Epoxides, from halohydrins

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

X = Cl. Br. T Treatment of polystyryl diphenylphosphine with CL, Br, or I, in CH,CL.24 Halohydrins from epoxides yields and selectivities compare well with those obtained with monomeric triphenyl phosphine-halogen complexes.2 11 Open and cyclic acetals and thioacetals by treatment of aldehydes and ketones with alcohols or thiols in the presence of polystyryl diphenylphosphine iodine complex.249... 

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... 

Nakamura T, Nagasawa T et al (1991) A new catalytic function of halohydrin hydrogen-halide-lyase, synthesis of P-hydroxynitriles from epoxides and cyanide. Biochem Biophys Res Commun 180 124—130... 

The conversion of halohydrins into epoxides by the action of base is simply an adaptation of the Williamson synthesis (Sec. 17.5) a cyclic compound is obtained because both alcohol and halide happen to be part of the same molecule. In the presence of hydroxide ion a small proportion of the alcohol exists as alkoxide this alkoxide displaces halide ion from another portion of the same molecule to yield the cyclic ether. 

P-Halohydrins are formed readily from epoxides on contact with silica-gel-supp)orted LiX.- ... 

As an alternative to the enzymatic hydrolysis of epoxides, nonracemic vicinal diols may be obtained from epoxides via the nucleophilic ring-opening by nitrite catalyzed by halohydrin dehalogenase (a lyase). The corresponding nitrite-monoesters are spontaneously hydrolyzed to yield diols. For the application of this technique see Sect. 2.7.2. 

Lewis acid with Halides. Ag20 can activate halide as a leaving group by coordination. Methylation of carbohydrates and 5-benzyUdenebarbituric acid have been carried out with Ag20 and iodomethane in DMF. Triphenyltin trifluoroacetate was obtained from the reaction of triphenyltin iodide and trifluoroacetic acid in the presence of silver oxide. Ag20 converts trawi -halohydrins to epoxides(eq 12) or rearranged products (eq 13). It is reported that the reaction is nonstereoselective, and that tra/w-epoxides are the major products when c/s-alkenes are the reactants. ... 

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 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. 

Chiral halohydrins epoxides.1 The esters (2) of the chiral alcohol 1 derived from camphor-10-sulfonic acid, are converted to a-chloro esters (3) by O-silylation and reaction with NCS with high diastereoselectivity. Reduction of 3 with Ca(BH4)2 results in the recovered auxiliary and the chlorohydrin 4 with clean retention. Cyclization of 4 to the terminal epoxide 5 proceeds with clean inversion. 

Enantioselective reductions. The neat reagent (1), prepared from ( + )-< -pinene, reduces aryl a-halomethyl ketones slowly but in high chemical yield to (R)-halohydrins in 90-96% ee, but optical induction is mediocre in the case of aliphatic a-halo ketones (35-66% ee). The chiral halohydrins are useful precursors to chiral epoxides. 

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. 

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