Stereoselective oxidative

Oxidation of olefins and dienes provides the classic means for syntheses of 1,2- and 1,4-difunctional carbon compounds. The related cleavage of cyclohexene rings to produce 1,6-dioxo compounds has already been discussed in section 1.14. Many regio- and stereoselective oxidations have been developed within the enormously productive field of steroid syntheses. Our examples for regio- and stereoselective C C double bond oxidations as well as the examples for C C double bond cleavages (see p. 87f.) are largely selected from this area. 

Chiral Alcohols and Lactones. HLAT) has been widely used for stereoselective oxidations of a variety of prochiral diols to lactones on a preparative scale. In most cases pro-(3) hydroxyl is oxidized irrespective of the substituents. The method is apphcable among others to tit-1,2-bis(hydroxymethyl) derivatives of cyclopropane, cyclobutane, cyclohexane, and cyclohexene. Resulting y-lactones are isolated in 68—90% yields and of 100% (164,165). 

On the basis of precedent established previously by Still,9 the C2-C3 enone double bond in 18 is stereoselectively oxidized with potassium terf-butylperoxide to give a 4 1 mixture of stereoiso-... 

Metalloporphyrins as catalysts of chain transfer in radical polymerisation and stereoselective oxidation. L. Karmilova, G. V. Ponomarev, B. R. Smirnov and I. M. Bel yovskii, Russ. Chem. Rev. (Engl. Transl), 1984, 53,132 (44). 

Although a maj ority of research activities were dedicated to cycloketone converting BVMOs, the recently discovered novel MOs also enable stereoselective oxidation of noncyclic ketones to esters. An aliphatic open-chain monooxygenase (AOCMO) from Pseudomonas Jluorescens DSM 50106 displays stereoselective biooxidation of terminal acyl-groups in proximity to hydroxyls (Scheme 9.23). The biooxidation gives acetic... 

Of several procedures for the stereoselective oxidation of sulfides using organometallic complexes, two adaptations of Kagan s original process have gained prominence. In the first method the diol (36) is reacted with Ti(0 Pr)4 to form the catalyst. With cumyl hydroperoxide as the stoichiometric oxidant, methyl para-tolyl sulfide was converted into the optically active sulfoxide in 42 % yield (98 % ee)[109]. 

Nakamura, K., Inoue, Y., Matsuda, T. and Misawa, I., Stereoselective oxidation and reduction by immobilized Geotrichum candidum in an organic solvent. J. Chem. Soc. Perkin Trans. 1, 1999, 2397-2402. 

Figure 25 Stereoselective oxidation of sulfides catalyzed by SN37.4-Ru(ToCPP) and 3A3-MP8 complexes and regioselective nitration of phenol catalyzed by 3A3-MP8.

Several microbial alcohol oxidoreductases can catalyze the stereoselective oxidation of GLD. Quinohaemoprotein ethanol dehydrogenase of Acetobacter pasteurianus is able to oxidize stereospecifically (S)-GLD to (l )-glycidic acid in racemic GLD [41,42 ]. When washed cells of A. pasteurianus were incubated with 4.8 mg ml" of racemic GLD, (1 )-GLD was obtained, with an optical purity of 99.5% e. e. and 64% conversion. 

Stereoselective oxidation of alkyl-silanes into the corresponding alkyl-alcohols using peracids. 

Agusta A et ai. Stereoselective oxidation at C-4 of flavans by the endophytic fungus Diaporthe sp isolated from a tea plant, Chem Pharm Bull 53 1565—1569, 2005. 

The most widely studied transition metal is titanium. At various times, all oxidation states of titanium (II, III, IV) have been proposed for the active site of titanium-based initiators. Most of the evidence points to titanium (HI) as the most stereoselective oxidation state, although not necessarily the most active nor the only one [Chien et al., 1982]. (Data for vanadium systems indicate that trivalent vanadium sites are the syndioselective sites [Lehr, 1968].) Initiators based on the a-, y-, and 8-titanium trihalides are much more stereoselective (iso-selective) than those based on the tetrahalide or dihalide. By itself, TiCl2 is inactive as an initiator but is activated by ball milling due to disproportionation to TiCl3 and Ti [Werber et al., 1968]. The overall stereoselectivity is usually a-, y-, 8-TiCl , > TiCL > TiCLj P-TiCl3 [Natta et al., 1957b,c],... 

