Oxidation enantioselectivity

The most common oxidation states and the corresponding electronic configurations of osmium ate +2 and + (t5 ), which ate usually octahedral. Stable oxidation states that have various coordination geometries include —2 and 0 to +8 (P] The single most important appHcation is OsO oxidation of olefins to diols. Enantioselective oxidations have also been demonstrated. 

In a first step, JS ocardia asteroides selectively oxidizes only (3)-pantolactone to ketopantolactone (19), whereas the (R)-pantolactone remains unaffected (47). The accumulated ketopantolactone is stereospecificaHy reduced to (R)-pantolactone in a second step with Candidaparapsilosis (product concentration 72 g/L, 90% molar yield and 100% ee) (48). Racemic pantolactone can also be converted to (R)-pantolactone by one single microbe, ie, Jiodococcus erythropolis by enantioselective oxidation to (3)-pantolactone and subsequent stereospecific reduction in 90% yield and 94% ee (product concentration 18 g/L) (40). 

Although alcohol dehydrogenases (ADH) also catalyze the oxidation of aldehydes to the corresponding acids, the rate of this reaction is significantly lower. The systems that combine ADH and aldehyde dehydrogenases (EC 1.2.1.5) (AldDH) are much more efficient. For example, HLAD catalyzes the enantioselective oxidation of a number of racemic 1,2-diols to L-a-hydroxy aldehydes which are further converted to L-a-hydroxy acids by AldDH (166). 

Enantioselective oxidation of cyclic dithioacetals to monosulfoxides catalyzed by bacterial cyclohexanone monooxygenases 96CC2303. 

Advances in synthesis and research of oligomeric tetrathiafulvalenes 97MI9. Enantioselective oxidation of 1,3-dithiolanes to corresponding S-oxides and S,S-dioxides by designer yeast 99JHC1533. 

This thoroughly revised and updated new edition is a must for every synthetic organic chemist. New material has been added on homogene- ous diastereoselective hydrogenations, enantioselective oxidations, and novel chiral auxiliaries. 

Recently, one of the evolved MAO-N variants has been shown to catalyze the enantioselective oxidation of O-methyl-N-hydroxylamines (e.g. 16) yielding the corresponding -oxime (17) and recovered (R)-hydroxylamine (16) in enantiomeri-cally pure form (Figure 5.11) [21]. 

Encapsulation in Y zeohte was also the method chosen to immobihze Mn complexes of C2-symmetric tetradentate hgands (Fig. 24) [75]. These materials were used as catalysts for the enantioselective oxidation of sulfides to sulfoxides with NaOCl. The lack of activity when the larger io-dosylbenzene was used as an oxidant was interpreted as an indication that the reaction took place inside the zeolite microporous system. Both the chemo- and enantioselectivity were dependent on the structure of the sulfide. (2-Ethylbutyl)phenylsulfide led to better results than methylphenylsulfide, although in all cases the enantioselectivity was low (up to 21% ee). 

Figure 6 Enantioselective oxidation of secondary alcohols with secondary alcohol dehydrogenase (SADH), from a thermophihc bacterium (log E is used in...

Ohta and coworkers used a bacterium, Corynebacterium equi IFO 3730, rather than a fungus, to oxidize eight alkyl phenyl and p-tolyl sulfides to their respective sulfoxides (119, 120) of configuration R. Virtually all of the sulfur compounds were accounted for as the sum of uncreacted sulfide, sulfoxide and sulfone. The enantiomeric purities of the sulfoxides obtained were quite good and are shown below in parentheses. The formation of the allyl sulfoxides in high optical purity is noteworthy. The authors believe that the sulfoxides were formed by enantioselective oxidation of the sulfides rather than by enantioselective oxidation of racemic sulfoxides, since the yield of sulfoxides was greater than 50% in five of the ten oxidations reported (see also Reference 34). 

The mechanism by which the enantioselective oxidation occurs is generally similar to that for the vanadium-catalyzed oxidations. The allylic alcohol serves to coordinate the substrate to titanium. The tartrate esters are also coordinated at titanium, creating a chiral environment. The active catalyst is believed to be a dimeric species, and the mechanism involves rapid exchange of the allylic alcohol and /-butylhydroperoxide at the titanium ion. 

These reagents exhibit good stereoselectivity toward chiral reactants, such as acylox-azolidinones.253 Chiral oxaziridine reagents have been developed that can achieve enantioselective oxidation of enolates to a-hydroxyketones.254... 

Mejorado, L. H. Pettus, T. R. R. Total synthesis of (+ )-rishirilide B development and application of general processes for enantioselective oxidative dearomatization of resorcinol derivatives. J. Am. Chem. Soc. 2006, 128, 15625-15631. 

Optically active oxaziridines are useful reagents for the enantioselective oxidation of olefins 37 39). The following three preparative methods to make this reagent available have been reported enantioselective oxidation of an imine by (-)-peroxycam-phoric acid 37,38), photocyclization of a nitrone which has a chiral substituent39), and photocyclization of a nitrone in an optically active solvent 39). However, an... 

Clark, D.S., Geresh, S. and DiCosimo, R. (1995) Enantioselective oxidation of 2-methyl-1-alkanols by alcohol oxidase from methylotrophic yeasts. Bioorganic Medicinal Chemistry Letters, 5 (13), 1383-1388. 

Pezzotti, F. and Therisod, M. (2007) Enantioselective oxidation of thioanisole with an alcohol oxidase/ peroxidase bienzymatic system. Tetrahedron Asymmetry, 18 (6), 701-704. 

Adam, W., Lazarus, M., Boss, B. et al. (1997) Enzymic resolution of chiral 2-hydroxy carboxylic acids by enantioselective oxidation with molecular oxygen catalyzed by the glycolate oxidase from spinach (Spinacia oleracea). The Journal of Organic Chemistry, 62 (22), 7841-7843. 

Adam, W., Lazarus, M., Saha-Moller, C.R. and Schreier, P. (1998) Quantitative transformation of racemic 2-hydroxy acids into (R)-2-hydroxy acids by enantioselective oxidation with glycolate oxidase and subsequent reduction of 2-keto acids with D-lactate dehydrogenase. Tetrahedron Asymmetry, 9 (2), 351-355. 

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