Epoxidation of allylic alcohols, stereoselectivity

Stereoselective epoxidation of allylic alcohols (5,76 7,62-63). The effect of an allylic hydroxyl group on the stereoselectivity of epoxidation of a double bond is reinforced by a neighboring ether group. Typical examples are shown in equations (I) and (II). ... 

Takai, Oshima, and Nozaki achieved stereoselective epoxidation of allylic alcohols with Al(OBu )3-f-BuOOH (Sch. 16) ( )-allylic alcohols were converted predominantly into threo epoxy alcohols these are unfavorable products in epoxidations with VO(acac)2-f-BuOOH [36]. 

Molybdenum hexacarbonyl, Mo(CO)6 [328], and molybdenum ace-tylacetonate, Mo02(acac)2 [530], catalyze the stereoselective epoxidation of allylic alcohols with tert-butyl hydroperoxide. 

W. Adam, P. L. Alsters, R. Neumann, C. R. Saha-Moller, D. Seebach, A. K. Beck, R. Zhang, Chiral hydroperoxides as oxygen source in the catalytic stereoselective epoxidation of allylic alcohols by sandwich-type polyoxometalates Control of enantioselectivity through a metal-coordinated template, /. Org. Chem. 68 (2003) 8222. 

Both approaches are based on the stereoselective epoxidation of allylic alcohol 26.10 with mCPBA. Compound 26.10 was obtained from the known (ib)methyl bicyclofarnesate 26.1 and from (—)-drimenol (3.7), respectively, by the reaction sequences shown in Schemes 24 and 25, respectively. 

The generalised stereoselective epoxidation of allylic alcohols 1 by r-butyl hydroperoxide in the presence of titanium(IV) isopropoxide and tartrate esters to the epoxides 2 (Scheme 1.1) constitutes a seminal landmark in metal-mediated asymmetric oxidations. The catalytic version of this reaction is often the most effective procedure and is especially useful for the kinetic... 

Stereoselective epoxidation of allylic alcohols and oxidation of secondary alcohols have been achieved using organoaluminium peroxides prepared in situ (Scheme 11). Several types of organometallic reagent including triorgano-... 

Enantioselective epoxidation of allylic alcohols using hydrogen peroxide and chiral catalysts was first reported for molybdenum 7B) and vanadium 79) complexe. In 1980, Sharpless 80) reported a titanium system. Using a tartaric acid derivative as chiral auxiliary it achieves almost total stereoselection in this reaction. 

Chiral alkenyl and cycloalkenyl oxiranes are valuable intermediates in organic synthesis [38]. Their asymmetric synthesis has been accomplished by several methods, including the epoxidation of allyl alcohols in combination with an oxidation and olefination [39a], the epoxidation of dienes [39b,c], the chloroallylation of aldehydes in combination with a 1,2-elimination [39f-h], and the reaction of S-ylides with aldehydes [39i]. Although these methods are efficient for the synthesis of alkenyl oxiranes, they are not well suited for cycloalkenyl oxiranes of the 56 type (Scheme 1.3.21). Therefore we had developed an interest in the asymmetric synthesis of the cycloalkenyl oxiranes 56 from the sulfonimidoyl-substituted homoallyl alcohols 7. It was speculated that the allylic sulfoximine group of 7 could be stereoselectively replaced by a Cl atom with formation of corresponding chlorohydrins 55 which upon base treatment should give the cycloalkenyl oxiranes 56. The feasibility of a Cl substitution of the sulfoximine group had been shown previously in the case of S-alkyl sulfoximines [40]. 

Asymmetric epoxidation of ailylic alcohols.1 Epoxidation of allylic alcohols with r-bulyl hydroperoxide in the presence of titanium(lV) isopropoxide as the metal catalyst and either diethyl D- or diethyl L-tartrate as the chiral ligand proceeds in > 90% stereoselectivity, which is independent of the substitution pattern of the allylic alcohol but dependent on the chirality of the tartrate. Suggested standard conditions are 2 equivalents of anhydrous r-butyl hydroperoxide with 1 equivalent each of the alcohol, the tartrate, and the titanium catalyst. Lesser amounts of the last two components can be used for epoxidation of reactive allylic alcohols, but it is important to use equivalent amounts of these two components. Chemical yields are in the range of 70-85%. 

Selective epoxidation of allylic alcohols. This reagent is particularly useful for completely selective epoxidation of a double bond allylic to a hydroxyl group in the presence of another double bond. In this respect it is superior to t-BuOOH in combination with VO(acac)2, Al(0-f-Bu)3, or Ti(0-/-Pr)4. The stereoselectivity with 1 is fairly similar to that of f-BuOOH-VO(acac)2. 

There are also several situations where the metal can act as both a homolytic and heterolytic catalyst. For example, vanadium complexes catalyze the epoxidation of allylic alcohols by alkyl hydroperoxides stereoselectively,57 and they involve vanadium(V) alkyl peroxides as reactive intermediates. However, vanadium(V)-alkyl peroxide complexes such as (dipic)VO(OOR)L, having no available coordination site for the complexation of alkenes to occur, react homolyti-cally.46 On the other hand, Group VIII dioxygen complexes generally oxidize alkenes homolytically under forced conditions, while some rhodium-dioxygen complexes oxidize terminal alkenes to methyl ketones at room temperature. 

