Amine oxides asymmetric epoxidation

Zhang Z, Tang J, Wang X, Shi H. Chiral ketone- or chiral amine-catalyzed asymmetric epoxidation of cis-l-propenyl-phosphonic acid using hydrogen peroxide as oxidant. J. Mol. Catal. A Chem. 2008 285 68-71. 

In conjunction with the chiral anion TRIP (156) (10 mol%), diamine 157 (10 mol%) can be used in the catalytic asymmetric epoxidation of a,p-unsaturated ketones (>90% ee) [196], while the secondary amine 158 (10 mol%) can be used for the epoxidation of both di- and trisubstituted a,P-unsaturated aldehydes (92-98% ee) (Fig. 15) [211], The facile nature of these reactions, using commercially available peroxides as the stoichiometric oxidant, together with the synthetic utility of the epoxide products suggests application in target oriented synthesis. 

The [3-hydroxy amines are a class of compounds falling within the generic definition of Eq. 6A.6. When the alcohol is secondary, the possibility for kinetic resolution exists if the Ti-tartrate complex is capable of catalyzing the enantioselective oxidation of the amine to an amine oxide (or other oxidation product). The use of the standard asymmetric epoxidation complex (i.e., T2(tartrate)2) to achieve such an enantioselective oxidation was unsuccessful. However, modification of the complex so that the stoichiometry lies between Ti2 (tartrate) j and Ti2(tartrate)1 5 leads to very successful kinetic resolutions of [3-hydroxyamines. A representative example is shown in Eq. 6A.11 [141b,c]. The oxidation and kinetic resolution of more than 20 secondary [3-hydroxyamines [141,145a] provides an indication of the scope of the reaction and of some... 

In practice in the literature of the past 20 years the important results with ruthenium in epoxidation are those where ruthenium was demonstrated to afford epoxides with molecular oxygen as the terminal oxidant. Some examples are presented (see later). Also ruthenium complexes, because of their rich chemistry, are promising candidates for the asymmetric epoxidation of alkenes. The state of the art in the epoxidation of nonfunctionalized alkenes is namely still governed by the Jacobsen-Katsuki Mn-based system, which requires oxidants such as NaOCl and PhIO [43,44]. Most examples in ruthenium-catalysed asymmetric epoxidation known until now still require the use of expensive oxidants, such as bulky amine oxides (see later). 

Asymmetric epoxidation of unfunctionalized alkenes by NaOCl with 9.38B-type catalysts is found to be substantially accelerated in the presence of near-catalytic quantities of amine N-oxides. What is the mechanistic significance of this observation ... 

Permanganate oxidation of 1,5-dienes to prepare f r-2,5-disubstituted tetrahydrofurans is a well-known procedure (Equation 80). The introduction of asymmetric oxidation methodology has revived interest in this area. Sharpless-Katsuki epoxidation has found widespread application in the catalytic enantioselective synthesis of optically active tetrahydrofurans and the desymmetrization of w ro-tetrahydrofurans <2001COR663>. A general stereoselective route for the synthesis of f-tetrahydrofurans from 1,5-dienes has been developed which uses catalytic amounts of osmium tetroxide and trimethyl amine oxide as a stoichiometric oxidant in the presence of camphorsulfonic acid <2003AGE948>. 

Alcohols can be obtained from many other classes of compounds such as alkyl halides, amines, al-kenes, epoxides and carbonyl compounds. The addition of nucleophiles to carbonyl compounds is a versatile and convenient methc for the the preparation of alcohols. Regioselective oxirane ring opening of epoxides by nucleophiles is another important route for the synthesis of alcohols. However, stereospe-cific oxirane ring formation is prerequisite to the use of epoxides in organic synthesis. The chemistry of epoxides has been extensively studied in this decade and the development of the diastereoselective oxidations of alkenic alcohols makes epoxy alcohols with definite configurations readily available. Recently developed asymmetric epoxidation of prochiral allylic alcohols allows the enantioselective synthesis of 2,3-epoxy alcohols. 

A range of structurally different chiral primary amines was converted into the corresponding iminium tetraphenylborate salts (Fig. 5.3) and tested in the asymmetric epoxidation of a standard test substrate, 1-phenylcyclohexene, using Oxone (4 equiv) as the stoichiometric oxidant, sodium carbonate (8 equiv) as base, in acetonitrile/water (2 1) at 0 °C (Table 5.1) [19,21]. 

More recently. List and co-workers [169], have reported the asymmetric epox-idation of cyclic enones, using a chiral primary diamine (111) and a phosphoric acid derived from BINOL (112) (Scheme 12.29). With H2O2 as oxidant, the epoxides were obtained in good yields (63-82%) and moderate to good enantioselectivities (78-98%). They also tested amine 113, which provided better ee s (92 to 99%) and slightly lower yields (49-85% (Scheme 12.29). 

Conjugated ester 6.271 was treated with diisobutylaluminum hydride to reduce the ester moiety to an alcohol. This allylic alcohol was subjected to Sharpless asymmetric epoxidation. 1 Opening the epoxide (5.272) with azide was followed by protection of the diol moiety to give 5.273.154 Reduction of the azide, protection of the amine, and oxidation gave lactam 5.274.154 Acid hydrolysis gave 4-amino-2,3-dihydroxy-3-methylbutanoic acid, 6.275, a degradation product of carzinophilin.i55[Pg.231]

Previous reviews have dealt with metal-catalyzed [93] and stoichiometric [94] oxidation of amines in a broad sense. This section will be limited to the selective oxidation of tertiary amines to N-oxides. Amine N-oxides are synthetically useful compounds [95, 96] and are frequently used as stoichiometric oxidants in osmium-[97-99] manganese- [100] and ruthenium-catalyzed [101,102] oxidations, as well as in other organic transformations [103-105]. Aliphatic tert-amine N-oxides are usefid surfactants [96] and are essential components in hair conditioners, shampoos, toothpaste, cosmetics, and so on [106]. Chiral N-oxides have been used in asymmetric catalysis involving metal-free catalytic transformations [107] as well as metal-catalyzed reactions where the N-oxide serves as a ligand [107, 108]. Chiral tertiary amine N-oxides were recently used as reagents in asymmetric epoxidation of a,(3-unsaturated ketones [109]. 

Some representative data showing the use of chiral Ru-porphyrins for asymmetric epoxidation using amine oxides or PtilO as 0-donors are given in Table 1. The Ru(porp-D4)CO/Cl2pyNO system effects epoxidation with 5-88% yields and 28-77% ee values. Either Ru(porp-D4)CO or Ru(porp-D4)-... 

---