Activation of mannuronic acid esters leads to a conformational mixture of alpha-anomeric triflates, in which the equatorial triflate (C-1(4), chair) is formed preferentially. This unexpected intermediate clearly opposes the anomeric effect and is mainly stabilized by the electron-withdrawing carboxylate function at C-5. Because the anomeric center carries a significant positive charge, the C-1(4), mannopyral chair approximates the favored H-3(4), half-chair oxacarbenium ion conformation. The excellent B-selectivity in glycosytations of mannuronates is postulated to originate from the cooperative action of the triflate counterion and the (stereo)etectronic effects governing oxacarbenium ion stabilization in the transition state leading to the 1,2-cis product.