1,720,979 research outputs found
Sintesi regioselettiva di 4-desossi- e 2-ammino-2,4-dideossi-beta-D-treo-3-esenopiranosidi a partire da beta-D-galattipiranosidi.
No full textUn nuovo metodo di sintesi stereocontrollata di derivati beta-D-mannopiranosidici ortogonalmente protetti.
No full textAn efficient and highly regioselective synthesis of 4-deoxy- and 2-acetamido-2,4-dideoxy-alfa-D-threo-hex-3-enopyranosides
No full textThe preparation of the previously undescribed class of 4-deoxy- and 2,4-dideoxy-2-acetamido-beta-D-threo-hex-3-enopyranosides
was accomplished with a very high yield and a complete regioselectivity by means of a simultaneous activation–elimination
process of the OH-4 group of -D-talopyranosides (5a,b) and 2-acetamido-2-deoxy-beta-D-talopyranosides (5c,d) with NaH/N,N'-sulfuryldiimidazole. The same reaction of analogous beta-D-galactopyranosides (5e,f) is not regioselective, leading to
mixtures of 3- and 4-hexeno derivatives. This difference is evidently determined by the orientation of the C-2 substituent, which,
in the talo series, is anti diaxially disposed to the H-3 eliminating group
Un nuovo metodo per la trasformazione stereoselettiva di beta-D-galattopiranosidi in beta-D-mannopiranosidi.
No full textBiselector enantioselective stationary phases for HPLC: dependence of the chiral discrimination properties on stereochemistry and chemical nature of each unity of the chiral auxiliary
No full text
A new stereocontrolled access to beta-D-mannopyranosides and 2-acetamido-2-deoxy-beta-D-mannopyranosides starting from beta-D-galactopyranosides
No full textA new stereocontrolled synthesis of BETA-D-mannopyranosides was defined relying on a high yielding sequence based on
the following three key steps: (a) a stereospecific inversion at C-2 of beta-D-galactopyranosides by an oxidation–reduction procedure;
(b) a regiocontrolled formation of 4-deoxy-alfa-D-threo-hex-3-enopyranosides; (c) a regio- and stereocontrolled hydroboration–oxidation
of the above enol ethers. The flexibility of this new method was demonstrated by its extension to the synthesis of
2-acetamido-2-deoxy-beta-D-mannopyranosides and of an orthogonally protected beta-D-mannopyranoside scaffold and, finally, by the
transformation of lactose into the two biologically relevant disaccharides with primary structure BETA-D-Manp-(1->4)-D-Glc and
beta-D-ManNAcp-(1->4)-D-Glc
Regiospecific synthesis of 4-deoxy-D-threo-hex-3-enopyranosides by simultaneous activation-elimination of the talopyranoside axial OH-4 with the NaH/Im2SO2 system: manifestation of the stereoelectronic effect
No full textA new and high-yielding method for the regioselective preparation of 4-deoxy- and 2,4-dideoxy-2-acetamido-beta-D-threohex-3-enopyranosides has been developed. The process involves a simultaneous activation–elimination of the OH-4 group of beta-D-talopyranosides and 2-acetamido-2-deoxy- beta-D-talopyranosides, mediated by the NaH/N,N-sulfuryldiimidazole system at –30 °C. The same reaction applied on the analogous beta-D-galactopyranosides takes place without any regioselectivity, affording mixtures of hex-3- and hex-4-enopyranosides. In the case of the methyl 2,3,6-tri-O-benzyl-alfa-D-talo- and alfa-D-galactopyranosides, the corresponding 4-O-imidazylates can be isolated by quenching the reactions at –30 °C. Upon warming these crude products to room temperature, the alfa -talo-4-O-imidazylate gives the corresponding hex-3-eno derivative in very high yield, but its alfa-galacto analogue gives the hex-4-enopyranoside enol ether in poor yield. The different regiochemical outcome between the talo and the galacto series has been attributed to the stereoelectronic effect exerted, exclusively in talo-configured compounds, by the axially disposed C-2 electronegative substituent, which selectively accelerates the breaking of the antiperiplanar C(3)–H bond
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