Cyclic epoxidation

To extend the scope of asymmetric transannular C-H insertions, more highly functionalized medium-sized cyclic epoxides have been investigated. A triad of cydooctene oxides 34, 36, and 38, possessing protected diol units, gave the expected alcohols 35, 37, and 39 (Scheme 5.10) [17, 18] an asymmetric synthesis of (-)-xialenon A has been achieved starting from alcohol 39 [19]. In comparison,... 

Andersson also showed that, in addition to meso-desymmetrization, kinetic resolution of some cyclic epoxides by use of the first-generation catalyst was also possible, giving both epoxides and allylic alcohols in good yields (Scheme 7.51) [108], Kozmin reported the effective use of the same catalyst in the desymmetrization of diphenylsilacyclopentene oxide. The resulting products could be used in the ster-eocontrolled syntheses of various acyclic polyols (Scheme 7.52) [109]. 

In a reaction closely related to the latter, pyranylidene derivatives are obtained by the intermolecular radical coupling of alkynyl- or alkenylcarbene complexes and epoxides. Good diastereoselectivities are observed when cyclic epoxides are used. Moreover, the best results are reached by the generation of the alkyl radical using titanocene monochloride dimer [90] (Scheme 43). 

Cyclic epoxides such as 124 can react in two ways with strong bases (a) via abstraction of a /3-proton to form allylic alcoholates 125 or (b) by deprotonation at the epoxide carbon atom forming the intermediate 126 and, after electrophilic substitution, the epoxides 128. If there is a suitable C—H bond in the vicinity of the C-Li moiety, intramolecular carbenoid insertion reactions to 127 may take place (equation 27) ° . ... 

The second altogether different strategy does not involve a cyclic amino alcohol but an acyclic one, and its reaction with a cyclic epoxide or dielectrophile. The reactions have been shown in Scheme 28 for both ds- or trans-iuscA tetrahydrooxazines (263 to 109 and 264 to 110) <1978CB1164>, in Scheme 38 for r-fused tetrahydrooxazines of the type 344 <1990AP43>, and in Equation (50) for a /ra r-fused tetrahydrooxazine 349 <1987AP625>. 

Another approach to the preparation of enantiomerically-pure ring systems is to start with the. prochiral preformed ring, and perform an asymmetric transformation. Mauro Pineschi of the Univcrsita di Pisa has reported (7. Org. Chem. 2004, 69,2099) a study of a series of racemic cyclic epoxides, represented here by 6. Exposure of 6 to Bu,Zn in the presence of an enantiomerically-pure Cu complex derived from 7 converts one enantiomer of 6 to 8, and the other enantiomer to 9, each in high enantiomeric excess. Several other classes of cyclic epoxides showed similar selectivity. 

We have found that the peroxide is capable of efficiently transferring oxygen to alkenes, forming cyclic epoxides in excellent yields and selectivity. Acyclic alkenes are less easily epoxidised with this reagent. 

Cyclic epoxide derivatives, such as 2,3-epoxy-c/s-pinane 210 [Eq. (5.299)]826 and isomeric caryophyllene diepoxides 211 [Eq. (5.300)],827 undergo varied transformations including ring contraction under similar conditions. 

Bi(OTf)3, cyclic epoxides also react with amines, Me3SiN3, and Me3SiCl to afford the corresponding a-functionalized alcohols (Equation (82)).138,138a BiCl3 and Bi(OTf)3 catalyze aza-Diels-Alder reactions of A-arylimines with both Danishefsky s diene and dienophiles (Equation (83))139 and the intramolecular aza-Diels-Alder reaction of A-vinyloxypropyl-2-cyano-l-azadienes (Equation (84)).140... 

The reduction of a, (3-epoxy ketones using Sml2 is also possible. A range of acyclic and cyclic epoxides with different substitution patterns underwent reduction to give the desired aldol products in good yields (Scheme 4.26).31... 

Lithium enolates of ketones react with cyclic epoxides in the presence of boron trifluoride etherate <2003JOC3049>. The reactions between carbonyl enolate ions and epoxides have been reviewed <2000T1149>. The pseudoephedrine... 

Lanthanide catalysis was again effective in ring-opening reactions of cyclic epoxides (eq 8). Finally, MAO-activated l-RuCL provides block copolymers of ethylene and hex-l-ene. ... 

