Epoxidation and cis-Dihydroxylation of Alkenes

Epoxides are an important and extremely versatile class of organic compounds, and the development of new methods for the selective epoxidation of alkenes continues to be a major challenge [2, 12, 43]. The epoxidation of alkenes can be achieved by 

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Epoxidation and cis-DIhydroxYlation of Alkenes 385 Table 11.3 Epoxidation of alkenes by Mn(lll)-salen complexes employing H2O2 as oxidant. 

The addition of borane to alkenes is stereospecifically cis and leads to the formation of tri-alkylboranes. These may be oxidized to alcohols using the anion of hydrogen peroxide. Overall addition of water is achieved, in a c/s-stereospecific, anti-Markovnikov manner. Hydroboration/oxidation of alkynes gives ketones, after tautomerization of the enol formed. c/s-Dihydroxylation of alkenes is accomplished with catalytic OSO4 plus an oxidant such as NMO or K3[Fe(CN)g]. This contrasts with the formation of frans-diols by epoxidation of alkenes followed by the opening of the epoxide with hydroxyl ion. 

Abstract This chapter covers one of the most important areas of Ru-catalysed oxidative chemistry. First, alkene oxidations are covered in which the double bond is not cleaved (3.1) epoxidation, cis-dihydroxylation, ketohydroxylation and miscellaneous non-cleavage reactions follow. The second section (3.2) concerns reactions in which C=C bond cleavage does occur (oxidation of alkenes to aldehydes, ketones or carboxylic acids), followed by a short survey of other alkene cleavage oxidations. Section 3.3 covers arene oxidations, and finally, in section 3.4, the corresponding topics for aUcyne oxidations are considered, most being cleavage reactions. 

The aminocyclitol moiety was synthesized in a stereocontrolled manner from cis-2-butene-l,4-diol (Scheme 40)112 by conversion into epoxide 321 via Sharpless asymmetric epoxidation in 88% yield.111 Oxidation of 321 with IBX, followed by a Wittig reaction with methyl-triphenylphosphonium bromide and KHMDS, produced alkene 322. Dihydroxylation of the double bond of 322 with OSO4 gave the diol 323, which underwent protection of the primary hydroxyl group as the TBDMS ether to furnish 324. The secondary alcohol of 324 was oxidized with Dess-Martin periodinane to... 

From Achiral Non-carbohydrates. — 3-Deoxy-3-guanidino-D-threose 48 equilibrates with 49. a transition state inhibitor for galactosidase. It was synthesized as shown in Scheme 12 from epoxide 47, which was obtained by porcine pancreatic lipase catalysed enantioselective esterification of the racemic epoxy-alcohol precursor. 6-Deoxy-L-talonolactone 50 was synthesized by an asymmetric aldol condensation - dihydroxylation sequence (Vol.24, p.lS2) in improved diastereoselectivity and was converted into 2-acetamido-2,6-dideoxy-L-fucose (shown as its furanose isomer 51 in Scheme 13), 3-acetamido-3,6Hlideoxy-L-idose and 5-acetamido-S,6-dideoxy-D-allose by S 2 displacements of triflate with azide ion. 4-Amino-4-deoxy-DL-erthrose 53 was obtained from the hetero-Diels-Alder adduct 52 by a sequence of reactions including cis-dihydroxylation (OSO4, NMNO) of the alkene moiety (Scheme 14). The synthesis of a racemic branched-chain lactam is covered in Chapter 16. 

We already know how to produce 1,2-diols from the reactions of alkenes we studied in Chapter 11. We can make cis-diols using osmium tetroxide-mediated dihydroxylation, tra s-diols by ring opening of epoxides, and chiral diols by Sharpless asymmetric dihydroxylation (Figure 20.1). tra s-l,2-Amino alcohols can be prepared by ring opening of epoxides, and there is a version of the Sharpless dihydroxylation that leads to chiral amino alcohols. 

Jacobsen epoxidation turned out to be the best large-scale method for preparing the cis-amino-indanol for the synthesis of Crixivan, This process is very much the cornerstone of the whole synthesis. During the development of the first laboratory route into a route usable on a very large scale, many methods were tried and the final choice fell on this relatively new type of asymmetric epoxidation. The Sharpless asymmetric epoxidation works only for allylic alcohols (Chapter 45) and so is no good here. The Sharpless asymmetric dihydroxylation works less well on ris-alkenes than on trans-alkenes, The Jacobsen epoxidation works best on cis-alkenes. The catalyst is the Mn(III) complex easily made from a chiral diamine and an aromatic salicylaldehyde (a 2-hydroxybenzaldehyde). 

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