Epoxidation conjugated enyne

Subsequently, high chemoselectivity and enantioselectivity have been observed in the asymmetric epoxidation of a variety of conjugated enynes using fructose-derived chiral ketone as the catalyst and Oxone as the oxidant. Reported enantioselectivities range from 89% to 97%, and epoxidation occurs chemoselectively at the olefins. In contrast to certain isolated trisubstituted olefins, high enantioselectivity for trisubstituted enynes is noticeable. This may indicate that the alkyne group is beneficial for these substrates due to both electronic and steric effects. 

Wang, Z.-X., Cao, G.-A., Shi, Y. Chiral Ketone Catalyzed Highly Chemo- and Enantioselective Epoxidation of Conjugated Enynes. J. Org. 

A one-step Lewis acid-catalysed intermolecular 4- -3-cycloaddition of aromatic a,)3-unsaturated aldehyde and ketones (105) with epoxides (106) formed seven-membered oxacycles (107) under mild conditions (Scheme 34).The effect of oxygen-, sulfur-, and halogen-substituents on the reactivity of nitrogen-stabilized oxyallyl cations in 4- -3-cycloaddition reactions has been extensively investigated. Aza-oxyallyl cationic intermediates react with cyclopentadiene and furan via an aza-4 -I- 3-cycloaddition reaction to form bicyclic cycloadducts in moderate yields. The intramolecular formal 4- -4-cycloaddition of conjugated enynes with an e-deflcient cyclobutene (108) yielded a strained six-membered cyclic allene (109) that isomerized to a 1,3-cyclohexadiene (110). This intermediate underwent a thermal or acid-promoted six-electron electrocyclic ring opening to yield a 2,4,6-cyclooctatrienone (111) (Scheme 35). ... 

Wang ZX, Cao GA, Shi Y. Chiral ketone catalyzed highly chemo- and enantioselective epoxidation of conjugated enynes. J. Org. Chem. 1999 64(20) 7646-7650. 

In 1996, Shi et al. reported fructose-derived ketone 376 as a highly reactive and enantioselective (<90% ee) catalyst for the epoxidation of tran -olefins, trisubsti-tuted olefins, fluorooleflns, conjugated enynes, and silyl enol ethers with20-30mol% catalyst loading at an elevated pH of 10 [256]. This ketone can be prepared from... 

Styrenes [103], conjugated aT-dienes [107], and aT-enynes [108] are also epoxidized with ketones 57 in high ees (Table 5, entries 9-14). No isomerization of the epoxides was observed therefore only c/x-epoxides were obtained from cis-olefins. Alkenes and alkynes appear to be effective directing groups to favor the desired transition states T and V (Fig. 19). 

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. 

Asymmetric Epoxidation of Representative Conjugated Dienes and Enynes by Ketone 1... 

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). 

Epoxidation and other oxidation reactions. Regenerative pie-oxidants of the type 156A are derived from a pyranose. They are employed in conjunction with a expendable agent, such as oxone for epoxidation of conjugated cw-enynes, and H2O2 to epoxidize alkenes. ... 

Asymmetric epoxidation of 1,3-dienes and eynes. Jacobsen has extended his method for effecting asymmetric epoxidation of alkencs with a chiral (salen)Mn (II) complex (16, 157-158) to conjugated dienes and enynes by use of the chiral (salcn)Mn(IH) complex 2 ... 

Disubstituted vinyl silanes are epoxidized in high ee s and enantiomerically enriched 1,1-disubstituted epoxides can be obtained via the desilylation of these epoxides e.g., 18). Allylic alcohols and conjugated dienes and enynes are effective substrates (e.g. 19 and 20). ° The epoxidations of enol ethers and enol esters were also studied. The resulting epoxides (e.g., 21) from enol esters can undergo stereoselective rearrangement to give optically active a-acyloxy ketones, (5)-22 or (i )-23, under different acidic conditions. 

This catalytic system provides high enantioselectivities for a range of epoxides (Figure 19.2), including those derived from trisubstituted and traus-1,2-disubstituted alkenes, with complete stereospecificity (retention of the alkene geometry in the epoxide product) [17]. The reaction has been shown to be chem-oselective for the alkene of enynes [18], provided monoepoxides upon reaction with conjugated trans-dienes [19] and afforded up to 93% ee for the asymmetric epoxidation of fluoro-olefins [20]. However, decreased enantioselectivity was observed for both cis- and terminal alkenes. The catalytic system has also been applied to the resolution and desymmetrization of cyclic trisubstituted alkenes [21]. 

The development of the oxazolidinone catalyst has allowed Shi to further increase the scope of the epoxidation to a range of new substrates. Using ketone catalyst 5 styrene is epoxidized with 24% ee, while the use of oxazolidinone 13a for the same transformation gave 86% ee, with further improvement in enantioselectivity seen with the inclusion of electron-donating substituents on the aromatic ring of the styrene substrates [43]. ds/tra s-Dienes and trienes were epoxidized regioselectivity at the ds-alkene and no isomerization or further epoxidation of the products were observed [44], while epoxidation of conjugated dx-enynes resulted in chemoselective epoxidation of the alkene [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],... 

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