title: Influence of O6 in mannosylations using benzylidene protected donors: stereoelectronic or conformational effects?
abstract: The stereoselective synthesis of β-mannosides and the underlying reaction mechanism have been thoroughly studied, and especially the benzylidene-protected mannosides have gained a lot of attention since the corresponding mannosyl triflates often give excellent selectivity. The hypothesis for the enhanced stereoselectivity has been that the benzylidene locks the molecule in a less reactive conformation with the O6 trans to the ring oxygen (O5), which would stabilize the formed α-triflate and subsequent give β-selectivity. In this work, the hypothesis is challenged by using the carbon analogue (C7) of the benzylidene-protected mannosyl donor, which is investigated in terms of diastereoselectivity and reactivity and by low-temperature NMR. In terms of diastereoselectivity, the C-7-analogue behaves similarly to the benzylidene-protected donor, but its low-temperature NMR reveals the formation of several reactive intermediate. One of the intermediates was found to be the β-oxosulfonium ion. The reactivity of the donor was found to be in between that of the "torsional" disarmed and an armed donor.
authors: Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen, Denmark.
authors: Tobias Gylling
authors: TG
authors: Frihed
authors: Marthe T C
authors: MT
authors: Walvoort
authors: Jeroen D C
authors: JD
authors: Codée
authors: Gijs A
authors: GA
authors: van der Marel
authors: Mikael
authors: M
authors: Bols
authors: Christian Marcus
authors: CM
authors: Pedersen
mesh_terms: D001597:Benzylidene Compounds / Q000737:chemistry*; D007477:Ions / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D008358:Mannose / Q000138:chemical synthesis* / Q000737:chemistry*; D008362:Mannosides / Q000138:chemical synthesis* / Q000737:chemistry*; D013237:Stereoisomerism; D013452:Sulfonium Compounds / Q000737:chemistry*
chemical_list: D001597:Benzylidene Compounds; D007477:Ions; D008362:Mannosides; D013452:Sulfonium Compounds; D008358:Mannose