title: The Origin of High Stereoselectivity in Di-tert-butylsilylene-Directed α-Galactosylation.
abstract: The origin of the high α(1,2-cis)-stereoselectivity in the reaction of galactosyl and galactosaminyl donors with a di-tert-butylsilylene (DTBS) group with several nucleophiles has been elucidated by means of experimental and computational approaches. DTBS overcomes any other cyclic protecting groups examined to date and the β(1,2-trans)-directing effect due to the neighboring participation by CO groups at C2. Requirements for the α(1,2-cis)-stereoselectivity are as follows: (1) generation of an oxocarbenium ion; (2) a galacto-type glycosyl donor with a cyclic protecting group bridging O4 and O6 to form a six-membered ring; (3) through-space electron donation from O4 and O6 into the empty p-orbital of the anomeric carbon to stabilize the oxocarbenium intermediate; (4) steric hindrance due to bulky alkyl substituents on the cyclic protecting group to prevent nucleophilic attack from the β-face; and (5) a 4,6-O-silylene structure. Furthermore, it was found that the strong stereodirecting effect of the DTBS group was unique and specific among the various cyclic protecting groups examined.
authors.affiliation: Department of Applied Bio-organic Chemistry, Faculty of Applied Biological Sciences, Gifu University , 1-1 Yanagido, Gifu 501-1193, Japan.
authors.forename: Akihiro
authors.initials: A
authors.lastname: Imamura
authors.affiliation: Department of Applied Bio-organic Chemistry, Faculty of Applied Biological Sciences, Gifu University , 1-1 Yanagido, Gifu 501-1193, Japan.
authors.forename: Naomi
authors.initials: N
authors.lastname: Matsuzawa
authors.affiliation: Department of Applied Bio-organic Chemistry, Faculty of Applied Biological Sciences, Gifu University , 1-1 Yanagido, Gifu 501-1193, Japan.
authors.forename: Shizuo
authors.initials: S
authors.lastname: Sakai
authors.affiliation: Department of Chemistry and Biomolecular Science, Faculty of Engineering, Gifu University , 1-1 Yanagido, Gifu 501-1193, Japan.
authors.forename: Taro
authors.initials: T
authors.lastname: Udagawa
authors.affiliation: Department of Applied Bio-organic Chemistry, Faculty of Applied Biological Sciences, Gifu University , 1-1 Yanagido, Gifu 501-1193, Japan.
authors.forename: Shinya
authors.initials: S
authors.lastname: Nakashima
authors.affiliation: Department of Applied Bio-organic Chemistry, Faculty of Applied Biological Sciences, Gifu University , 1-1 Yanagido, Gifu 501-1193, Japan.|Institute for Integrated Cell-Material Sciences (WPI-iCeMS), Kyoto University , Yoshida Ushinomiya-cho, Sakyo-ku, Kyoto 606-8501, Japan.
authors.forename: Hiromune
authors.initials: H
authors.lastname: Ando
authors.affiliation: Department of Applied Bio-organic Chemistry, Faculty of Applied Biological Sciences, Gifu University , 1-1 Yanagido, Gifu 501-1193, Japan.
authors.forename: Hideharu
authors.initials: H
authors.lastname: Ishida
authors.affiliation: Department of Applied Bio-organic Chemistry, Faculty of Applied Biological Sciences, Gifu University , 1-1 Yanagido, Gifu 501-1193, Japan.|Institute for Integrated Cell-Material Sciences (WPI-iCeMS), Kyoto University , Yoshida Ushinomiya-cho, Sakyo-ku, Kyoto 606-8501, Japan.
authors.forename: Makoto
authors.initials: M
authors.lastname: Kiso