Brevetoxin B (79) is one of the most representative polyether compounds, which was isolated by Nakanishi, Clardy, and co-workers, from Gymnodynium breve Davis [36]. Nakanishi and co-workers postulated the biosynthetic pathway of 79 as shown in Scheme 7.21, that is, stereoselective oxidation of polyene 80 would give... 

The enzyme-catalyzed stereoselective oxidations of 1,2-dithiane and l,4-dihydro-2,3-benzodithiin were also investigated <2002CC1452>. Using naphthalene dioxygenase and chloroperoxidase, enantiomerically enriched sulfoxides (1,2-dithiane 1-oxides) were obtained l,4-dihydro-2,3-benzodithiin yielded a product of 32—47% ee with an excess of the (6)-configuration while 1,2-dithiane yielded almost enantiopure (96% ee) (R)-configured 1-oxide. Finally, 1,4-dihydro-2,3-benzodithiin 2-oxide was also prepared by perborate oxidation <1988JOC2608>. 

Horseradish peroxidase catalysed kinetic resolution of racemic secondary hydroperoxides has been described by Adam et al. [79]. The reaction yields (i )-hy-droperoxides up to ee>99% and (S)-alcohols up to ee>97%. Optically active hydroperoxides as potential stereoselective oxidants can be obtained by this process. 

BTSP also converts the thioxophosphoryl function to the oxophosphoryl function with inversion of configuration at P. For example, oxidation of tra .y-2-thio-2-chloro-5-methyl-1,3-dioxaphosphorinane 41 by BTSP in the presence of AICI3 gives the cw-isomer of the corresponding product of the oxygenation accompanied by less than 5% of the trans-isomer 42 (equations 64). P(III) esters can also be stereoselectively oxidized by BTSP, as demonstrated by the transformation of 2-chloro-4-methyl-l,3-dioxaphosphorinane to its oxygen derivative (equation 65)58. 

Dehydrogenases also represent a class of interesting enzymes since enantiose-lective reduction of ketones can lead to the production of enantiomerically pure secondary alcohols for the fine chemicals industry. Compared to liquid systems, in which the cofactor is often eliminated by the circulating phase in continuous systems, solid/gas catalysis can be highly suitable since it has been demonstrated that the cofactor is stable and its regeneration effective by addition of a second substrate. Also, stereoselective oxidation of secondary alcohols by these systems can help in the resolution of racemic mixtures. 

Intramolecular cyclization of ally lie stannanes.1 The pyrrolizidine alkaloid isoretronecanol (3) can be synthesized efficiently by cyclization of the mesylate of the hydroxylactam 1 to give 2 stereoselectively. Oxidative cleavage followed by reduction of the keto group gives 3. 

Epoxides can also be formed from the oxidation of alkenes by molecular oxygen via in situ generation of hydroperoxides by autoxidation.251,252 An interesting example is the direct stereoselective oxidation of cyclohexene by 02 to syn-l,2-epoxycyclohexan-3-ol catalyzed by CpV(CO)4 with a 65% yield and 99% stereoselectivity (equation 78).253... 

It should be noted that the related imine-oxaziridine couple E-F finds application in asymmetric sulfoxidation, which is discussed in Section 10.3. Similarly, chiral oxoammonium ions G enable catalytic stereoselective oxidation of alcohols and thus, e.g., kinetic resolution of racemates. Processes of this type are discussed in Section 10.4. Whereas perhydrates, e.g. of fluorinated ketones, have several applications in oxidation catalysis [5], e.g. for the preparation of epoxides from olefins, it seems that no application of chiral perhydrates in asymmetric synthesis has yet been found. Metal-free oxidation catalysis - achiral or chiral - has, nevertheless, become a very potent method in organic synthesis, and the field is developing rapidly [6]. 

---