Enhanced diastereoselectivity in beta-mannopyranosylation through the use of sterically minimal propargyl ether protecting groups.
[reaction: see text] 2-O-Propargyl ethers are shown to be advantageous in the 4,6-O-benzylidene acetal directed beta-mannosylation reaction. The effect is most pronounced when the O3 protecting group is a bulky silyl ether or a glycosidic bond; however, even with a 3-O-benzyl ether, the use of a 2-O-propargyl ether results in a significant increase in diastereoselectivity. The beneficial effect of the propargyl ether is thought to be a combination of its minimal steric bulk, as determined by a measurement of the steric A-value and of its moderately disarming nature, as reflected in the pKa of propargyl alcohol. Conversely, the application of a 3-O-propargyl ether in the benzylidene acetal directed mannosylation has a detrimental effect on stereoselectivity, for which no explanation is at present available. Deprotection is achieved by base-catalyzed isomerization of the propargyl ether group to the corresponding allenyl ether, followed by oxidative cleavage with N-methylmorpholine N-oxide and catalytic osmium tetroxide.
Crich
David
D
Department of Chemistry, University of Illinois at Chicago, 845 West Taylor Street, Chicago, Illinois 60607-7061, USA. dcrich@uic.edu
Jayalath
Prasanna
P
Hutton
Thomas K
TK
D004987:Ethers / Q000737:chemistry*; D008362:Mannosides / Q000737:chemistry*; D015394:Molecular Structure; D010293:Pargyline / Q000031:analogs & derivatives* / Q000737:chemistry*; D013237:Stereoisomerism
D004987:Ethers; D008362:Mannosides; D010293:Pargyline