Epoxide from halohydrins

The final enzyme discussed in the chapter, haloalcohol dehalogenase, catalyzes the formation of epoxides from halohydrins. However, the reverse reaction, the ring opening of epoxides with nonhalide nucleophiles such as cyanide, azide, and nitrite, has been exploited commercially to generate /3-substituted alcohols that are potential synthons. For this reason, the regioselectivity of the enzymatic ring opening reaction is of... 

As anticipated by the formation of oxiranes (oxacyclopropanes, epoxides) from halohydrins, the four-membered oxygen-containing ring, oxetane (oxacyclobutane), can be prepared by heating 3-chloropropanol (HOCH2CH2CH2CI) with potassium hydroxide (KOH) (Equation 8.51). 

The activities of both haloalkanol dehalogenase (halohydrin hydrogen lyase) that catalyzes the formation of epoxides from alkanes with vicinal hydroxyl and halogen groups, and epoxide hydrolase that brings about hydrolysis of epoxyalkanes to diols are involved in a number of degradations that involve their sequential operation. 

Bartnicki EW, CE Castro (1969) Biodehalogenation. The pathway for transhalogenation and the stereochemistry of epoxide formation from halohydrins. Biochemistry 8 4677-4680. 

Ketones from halohydrins. Palladium acetate complexed with a triarylphos-phine, particularly tri-o-tolylphosphine, converts halohydrins into ketones in the presence of K2C03. Yields are about 70-85% for substrates in which the halogen is secondary or tertiary, but less than 50% when the halogen is primary because of epoxide formation. The reaction is useful for conversion of alkenes to ketones in those instances in which halohydrins are formed regioselectively. 

Since esmolol is marketed as its racemic mixture, there were no stereochemical issues associated with the Payne-like [21] formation of the intermediate aryloxyep-oxide, particularly when the coupling reaction was carried out under the K2C03/ acetone conditions conducive to the formation of epoxides from alpha-halohydrins (see discussion in Chapter II-8). Likewise, even though over-alkylation had been a major problem when preparing the N-extemal ester targets because the N-alkyl-... 

Synthesis of Epoxides from Chiral Chlorohydrins. Asymmetric halogenation of CSA-derived esters allows for the formation of enantiomerically pure halohydrins and terminal epoxides (eq 23). ... 

One method for the preparation of epoxides from alkenes involves (1) treating the alkene with chlorine or bromine in water to form a chlorohydrin (or bromohydrin) followed by (2) treating the halohydrin with a base to bring about intramolecular displacement of Cl . These steps convert propene first to l-chloro-2-propanol and then to methyloxirane (propylene oxide). 

Synthesis of Epoxides from Haiohydrins (Section 11.8C) Formation of the halohydrin and the following intramolecular Sf 2 reaction are both stereoselective (the configuration of the alkene is retained in the epoxide) and stereospecific (for alkenes that show ds, trans isomerism, the configuration of the epoxide depends on the configuration of the alkene). 

Oxirans. - The synthesis of l,2-anhydro-3,4-di-0-benzyl-6-deoxy-a-D-glucopyranose and its conformational analysis have been reported. A range of epoxides have been prepared by base treatment of bromohydrins, which were made by reaction of hydrogen bromide with aldonolactones. A one-pot conversion of vicinal diols into epoxides employs halohydrin ester intermediates generated from cyclic orthoacetates and either acetyl bromide or trimethylsilyl chloride. Levoglucosenone has been transformed into l,6 3,4-dianhydro-p-D-talopyranose by way of a trn/w-iodo-acetoxylation of the alkene moiety... 

Epoxides can also be prepared from halohydrins, themselves produced by electrophilic addition of ffO X to alkenes (Section 8.3). When a halohydrin is treated with base, ffX is eliminated and an epoxide is produced by an intramolecular Williamson ether synthesis. That is, the nucleophilic alkoxide ion and the electrophilic alkyl halide are in the same molecule. 

Base promoted cyclization of vicinal halohydrms (Section 16 10) This reaction is an intramolecu lar version of the Williamson ether synthesis The alcohol function of a vicinal halohydrin is con verted to its conjugate base which then displa ces halide from the adjacent carbon to give an epoxide... 

The following section describes the preparation of epoxides by the base-promoted ring closure of vicinal halohydrins. Because vicinal halohydrins are customarily prepared from alkenes (Section 6.17), both methods—epoxidation using peroxy acids and ring closure of halohydrins—are based on alkenes as the starting materials for preparing epoxides. 

The formation of vicinal halohydrins from alkenes was described in Section 6.17. Halohydrins are readily converted to epoxides on treatment with base ... 

Silica gel-based catalytic systems have been described as efficient promoters for a number of organic reactions.28 Illustrative examples include the oxidative cleavage of double bonds catalyzed by silica-supported KM11O4,29 reaction of epoxides with lithium halides to give /i-halohydrins performed on silica gel,30 selective deprotection of terf-butyldimethylsilyl ethers catalyzed by silica gel-supported phosphomolybdic acid (PMA),31 and synthesis of cyclic carbonates from epoxides and carbon dioxide over silica-supported quaternary ammonium salts.32... 

Formation of Halohydrins from Epoxides (3) OC-seco-Halo-de-alkoxylation... 

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