynes is reported. The reaction sequence is regulated by an acetonide protecting group which undergoes extrusion of acetone to deliver the desired spiroketals in good yields and diastereoselectivities. The reaction, which is carried out under very mild conditions employing AuCl as the catalyst, should be widely applicable in the synthesis of a broad range of spiroketals.
A cationic gold(l) complex with a semihollow-shaped trialkynylphosphine catalyzed 5-exo-dig and 6-endo-dig cyclizations of various internal alkynic beta-keto esters, showing a marked advantage over a gold(I)-PPh3 complex with respect to the rates of the reactions and the product yields. It is proposed that the gold-bound alkynic substrate in a catalytic pocket must be somewhat folded and that such a steric effect makes the carbon-carbon bond formation entropically more favorable.