Epoxidation of conjugated dienes

Epoxidation of conjugated dienes can be regioselective when one double bond is more electron-rich than the other otherwise mixtures of mono- and diepoxides will be obtained. When the alkene contains an adjacent stereocenter, the epoxidation can be diastereoselective [2]. Hydroxy groups can function as directing groups, causing the epoxidation to take place syn to the alcohol [2, 3]. 

Asymmetric epoxidation of conjugated dienes and enynes catalysed by the chiral man-ganese(EI) complex 232 give monoepoxides exclusively reactions of cw-enynes give /raw.v-alkynyl epoxides as the major products with a high level of asymmetric induction362. 

Epoxidations. Further utilities for the dioxirane derived from fructose is the regio-and enantioselective epoxidation of conjugated dienes and enynes. Continuing explorations of the (salen)manganese(III) systems have uncovered the catalyzed epoxidation of 2-sulfonyl-1,3-dienes at the nondeactivated sites, and the advantage of added carboxylate salt as cocatalyst. ... 

Compelling evidence for stepwise C-O bond formation in [Mn(salen)] -catalyzed epoxidation is found in the formation of both cis- and trans-epoxides as primary products from acyclic ds-olefins [68]. The extent of frans-epoxide formation depends strongly on the nature of the substrate. Whereas simple alkyl-substituted ds-olefins are epoxidized stereospecifically, aryl-substituted cis-olefins afford mixtures of cis- and trans-epoxides with the ds-isomers being formed selectively. Epoxidations of conjugated dienes and enynes also afford cis/trans mixtures, with the frans-epoxide product predominating. These observations may be interpreted according to a stepwise mechanism in which a discrete radical intermediate undergoes competitive collapse to ds-epoxide and rotation/collapse to frans-epoxide (Scheme 4). 

Tan, H., and I.H. Espenson, Kinetics and Mechanism of the Dihydroxylation and Epoxidation of Conjugated Dienes with Hydrogen Peroxide Catalyzed by Methyhhenium Trioxide, Inorg. Chem. 37 467-472 (1998). 

Scheme 3.32 Epoxidation of conjugated dienes with ketone 39 [45]...

After experimentation it was found that Baeyer-Villiger oxidation could be suppressed and the amount of catalyst could be reduced to 20 mol% if the reaction was carried out between pH 10 and 11 (Scheme 1.13) [33,34], Yields were increased (65-95%) and the catalytic system resulted in slightly higher ees (91-97% ee). The synthetic utility of this system has been widely explored with the successful asymmetric epoxidation of various hydroxyalkenes (90-94% ee) [35], enol ethers and enol esters (80-91% ee) [36], enynes (90-97% ee) [37], vinylsilanes (84-94% ee) [38], cis-aUcenes (84-97% ee)[39, 40], terminal alkenes (30-94% ee) [41], and mono-epoxidation of conjugated dienes (90-97% ee) [42],... 

Conjugated dienes can be epoxidized to provide vinylepoxides. Cyclic substrates react with Katsuki s catalyst to give vinylepoxides with high ees and moderate yields [17], whereas Jacobsen s catalyst gives good yields but moderate enantiose-lectivities [18]. Acyclic substrates were found to isomerize upon epoxidation (Z, )-conjugated dienes reacted selectively at the (Z)-alkene to give trans-vinylepoxides (Scheme 9.4a) [19]. This feature was utilized in the formal synthesis of leuko-triene A4 methyl ester (Scheme 9.4b) [19]. 

Some derivatization methods mentioned in other sections of this review include chemical ionization by nitric oxide (MS) or epoxidation (MS), formation of jr-complexes for NMR (shift agents) etc. Also, the Diels-Alder reaction, which was mentioned several times as a tool for derivatization of conjugated dienes and polyenes, was extensively described and reviewed in the literature. 

Asymmetric monoepoxidation of conjugated dienes has been accomplished via (salen)Mn(III)-catalyzed [salen = A,Ar/-bis(salicylidene)ethylenediamine] oxidation. The reaction exhibits regioselectivity for attack at cis double bonds of cA.traws-conjugated dienes, and affords trans epoxides as the major products from cis olefins33. Thus, diene 14 gave optically active fraws-vinylepoxide 15 as the major product with 87% ee as shown in equation 16. 

In a related study, Jorgensen has examined the regio- and enantioselective catalytic epoxidation of conjugated aliphatic dienes using achiral and chiral manganese salen complexes and sodium hypochlorite or iodosylbenzene as the terminal oxidant. For most substrates, the less substituted diene is epoxidized however, in the case of isoprene, the more highly substituted double bond is the more reactive. Jorgensen proposes an intermediate of type 11, the... 

The thermal rearrangement of unsaturated bicyclic 1,4-peroxides, readily available from the reaction of conjugated dienes with singlet oxygen, is a convenient route fw the preparation of bisepoxides. Epoxidation of cholesteryl acetate (176) with air in the presence of a catalytic amount of dioxo(tetra-mesitylporphyrinato)ruthenium(VI) fimiishes in 8S% yield a 99 1 mixture the epoxides (177a) aitd (177b)." >... 

Hie preparation of allylic epoxides from conjugated dienes requires careful control of pH, since allylic epoxides undergo ring opening by nucleophiles at acidic pH. The allylic epoxide (12) has been prepared from the diene (11 equation 5) however, the ring opening of (12) cannot be totally suppressed. 

However, 1,2-diamino-l,2-di-(cr(-butylethane (3) holds particular interest because of its increased steric bulk and the absence of benzylic protons. Its recent ready availability should render it as attractive as the frequently used vicinal diamines 7 and 8. To our knowledge, only one application of this diamine has been previously described in the literature (eq 5), where the regio- and enantioselective epoxidation of conjugated aliphatic dienes were studied using the chiral manganese salen complex (9). 

Difunctionalization of conjugated dienes. Dienes are metallated by treatment with active Mg. A variety of compounds can be prepared by the subsequent reaction with electrophiles such as epoxides and CO2. 

The success of the enantioselective Sharpless epoxidation to a large extent depends on the configuration of the double bond of an available substrate. Shibasaki et al. developed an efficient method of stereospecific synthesis of exocychc double bonds using 1,4-hydrogenation of conjugated dienes catalyzed by an arene-Cr(CO)3 complex [43]. Allylic alcohol 31b [44,45], a substrate for Sharpless epoxidation, was obtained using this procedure. Alcohol... 

Sharifi, M. and J. Einhorn, Isomeric Differentiation of Conjugated Diene Epoxides by Polar... 

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