title: Absence of Stereodirecting Participation by 2-O-Alkoxycarbonylmethyl Ethers in 4,6-O-Benzylidene-Directed Mannosylation.
abstract: The preparation of a series of mannopyranosyl donors carrying 2-O-(2-oxoalkyl) ethers and their use in glycosylation reactions are described. The formation of cyclic products with the simple 2-O-phenacyl ether and with the 2-O-(t-butoxycarbonylmethyl) ether establishes the stereoelectronic feasibility of participation in such systems. The high β-selectivities observed with the bis-trifluoromethyl phenacyl ether indicate that participation can be suppressed through the introduction of electron-withdrawing substituents. The high β-selectivities and absence of cyclic products observed with the 2-O-(methoxycarbonylmethyl) ether exclude the effective participation of esters through six-membered cyclic intermediates in this series. The results are discussed in terms of the conformation of cyclic dioxenium ions (E,E-, E,Z-, or Z,Z-) and in the context of "neighboring group" participation by nonvicinal esters in glycosylation. Methods for the deprotection of the 2-O-phenacyl and 2-O-(methoxycarbonylmethyl) ethers are described.
authors.affiliation: Department of Chemistry, Wayne State University , Detroit, Michigan 48202, United States.
authors.forename: Peng
authors.initials: P
authors.lastname: Wen
authors.affiliation: Department of Chemistry, Wayne State University , Detroit, Michigan 48202, United States.
authors.forename: David
authors.initials: D
authors.lastname: Crich
mesh_terms: D001597:Benzylidene Compounds / Q000737:chemistry*; D004952:Esters / Q000737:chemistry*; D004987:Ethers / Q000737:chemistry; D006031:Glycosylation; D008968:Molecular Conformation; D013237:Stereoisomerism
chemical_list: C000607872:2-O-(t-butoxycarbonylmethyl)ether; D001597:Benzylidene Compounds; D004952:Esters; D004987:Ethers