Alkenylsilanes and -stannanes, and arylsilanes and -stannanes are useful reagents for transfer of an sp -carbon unit to electrophiles under titanium catalysis. Epoxides are opened by TiCE to generate cationic carbon, which is successfully trapped with bis(trimethylsilyl)propene as an aUcenylsilane (Eq. 122) [305]. Other Lewis acids, for example ZnCla, SnCU, and BF3 OEt2, proved less satisfactory. Cyclic epoxides such as cyclopentene and cyclohexene oxides gave poorer yields. An intramolecular version of this reaction proceeded differently (Eq. 123) [305]. Eqs (124) and (125) illustrate diastereoselective alkenylation and arylation of (A,0)-acetals that take advantage of the intramolecular delivery of alkenyl and aryl groups [306], Cyclic ethers... 

See Healy, E.F. Lewis, J.D. Minniear, A.B. Tetrahedron Lett. 1994, 35, 6647 for a discussion of the LiAlH4 reduction of unsaturated cyclic epoxides. 

Trans 1,2-dioIs are produced from cyclic epoxides, (d) Acid-induced epoxide ring opening (Section 18.8)... 

Stereospecific S 2 opening of cyclic epoxides with cyanocuprates furnishes, after workup, the rran5-2-hydroxy-alkylated products. 

Trans 1,2-diols are produced from cyclic epoxides. 

It has recently been reported that selenoamide is a useful reagent for the stereospecific deoxygenation of epoxides under mild conditions.This deoxygenation can be applied to various epoxides, i.e. mono-, di- and tri-substituted epoxides and cyclic epoxides. A suggested reaction path is illustrated in Scheme 34. 

Like chemoselectivity, regioselectivity is also governed largely by steric factors, with attack occurring preferentially at the least hindered epoxide center. This generalization holds true even for reactions performed in the presence of BP3-OEt2. However, stereoelectronic factors predominate in reactions of cyclic epoxides, which generally proceed predictably in a rrans-diaxial fashion. 

Qccurance and Identification. An early report of cotton volatile composition by Minyard et al. (44) involved steam distillation of large quantities of leaves and flowers. Major compounds identified included the monoterpenes a-pinene, B-pinene, myrcene, trans-B-ocimene, and limonene ( 4). Several other monoterpene hydrocarbons were also present in low concentration. Since that report, many other terpenes have been identified in cotton essential oil steam distillates and solvent extracts. These compounds include cyclic hydrocarbons such as bisabolene, caryophyllene, copaene and humulene (45-47), the cyclic epoxide caryophyllene oxide (45), cyclic alcohols such as bisabolol, spathulenol, and the aromatic compound... 

Nugent adapted the zirconium trialkanolamine complex developed for the enantioselective addition of TMSN3 (Scheme 1) to catalyze the addition of bromide, where substitution of bromide for azide at the metal center was proposed to account for the epoxide halogenation product (Scheme 15) [29]. When a large excess of the bromide source was used to suppress azido alcohol formation, a wide range of cyclic epoxides reacted with 5 mol % of the catalyst to afford good yields of the bromohydrins in 84-96% ee. 

Reaction with cyclic epoxides. The reaction of epoxides of medium-size rings with base (lithium di- -alkylamides) has been extensively studied by Cope and by Crandall (1, 610 611 2, 247). Two products are usually obtained an allylic alcohol formed by -elimination and a bicyclic alcohol derived from a carbene intermediate formed by a-elimination. Boeckman has now found that the ciirbcnc pathway is highly favored if n-butyllithium (3 equiv.) is used as base and if (he reaction is conducted at low temperatures, —78 to 25° in the case of cis-eyelooctene oxide (1) and m-cyclodecene oxide. The oxide of cyclododecene,... 

Regioselectivity may also be controlled by n-interaction, as seen in the aluminum hydride reduction of unsaturated cyclic epoxides (e.g., 52). The observed regiochemical outcome was explained by an intermediate n-complex (53) in which the substrate is essentially planar. This model, which is supported by semiempirical calculations, minimizes axial-attack effects and emphasizes subtle electronic factors as well as hydride donor-carbon distances [94TL6647]. 

Solvay and Cie, Cyclic Epoxidation Process for Unsatiuated Fatty Acid, DE Patent 1,257,767 (1962). 

Synonyms Condensation prods., epoxy Epoxides, polymers, epoxy resins Epoxy compds. Ethers, cyclic, epoxides, polymers Plastics, epoxy Polyethers, epoxy resins Classification Polymer... 

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