Adams catalyst epoxidation

Regardless of the mechanism, the chiral (salen)Mn-mediated epoxidation of unfunctionalized alkenes represents a methodology with constantly expanding generality. Very mild and neutral conditions can be achieved, as illustrated by Adam s epoxidation of chromene derivatives 12 using Jacobsen-type catalysts and dimethyldioxirane as a terminal oxidant [95TL3669]. Similarly, periodates can be employed as the stoichiometric oxidant in the epoxidation of cis- and tram-olefins [95TL319],... 

In reviewing the earlier chemical and spectral data, the presence of the epoxide moiety explains several observations. Excelsine is reduced with Raney nickel in methanolic base to lapaconidine (26), but is inert to reduction with Adams catalyst, sodium borohydride, or lithium aluminium hydride. Treatment of excelsine with boiling aqueous HCl gives an epimeric mixture of chlorohydrins. Hydrolysis with... 

The presence of the epoxide moiety at C-3 and C-4 in excelsine explained the interesting chemical reactions observed earlier. On treatment with acetic anhydride and p-toluenesulfonic acid, excelsine yielded a triacetate derivative, while treatment with acetyl chloride afforded a tetraacetate derivative. On reduction with Raney nickel in methanolic base, excelsine yielded lapaconidine (92), but was inert toward other reducing agents, e.g., lithium aluminum hydride, sodium borohydride, and Adams catalyst. Treatment of excelsine with boiling aqueous hydrochloric acid yielded an epimeric mixture of chlorohydrins with molecular formula C22H34NO6CI. These epimers were hydrolyzed to the crystalline compound C22H33NO6 when treated with aqueous sulfuric acid. This compound formed a tetraacetate derivative for which structure 105 was proposed on the basis of spectral data. 

Binaphthol- and biphenyl-derived ketones (9 and 10) were reported by Song and coworkers in 1997 to epoxidize unfunctionalized alkenes in up to 59% ee (Fig. 3, Table 1, entries 9, 10) [37, 38]. Ketones 9 and 10 were intended to have a rigid conformation and a stereogenic center close to the reacting carbonyl group. The reactivity of ketones 9 and 10 is lower than that of 8, presumably due to the weaker electron-withdrawing ability of the ether compared to the ester. In the same year, Adam and coworkers reported ketones 11 and 12 to be epoxidation catalysts for several trans- and trisubstituted alkenes (Table 1, entries 11,12). Up to 81% ee was obtained for phenylstilbene oxide (Table 1, entry 25) [39]. 

Compared to metal-catalyzed asymmetric epoxidation reactions, asymmetric versions of this reaction without the need of a catalyst (apart from a base) are rarely known. In 2000 Adam and coworkers reported a method for the asymmetric Weitz-Scheffer epoxidation of substituted enones 91 by the secondary, optically active hydroperoxide (5 )-(l-phenyl)ethyl hydroperoxide (equation 27, Table 10). ... 

Many other variations of the basic structure 10 have been explored, including an-hydro sugars and carbocyclic analogs, the latter derived from quinic acid 13 [23-26]. In summary, the preparation of these materials (e.g. 14-16) requires more synthetic effort than the fructose-derived ketone 10. Occasionally, e.g. when using 14, catalyst loadings can be reduced to 5% relative to the substrate olefin, and epoxide yields and selectivity remain comparable with those obtained by use of the fructose-derived ketone 10. Alternative ex-chiral pool ketone catalysts were reported by Adam et al. The ketones 17 and 18 are derived from D-mannitol and tartaric acid, respectively [27]. Enantiomeric excesses up to 81% were achieved in the epox-idation of l,2-(E)-disubstituted and trisubstituted olefins. 

The N-benzylcinchonidinium catalyst 70 was successfully employed by Barrett et al. in the synthesis of (—)-preussomerin G [112]. As a key step the epoxide 72 was obtained from the quinone acetal 71 in 81% yield and with 95% ee in the presence of 10 mol% of the ammonium salt 70 (Scheme 10.15). Adam et al. recently reported the highly enantioselective epoxidation of isoflavones [113]. The best results, i.e. ee up to 98% with essentially quantitative yields, were achieved when... 

Epoxy alcohols. A few years ago Mihelich1 was granted a patent for preparation of epoxy alcohols by photooxygenation of alkenes in the presence of titanium or vanadium catalysts. Adam et al.2 have investigated this reaction in detail and find that Ti(IV) isopropoxide is the catalyst of choice for epoxidation of di-, tri-, and tetrasubstituted alkenes, acyclic and cyclic, to provide epoxy alcohols. When applied to allylic alcohols, the reaction can be diastereo- and enantioselective. The reaction actually proceeds in two steps an ene reaction to provide an allylic hydroperoxide followed by intramolecular transfer of oxygen catalyzed by Ti(0-i-Pr)4. The latter step is a form of Sharpless epoxidation and can be highly stereoselective. 

Adam et al. also successfully applied Wynberg s condition to the asymmetric epoxidation of isoflavones [22], Using the chiral PTC 18 and cumyl hydroperoxide (CHP) as an oxidant, the isoflavone epoxide was obtained almost quantitatively and with excellent enantio selectivity (up to 98% ee) even at a low catalyst loading (1 mol%) (Scheme 5.16). 

A new variant of the Sharpless epoxidation is a one-pot procedure. It is known that on photochemical oxidation of olefins in the presence of tetraphenylporphine the corresponding allyl hydroperoxides are formed. The latter were partly reduced into allylic alcohols, which were epoxidized by the remaining hydroperoxide in the presence of transition metal catalysts. By addition of l-(+)-DET to the photochemical oxidation of 2,3,3-trimethyl but-l-ene, Adam et al. [16] succeded in preparing the (S)-epoxy alcohol in a good yield, while the epoxidation under standard conditions delivers only a smaller amount of product (Scheme 10). 

Adam W, Rao PB, Degen HG, Levai A, Patonay T, Saha-Moller CR. Asymmetric Weitz-Scheffer epoxidation of iso-flavones with hydroperoxides mediated by optically active phase-transfer catalysts. J. Org. Chem. 2002 67(l) 259-264. 

In ketones 39, 57, and 68, the ketals or oxazolidinone are placed at a-positions of the carbonyl. Studies with ketones 71-73 showed that moving the ketal from the a-to P-positions lowered the enantioselectivity for the epoxidation, su esting that placing the stereogenic centers close to the carbonyl is important for an efficient stereochemical communication between the substrate and the catalyst (Scheme 3.52) [73, 86]. Adam and coworkers also reported their studies on ketones 71 and 74, and up to 87% ee was obtained with 71 (Scheme 3.53) [87]. 

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