Regiospecific Synthesis of 4‐Deoxy‐D‐threo‐hex‐3‐enopyranosides by Simultaneous Activation–Elimination of the Talopyranoside Axial 4‐OH with the NaH/Im₂SO₂ System: Manifestation of the Stereoelectronic Effect

Abstract

A new and high‐yielding method for the regioselective preparation of 4‐deoxy‐ and 2,4‐dideoxy‐2‐acetamido‐β‐D‐threo‐hex‐3‐enopyranosides has been developed. The process involves a simultaneous activation–elimination of the OH‐4 group of β‐D‐talopyranosides and 2‐acetamido‐2‐deoxy‐β‐D‐talopyranosides, mediated by the NaH/N,N′‐sulfuryldiimidazole system at –30 °C. The same reaction applied on the analogous β‐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‐α‐D‐talo‐ and α‐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 α‐talo‐4‐O‐imidazylate gives the corresponding hex‐3‐eno derivative in very high yield, but its α‐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. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Lactose as an inexpensive starting material for the preparation of aldohexos-5-uloses: synthesis of l-ribo and d-lyxo derivatives

“…First was prepared the alcohol 10, which was easily obtained in 90% yield by treatment (Scheme 1) of the known alcohol 9 10 with NaH and BnBr in dry DMF, followed by selective removal of the mixed acetal 6'-O protecting group by mild acid treatment of the crude benzylation product in a biphasic system (CH 2 Cl 2 -5% aqueous HCl).…”

Stereoselective entry into the d-GalNAc series starting from the d-Gal one: a new access to N-acetyl-d-galactosamine and derivatives thereof

“…A straightforward route to β-D-GalNAcp-(1→4)-D-Glcp (1a) was envisaged (Scheme 1) taking advantage from the easily availability of the tetraacetonide 5, 17 prepared from its C-2' lactose epimer (4) through a completely stereoselective, high yielding (92%) oxidation-reduction sequence. 17 The activation of 5 was made by treatment with imidazyl sulfate (Im 2 SO 2 ) and NaH in DMF at -30 °C, leading to the corresponding imidazylate 6, isolated pure by flash chromatography in 86% yield.…”

“…17 The activation of 5 was made by treatment with imidazyl sulfate (Im 2 SO 2 ) and NaH in DMF at -30 °C, leading to the corresponding imidazylate 6, isolated pure by flash chromatography in 86% yield. 6 was then subjected to a S N 2 substitution with NaN 3 in DMF at 100 °C, obtaining, after flash chromatography, the azido derivative 7 in 94% yield.…”

Synthesis and reactivity of imidazole-1-sulfonate esters (imidazylates) in substitution, elimination, and metal-catalyzed cross-coupling reactions: a review

“…Stereoelectronic interactions affecting the regioselectivity of 4-O-imidazylate eliminations. [16] In an effort to synthesize novel N-carbohydrate-derived heterocyclic compounds via nucleophilic substitution of carbohydrate imidazylates with pyrrole derivatives in the presence Scheme 8. Synthesis of 3-hydroxybiphenyl O-sulfamate [reagents and conditions: ii, PhB(OH) 2 , 2 M aq.…”

“…[11] These methods have been often cited as efficient protocols for the synthesis of imidazylates. [15][16][17][18][19][20] In the first two methods, (A and B), alkoxides are generated from 2 by either treatment with sodium hydride or via silyl ether 5 and underwent reaction with an Journal of Sulfur Chemistry 3 Scheme 1. Synthesis of alkyl imidazylates 6 from alcohols 2. excess of commercially available 1,1 -sulfonyldiimidazole (Im 2 SO 2 ) 4, [12,21] to give the corresponding alkyl imidazoylates 6 in good to high yields.…”