RESUMO
Cesium carbonate-mediated stereoselective anomeric O-alkylation of a 2N,3O-oxazolidinone-protected d-mannosamine with sugar-derived primary or secondary alkyl triflates afforded the corresponding 2-amino-2-deoxy-ß-d-mannosides in moderate to good yields and excellent stereoselectivity. The oxazolidinone ring can be opened with aqueous alkali hydroxide to liberate the amine functionality. This method has been successfully applied to the synthesis of the trisaccharide repeating unit of Streptococcus pneumoniae 19F polysaccharide.
Assuntos
Streptococcus pneumoniae , Trissacarídeos , Manosídeos , Polissacarídeos , AlquilaçãoRESUMO
Stereoselective synthesis of d-glycero- and l-glycero-ß-d-mannoheptosides has been achieved by cesium carbonate-mediated ß-selective anomeric O-alkylation of the corresponding d-mannoheptoses. In addition, this method has been utilized in the total synthesis of a tetrasaccharide repeat unit of Bacillus thermoaerophilus surface-layer glycoprotein.
Assuntos
Glicoproteínas de Membrana , Oligossacarídeos , Alquilação , Bacillales , GlicoproteínasRESUMO
Cesium carbonate-mediated anomeric O-alkylation of various protected 2-azido-2-deoxy-d-mannoses with primary triflate electrophiles afforded corresponding 2-azido-2-deoxy-ß-mannosides in good yields and excellent anomeric selectivity. In addition, 1,3-dibromo-5,5-dimethylhydantoin was found to be the optimal oxidant for preparation of those 2-azido-2-deoxy-d-mannoses from their corresponding thioglycosides. The utilization of this method was demonstrated in the synthesis of a tetrasaccharide fragment of Micrococcus luteus teichuronic acid containing N-acetyl-ß-d-mannosaminuronic acid (ManNAcA).
Assuntos
Manosídeos , Micrococcus luteus , Alquilação , Oligossacarídeos , Ácidos UrônicosRESUMO
A number of structurally diverse D-mannose-derived lactols, including various deoxy-D-mannoses and conformationally restricted bicyclic D-mannoses, have been synthesized and investigated in mechanistic studies of ß-mannosylation via Cs2CO3-mediated anomeric O-alkylation. It was found that deoxy mannoses or conformationally restricted bicyclic D-mannoses are not as reactive as their corresponding parent mannose. This type of ß-mannosylation proceeds efficiently when the C2-OH is left free, and protection of that leads to inferior results. NMR studies of D-mannose-derived anomeric cesium alkoxides indicated the predominance of the equatorial ß-anomer after deprotonation. Reaction progress kinetic analysis suggested that monomeric cesium alkoxides be the key reactive species for alkylation with electrophiles. DFT calculations supported that oxygen atoms at C2, C3, and C6 of mannose promote the deprotonation of the anomeric hydroxyl group by Cs2CO3 and chelating interactions between Cs and these oxygen atoms favour the formation of equatorial anomeric alkoxides, leading to the highly ß-selective anomeric O-alkylation. Based on experimental data and computational results, a revised mechanism for this ß-mannosylation is proposed. The utilization of this ß-mannosylation was demonstrated by an efficient synthesis of the hexasaccharide core of complex fucosylated N-linked glycans.
RESUMO
Synthesis of a variety of sugar lactols (hemiacetals) has been accomplished in moderate to excellent yields by using bromine-mediated oxidation of thioglycosides. It was found that acetonitrile is the optimal solvent for this oxidation reaction. This approach involving bromine as oxidant is superior to that using N-bromosuccimide (NBS) which produces byproduct succinimide often difficult to separate from the lactol products.