Achiral oxidants

Cyclodextrin mediated oxidation ofprochiral sulphides by achiral oxidation reagents leads also to optically active sulphoxides (e.e. up to 30%). When oxidation was carried out in pyridine the highest optical purities were obtained294 with hydrogen peroxide, whereas in water the best results were observed with m-chloroperbenzoic acid295.. 

As shown in Table 12,H202 and fBuOOH have been used frequently as oxygen donors in peroxidase-catalyzed sulfoxidations. Other achiral oxidants, e.g. iodo-sobenzene and peracids, are not accepted by enzymes and, therefore, only racemic sulfoxides were found (c.f. entries 34-36). Interestingly, racemic hydroperoxides oxidize sulfides to sulfoxides enantioselectively under CPO catalysis [68]. In this reaction, not only the sulfoxides but also the hydroperoxide and the corresponding alcohol were produced in optically active form by enzyme-catalyzed kinetic resolution (cf. Eq. 3 and Table 3 in Sect. 3.1). 

A catalytic asymmetric oxidation of mono-, di-, and tri-substituted alkenes using a chiral bishydroxamic acid (BHA) complex of molybdenum catalyst in air at room temperature leads to good to excellent selectivity. It has been suggested that the Mo-BHA complex combines with the achiral oxidant to oxidize the alkene in a concerted fashion by transfer of oxygen from the metal peroxide to the alkene.78 The chiral BHA-molybdenum complex has been used for the catalytic asymmetric oxidation of sulfides and disulfides, utilizing 1 equiv. of alkyl peroxide, with yields up to 83% and ees up to 86%. An extension of the methodology combines the asymmetric oxidation with kinetic resolution providing excellent enantioselectivity (ee = 92-99%).79... 

Selenoxides derived from unsymmetrical selenides are chiral and stable toward pyramidal inversion at room or even higher temperatures. They are produced enantioselectively by the use of chiral oxidants such as the Sharpless reagent or camphor-derived oxaziridines or diastereoselectively with achiral oxidants when one of the selenide substituents is itself chiral (see Section 9). Racemic selenoxides have been resolved by chromatography over chiral adsorbents. Chiral selenoxides racemize readily in water, particularly under acid-catalyzed conditions, presumably via the intermediacy of achiral selenoxide hydrates (equation 2). 

Achiral oxidants in the presence of chiral auxiliaries or other... 

Achiral Oxidants in the Presence of Chiral Auxiliaries or Other Chiral Oxidants... 

Unsaturated chiral a-hydroxy esters are of interest due to possible reactions with the carbon double bond. They can easily be obtained from optically active cyanohydrins according to the Pinner method [35a]. Epoxidation of the chiral alcohols 9 with achiral oxidants, e.g., m-chloroperoxybenzoic acid (m-CPBA), yields a mixture of both possible epoxides 10a and 10b. With chiral (Sharpless titanium tartrate system) oxidants stereoselective epoxidation results. Using (+) dimethyl tartrate [(+) DMT] only the erythro isomer 10a is obtained (Scheme 6) [35b]. 

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