Chlorohydrins into epoxides

The conversion of chlorohydrins into epoxides by the action of base is an adaptation of the Williamson synthesis of ethers. In the presence of hydroxide ion, a small proportion of the alcohol exists as alkoxide, which displaces the chloride ion from the adjacent carbon atom to produce a cycHc ether (2). 

Uses of Ghlorohydrins From a volume standpoint almost all of the chlorohydrins produced are immediately converted into epoxides such as propylene oxide and epicblorobydrin. The small quantity of various chlorohydrins sold in the merchant market are used in specialty appHcations. 

Chlorohydrins.1 Epoxides are converted into chlorohydrins by TiCl4 and DBU in CH2C12 at 25°. The conditions allow survival of acid-sensitive groups such as acetals. Yields are lower when DBU is replaced by DMAP. 

The transformation of chlorohydrins into the corresponding epoxides (Reaction VI) may be regarded as a special case of the Williamson reaction. Many epoxides have been made this way... 

Chlorohydrins from epoxides. Steroid epoxides are converted into chloro-hydrins by this reagent (benzene, 20°). The ring opening is of the usual trans, diaxial type conditions are so mild that side reactions observed with hydrochloric acid are avoided. ... 

The epoxidation is generally conducted in two steps (/) the polyol is added to epichlorohydrin in the presence of a Lewis acid catalyst (stannic chloride, boron triduoride) to produce the chlorohydrin intermediate, and (2) the intermediate is dehydrohalogenated with sodium hydroxide to yield the aliphatic glycidyl ether. A prominent side-reaction is the conversion of aliphatic hydroxyl groups (formed by the initial reaction) into chloromethyl groups by epichlorohydrin. The aliphatic glycidyl ether resins are used as flexibilizers for aromatic resins and as reactive diluents to reduce viscosities in resin systems. 

Whereas ethylene oxide gives with 17 at ambient temperature a quantitative yield of l-trimethylsilyloxy-2-iodoethane [5, 31], substituted epoxides such as 846b react with 17 to give 848 as the main product [32]. Excess 17, however, leads to the bis-iodo compounds 849 and HMDSO 7 [4, 5]. In the presence of DBU the epoxides 850 are converted by 17, which is generated in situ from hexamethyl-disilane 857 and I2, into the allyl alcohols 851 [4, 32] (Scheme 6.14). Cycloctene epoxide 852 is opened by SiCl4 at -78 °C in the presence of catalytic amounts of the asymmetric catalyst 853 to give 61% of the chlorohydrin 854 in 98% ee [33]. 

C-branched sugars or C-oligosaccharides are obtainable through indium-promoted Barbier-type allylations in aqueous media.151 Indium-mediated allylation of a-chlorocarbonyl compounds with various allyl bromides in aqueous media gave the corresponding homoallylic chlorohydrins, which could be transformed into the corresponding epoxides in the presence of a base (Eq. 8.62).152... 

In the five-membered series, Bartlett and White 6 converted 2-chlorocyclopentanone into a chlorohydrin on treatment with methyl-magnesium bromide, but this product too failed to yield an epoxide because of the cis-disposition of hydroxyl and chlorine substituents (Eq. 177). 

Addition of pyridine bases to the catalytic system caused a considerable increase in the rate and selectivity of the reaction, reaching 80% yield of styrene oxide for 100% styrene conversion (r.t., 30 min). In the presence of this pyridine-modified system, the reactivity of alkenes is in the order styrene > trisubstituted > cis-disubstituted > trans-disubstituted> monosubstituted. The epoxidation of alkenes is not stereoselective. In the absence of pyridine, cis-stilbene was converted into a 1.8 1 trans cis epoxide mixture, whereas the cis isomer prevails in the presence of excess pyridine ligands. Neither chlorohydrins nor pyridine N-oxides are involved in this catalytic system. Attempts to isolate the reactive intermediate led to the characterization of a relatively stable... 

In metal-free catalysis enantioselective ring-opening of epoxides according to Scheme 13.27 path B has been achieved both with chiral pyridine N-oxides and with chiral phosphoric amides. These compounds act as nucleophilic activators for tetrachlorosilane. In the work by Fu et al. the meso epoxides 71 were converted into the silylated chlorohydrins 72 in the presence of 5 mol% of the planar chiral pyridine N-oxides 73 (Scheme 13.36) [74]. As shown in Scheme 13.36, good yields... 

An inversion mechanism had been previously observed in the hydroxylation of caranine (2) to lycorine by Wildman and Heimer. They observed 7% incorporation of [2/ -3H] caranine into lycorine in Zephyranthes Candida Herb., the 3H being retained at C-2 of 1 as shown by the conversion into the inactive 385. The stereospecifically labeled precursor was obtained through LiAl3H4 reduction of lycorine-1,2-a-epoxide prepared from lycorine via its cis-chlorohydrin and chromatography on Florisil. The structure of the a-epoxide rests on physical and chemical grounds, whereas the stereochemistry of the... 

Cleavage and resolution of epoxides.1 The aluminum reagent obtained by reaction of (R)-l with diethyl- or dimethylaluminum chloride shows slight reactivity in reactions with epoxides, but the ate complex (2), prepared from 1, (C2H5)2A1C1, and lithium butoxide, converts cyclohexene oxide into the chlorohydrin (3) in 40% ee. 

Epoxides — chlorohydrins This reagent converts epoxides into chlorohydrins. The same cleavage of a-hydroxy epoxides results in antt-chlorohydrins. 

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