Equatorial anomeric triflates from mannuronic acid esters.
Activation of mannuronic acid esters leads to a conformational mixture of alpha-anomeric triflates, in which the equatorial triflate ((1)C(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 (1)C(4) mannopyranosyl chair approximates the favored (3)H(4) half-chair oxacarbenium ion conformation. The excellent beta-selectivity in glycosylations of mannuronates is postulated to originate from the cooperative action of the triflate counterion and the (stereo)electronic effects governing oxacarbenium ion stabilization in the transition state leading to the 1,2-cis product.
Walvoort
Marthe T C
MT
Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands.
Lodder
Gerrit
G
Mazurek
Jaroslaw
J
Overkleeft
Herman S
HS
Codée
Jeroen D C
JD
van der Marel
Gijsbert A
GA