Cis epoxidation

The second C-C bond forming step (step C), while occurring after the first irreversible ee determining step (step B), can affect the observed enantioselective outcome of the reaction. If the radical intermediate collapses without rotation (k3 Ict, k5 ke), then the observed ee would be determined by the first C-C bond forming step (ki vs. k2), that is the facial selectivity (Scheme 1.4.6). However, if rotation is allowed followed by collapse, then the rate of both trans pathways (Ic and k ) will proportionally effect the observed ee of the cis epoxide (ks vs. ks). Should bond rotation be permissible, the diastereomeric nature of the radical intermediates 9a and 9b renders the distinct possibility of different observed ee s for frany-epoxides and dy-epoxides. 

To a solution of m-ethyl cinnamate (44, 352 mg, 85% pure, 1.70 mmol) and 4-phenylpyridine-A-oxide (85.5 mg, 29 mol%) in 1,2-dichloromethane (4.0 mL) was added catalyst 12 (38.0 mg, 3.5 mol%). The resulting brown solution was cooled to 4°C and then combined with 4.0 mL (8.9 mmol) of pre-cooled bleach solution. The two-phase mixture was stirred for 12 h at 4°C. The reaction mixture was diluted with methyl-t-butyl ether (40 mL) and the organic phase separated, washed with water (2 x 40 mL), brine (40 mL), and then dried over Na2S04. The drying agent was removed by filtration the mother liquors concentrated under reduce pressure. The resulting residue was purified by flash chromatography (silica gel, pet ether/ether = 87 13 v/v) to afford a fraction enriched in cis-epoxide (45, cis/trans . 96 4, 215 mg) and a fraction enriched in trans-epoxide cis/trans 13 87, 54 mg). The combined yield of pure epoxides was 83%. ee of the cis-epoxide was determined to be 92% and the trans-epoxide to be 65%. 

Entry R1 (ylide) R HO Epoxide epoxycyclop. cyclop. Epoxide trans cis Epoxide ee trans (cis) (%)... 

Vinylepoxides can be obtained by various strategies, all with their inherent limitations. Racemic epoxidation of olefins is a straightforward route to epoxides, as pure trans- or cis-epoxides can be obtained from ( )- or (Z)-alkenes, respectively. Various oxidants - such as mCPBA and other peracids, H2O2, or VO(acac)2/TBHP - can all be employed in this transformation [1],... 

In the cyclization of the corresponding cis-epoxides, with the aim of obtaining the corresponding cis-2,3-disubstituted tetrahydropyrans, a similar trend was observed. For these systems, however, the 6-endo pathway was less favored, which was ascribed to difficulties in attaining a TS conformation that would allow for maximum stabilization of the developing p-orbital with the adjacent 7t-system. Alternatively, palladium-catalyzed cyclization of the tetrabutylammonium alkoxide derived from 33b results in the corresponding ris-2,3-disubstituted tetrahydro-pyran in excellent yield and selectivity (90%, dr >99 1), while the ris-epoxide gives stereoisomer 37b (86%, dr 98 2) [112]. 

The analogous process involving allylic epoxides is more complex, as issues such as the stereochemistry of substituents on the ring and on the alkene play major roles in determining the course of the reaction [38]. Addition of the Schwartz reagent to the alkene only occurs when an unsubstituted vinyl moiety is present and, in the absence of a Lewis acid, intramolecular attack in an anti fashion leads to cyclopropane formation as the major pathway (Scheme 4.10). cis-Epoxides 13 afford cis-cyclopropyl carbinols, while trans-oxiranes 14 give mixtures of anti-trans and anti-cis isomers. The product of (S-elimi-... 

Addition of l,3-bis(methylthio)allyllithium to aldehydes, ketones, and epoxides followed by mercuric ion-promoted hydrolysis furnishes hydroxyalkyl derivatives of acrolein5 that are otherwise available in lower yield by multistep procedures. For example, addition of 1,3-bis-(methylthio)allyllithium to acetone proceeds in 97% yield to give a tertiary alcohol that is hydrolyzed with mercuric chloride and calcium carbonate to saturated aldehyde.8 Similarly, addition of l,3-bis(methylthio)allyl-lithium to an epoxide, acetylation of the hydroxyl group, and hydrolysis with mercuric chloride and calcium carbonate provides a 5-acetoxy-a,/ -unsaturatcd aldehyde,6 as indicated in Table I. Cyclic cis-epoxides give aldehydes in which the acetoxy group is trans to the 3-oxopropenyl group. 

Five- and six-membered cyclic allenylidenesiloxanes have been prepared by internal displacements of siloxane epoxides by lithiated propargylic siloxanes (Eqs. 9.49-9.51) [58], The mode of attack is related to the stereochemistry of the epoxide. A trans epoxide (Eq. 9.49) give rise to a six-membered siloxane by a 6-endo pathway, whereas cis epoxides (Eqs. 9.50 and 9.51) undergo 5-exo cleavage. 

The second step of the procedure reported here is the usual epoxidation by meta-chloroperoxybenzoic acid. We have obtained the cis-epoxides (4) with different perfluoroalkyl (Rp) and aryl or alkyl (R) substituents 4... 

Crotti and co-workers work on regiochemical control of ring opening of epoxides by means of chelating agents has continued. Under standard conditions the regio-isomeric C(l) derivatives are the sole products from the trans epoxides (22a) and (22b) and are the predominant products from the cis epoxides (23a) and (23b). Under chelating conditions the cis epoxides unexpectedly show a consistent increase in C(2) selectivity. The results are discussed in terms of electronic and steric effects. 

Reaction conditions 0.25 mmol substrate, 0.25mmol Oxone , 30mg homogeneous catalyst or 300-500mg heterogeneous catalyst, 0.075mmol 4-PPNO, 4mL CHjCOOH, pH 7-8 by use of aqueous NaHCOj, RT, 25-50min. a cisitrans ratio, b [2R, 3R) (cis-epoxide). c (2S, 3R) (trans-epoxide). 

Stereospecific Epoxidation of 2-Butene. The hydroperoxide epoxidation reaction is stereospecific. Pure cis- and trans-2-butene were epoxi-dized separately by cumene hydroperoxide. The cis olefin gave exclusively cis epoxide, and the trans olefin gave exclusively trans epoxide. In both cases, the epoxide was the sole product formed from the olefin. They can be distinguished easily by their different retention times on a gas chromatography column of 20% diisodecyl phthalate on Chromosorb W(60-80 mesh). They were also identified by comparing their infrared spectra with authentic samples. 

The epoxidation of alkenes is completely stereoselective, i.e. cis-alkenes are transformed into cis-epoxides and frans-alkenes into trans-epoxides. 

Tungsten(VI)-peroxo complexes such as W0(02)2(HMPA)(H20) are also effective reagents for the selective transformation of alkenes to epoxides. cis-Alkenes such as m-2-butene are exclusively converted to cis-epoxides.123... 

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... 

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