A density functional study of the transition structures of Ti-catalyzed epoxidation of allylic alcohol was performed, which mimicked the dimeric mechanism proposed by Sharpless et al.5 Importance of the bulkiness of alkyl hydroperoxide to the stereoselectivity, the conformational features of tartrate esters in the epoxidation transition structure, and the loading of allylic alcohol in the dimeric transition structure model were pointed out. 

Epoxidation. Oxone decomposes in the presence of a ketone (such as acetone) to form a species, possibly a dioxirane (a), which can epoxidize alkenes in high yield in reactions generally conducted in CH2C12-H20 with a phase-transfer catalyst. An added ketone is not necessary for efficient epoxidation of an unsaturated ketone. The method is particularly useful for preparation of epoxides that are unstable to heat or acids and bases.3 The acetone-Oxone system is comparable to m-chloroperbenzoic acid in the stereoselectivity of epoxidation of allylic alcohols. It is also similar to the peracid in preferential attack of the double bond in geraniol (dienol) that is further removed from the hydroxyl group.4... 

The known allylic alcohol 9 derived from protected dimethyl tartrate is exposed to Sharpless asymmetric epoxidation conditions with (-)-diethyl D-tartrate. The reaction yields exclusively the anti epoxide 10 in 77 % yield. In contrast to the above mentioned epoxidation of the ribose derived allylic alcohol, in this case epoxidation of 9 with MCPBA at 0 °C resulted in a 65 35 mixture of syn/anti diastereomers. The Sharpless epoxidation of primary and secondary allylic alcohols discovered in 1980 is a powerful reagent-controlled reaction.12 The use of titanium(IV) tetraisopropoxide as catalyst, tert-butylhydro-peroxide as oxidant, and an enantiopure dialkyl tartrate as chiral auxiliary accomplishes the epoxidation of allylic alcohols with excellent stereoselectivity. If the reaction is kept absolutely dry, catalytic amounts of the dialkyl tartrate(titanium)(IV) complex are sufficient. 

It has recently been reported495 that the complex CsH5V(CO)4 (CSHS = cy-clopentadienyl) is an efficient catalyst for the stereoselective oxidation of cyclohexene to ris-l,2-epoxycyclohexane-3-ol in good yield (65% at 10% conversion). This high stereoselectivity is reminiscent of the highly selective vanadium-catalyzed epoxidations of allylic alcohols with alkyl hydroperoxides discussed earlier. The mechanism of reaction,... 

In the course of our earlier studies on terpenylboranes we developed a simple transformation of a- into 3-pinene," and a stereoselective synthesis of allylic alcohols by the reduction of vinylic epoxides.12 An extension of these studies to contrathermo-dynamic isomerization of a-thujene, 2- and 3-carene,13 and kinetic resolution of vinylic epoxides by the reduction with terpenylboranes,14 is described. 

Epoxidation of allylic alcohols with peracids or hydroperoxide such as f-BuOaH in the presence of a transition metal catalyst is a useful procedure for the synthesis of epoxides, particularly stereoselective synthesis [587-590]. As the transition metal catalyst, molybdenum and vanadium complexes are well studied and, accordingly, are the most popular [587-590], (Achiral) titanium compounds are also known to effect this transformation, and result in stereoselectivity different from that of the aforementioned Mo- and V-derived catalysts. The stereochemistry of epoxidation by these methods has been compared for representative examples, including simple [591] and more complex trcMs-disubstituted, rrans-trisubstituted, and cis-trisubstituted allyl alcohols (Eqs (253) [592], (254) [592-594], and (255) [593]). In particular the epoxidation of trisubstituted allyl alcohols shown in Eqs (254) and (255) highlights the complementary use of the titanium-based method and other methods. More results from titanium-catalyzed diastereoselective epoxidation are summarized in Table 25. 

Recent literature refers to the stereoselective and asymmetric epoxidation of allylic alcohols with organoaluminium peroxides. PhaSiOOH epoxidizes olefins with a stereoselectivity similar to that with peracid. Reports have been made of a-substituted hydroperoxides (acids, esters, ketones, amides, and nitriles) as effective epoxidizing reagents and the application of hexachloroacetone, tetrachloracetone, and hexafluoroacetone hydroperoxide, as well as the HaOa-Vilsmeier reagent system. ... 

The first practical asymmetric epoxidation of primary and secondary allylic alcohols was realized by Sharpless and Katsuki in 1980. They discovered that the use of titanium(IV) tetraisopropoxide, tert-butylhydroperoxide (TBHP), and an enantiopure chiral auxiliary (such as diethyl tartrate (6)) accomplishes the epoxidation of allylic alcohols with excellent stereoselectivity ee > 90 %) to give chiral 2,3-epoxy alcohols, which represent valuable building blocks. In acknowledgement of the discovery and of his outstanding contributions to the development of this reaction. Sharpless was awarded one half of the Nobel Prize in Chemistry in 2001. ... 

---