Alkenes allylic dihydroxylation

Allyl alcohols can be produced catalytically by oxidation of alkenes with TBHP in the presence of small amounts of Se02 (equation 128). In contrast to the stoichiometric reaction, this catalytic oxidation can be performed under mild conditions (r.t., CH2C12 solvent).356 Alkynic compounds undergo a predominant allylic dihydroxylation upon reaction with TBHP/CH2C12 (equation 129) 357... 

After the "asymmetric epoxidation" of allylic alcohols at the very beginning of the 80 s, at the end of the same decade (1988) Sharpless again surprised the chemical community with a new procedure for the "asymmetric dihydroxylation" of alkenes [30]. The procedure involves the dihydroxylation of simple alkenes with N-methylmorpholine A -oxide and catalytic amounts of osmium tetroxide in acetone-water as solvent at 0 to 4 °C, in the presence of either dihydroquinine or dihydroquinidine p-chlorobenzoate (DHQ-pClBz or DHQD-pClBz) as the chiral ligands (Scheme 10.3). 

The dihydroxylation of alkenes is a useful sttategy for the synthesis of polyols and these can be nitrated to the corresponding nitrate esters. Evans and Gallaghan " synthesized both the mono- (74) and di- (70) allyl ethers of pentaerythritol and used these for the synthesis of some novel nitrate ester explosives. 

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). 

Allylic alcohols represent a small fraction of the total population of alkenes found in organic molecules. Asymmetric epoxidation of allylic alcohols therefore taps only a small portion of the synthetic potential inherent in a completely general asymmetric epoxidation of isolated (nonfunctionalized) alkenes. A partial solution to this problem now exists. The recent development of a catalytic asymmetric process for the dihydroxylation of aUcenes provides an indirect route to epoxides or epoxide-like functionalization of alkenes. The stereochemistry of the process, the scope of enantioselectivity and chemical yield and a summary of key chemical transformations are presented in this section. Since this roach to alkene functionalization is at an early stage of development, the results sununarized here are certain to benefit from extensions and improvements as research in this area progresses. 

Control of enantioselectivity will be discussed in the corresponding sections on carbonyl reduction (Chapter 4) alkene hydrogenation, epoxidation, and dihydroxylation (Chapter 5) aldol condensation (Chapter 6) allylation and crotylation (Chapter 7) Claisen rearrangement (Chapter 8) and the Diels-Alder reaction (Chapter 9). 

The use of C2-symmetric aziridines as chiral auxiliaries has been reviewed by Tanner <94AG(E)599>. Chiral A-acylaziridines such as (143) have been used in enolate chemistry (see Section 1.01.8.2). Aziridine-containing ligands such as (144) have been used for a variety of metal-catalyzed reactions such as the asymmetric dihydroxylation of alkenes, the palladium-catalyzed allylation of nucleophiles, asymmetric cyclopropanation, and aziridination <94AG(E)599,94TL4631). 

Equation 12.16 is an example of the Sharpless-Katsuki asymmetric epoxi-dation of allylic alcohols, which is catalyzed by a Ti complex bound to a chiral tartrate ligand.38 A Mn-salen39 complex serves as catalyst for asymmetric epoxi-dation (Jacobsen-Katsuki reaction) of a wide variety of unfunctionalized alkenes, shown in equation 12.17.40 0s04 complexed with chiral alkaloids, such as quinine derivatives (equation 12.18), catalyzes asymmetric 1,2-dihydroxylation of alkenes (known as the Sharpless asymmetric dihydroxylation).41 The key step of all these transformations is the transfer of metal-bound oxygen, either as a single atom or as a pair, to one face of the alkene. 

A good illustration of many aspects of this chemistry comes from Koreeda s synthesis of shikimic acid.22 The silyl diene 123 adds cleanly to methyl acrylate to give essentially one isomer of the adduct 124 (9 1). Dihydroxylation with catalytic 0s04 and the stoichiometric oxidant NMO (A-methylmorpholine-A-oxide) occurs on the opposite face to all three substituents. The alkene 124 is an allylic acetate but this group has no attractive interaction with the reagent. 

The presence of the anfi-vicinal diol unit in the phytosphingosine 67 strongly suggests the application of the double diastereocontrolled AD to the Z-alkene 64 (Scheme 25). However, in order to access the desired product 64, the mismatched diol should be obtained. Horikawa and coworkers studied both the achiral and the chiral dihydroxylation of the optically active (E)- and (Z)-allylic... 

Epoxidation of allylic and homoallylic alcohols not part of the diene complex can be achieved using the Sharpless r-butyl hydroperoxide/vanadium acetylacetonate protocol. Dihydroxylation of alkenes adjacent to the diene complex using osmium tetraoxide-7-butyl peroxide has been reported... 

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