Ring-opening, oxirane

Oxirane ring opening reaction with a nucleophile mediated by alkali metal salts has been studied extensively. Several oxiranes react with ammonium halide [51], KCN [52], NaNs [53], lithium acetylide [54], amines [55], and ketone enolate [56] in the presence of alkali metal salts providing corresponding -functionalized alcohols. 

Entry Epoxide Nucleophile Ratio of regioselectivity Conditions Attack at Attack at the more the less hindered hindered carbon carbon Yield (%) 

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The oxirane ring-opening reaction requires the presence of a basic catalyst. An acidic catalyst also works, but the polymerization of the oxirane limits its usehilness. In the case of 2-mercaptoethanol (eq. 8), the product has been found to be autocatalytic, ie, the product is a catalyst for the reaction. 

A relatively few oxirane ring openings have been reported to give retention or a mixture of inversion and retention (see the general reviews on the mechanisms of oxirane ring opening, and the specialized one devoted to retention (68RCR448)). 

Introduction of fluorine by oxirane-ring opening is described first. The 1,2-oxirane 408, prepared from nitroalkene 407 and hydrogen peroxide, was treated with KHF2 (ethylene glycol, 20 min, 112°) to 2-... 

Deoxy-4-fluoro-D-fructose (552) was prepared (59%) by fermentation of 3-deoxy-3-fluoro-D-mannitol with Gluconobacter oxydans. The structure of 552 (fi-T) form) was confirmed by the n.m.r. spectrum, which resembles that of 4-deoxy-4-fluoro-Q -D-sorbopyranose (553) 552 was identical with one of the products obtained from the oxirane-ring opening of 3,4-anhy-dro-l,2-0-isopropylidene- -D-tagatopyranose with KHFj. 

Since acidity (Lewis or Brpnsted) impacts adversely on the yield of epoxides, Clerici and Ingallina (204) added basic compounds in low concentrations to TS-1 catalysts during epoxidation of alkenes to inhibit the oxirane ring opening and enhanced the epoxide yields. A comprehensive investigation of the influence of pH on product selectivity in epoxidation of allylalcohol, allylchloride, and styrene catalyzed by various titanosilicates was reported recently by Shetti et al (205). 

B. Oxirane Ring Opening with Low Basicity Organolithinm Reagents. . 1199... 

The oxirane ring opening reaction with organoUthium reagents is an important reaction as it gives direct access to -hydroxyalkylated products. This reaction has been reviewedseveral times, and only general trends will be described here. 

The oxirane ring opening reaction with alkynyllithiums is of considerable synthetic value, as stereodefined homopropargylie alcohols can be obtained. The lithium acetylide ... 

Nucleophilic fluorination is particularly useful in the synthesis of 2-fluoro-2-deoxypyranoses (e.g., DAST fluorination, displacement of a leaving group by a fluoride ion, oxirane ring opening with HF-amine complexes). Electrophilic fluorination also allows the synthesis of 2-fluoro-2-deoxypyranoses. 

Oxirane-ring opening of talopyranose (76) with (30 a) afforded a mixture of thiodisaccharides (77) and (78) in very good yield. Conventional treatment of (77) led to the allyl-2-thio-fucosyl-galactoside (79) (Scheme 23) [32]. 

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