Influence of O6 in mannosylations using benzylidene protected donors: stereoelectronic or conformational effects?
J Org Chem
; 78(6): 2191-205, 2013 Mar 15.
Article
em En
| MEDLINE
| ID: mdl-23336427
ABSTRACT
The stereoselective synthesis of ß-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 ß-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 ß-oxosulfonium ion. The reactivity of the donor was found to be in between that of the "torsional" disarmed and an armed donor.
Texto completo:
1
Base de dados:
MEDLINE
Assunto principal:
Compostos de Sulfônio
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Compostos de Benzilideno
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Íons
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Manose
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Manosídeos
Idioma:
En
Ano de publicação:
2013
Tipo de documento:
Article