Olefin diastereoselective dihydroxylation

A recent example L. Emmanuvel, T.M. A. Shaikh, A. Sudalai, Nal04 / LiBr-mediated diastereoselective dihydroxylation of olefins a catalytic approach to the Prevost-Woodward reaction, Org. Lett. 7 (2005) 5071-5074. 

One of the foremost methods for oxidation of olefins is stereospecific syn-di-hydroxylation by treatment with OSO4. Because of the toxicity and cost of osmium, however, the need for methods that would employ substoichiometric quantities of this reagent was identified early on [180], A number of stoichiometric oxidants for catalytic dihydroxylations were examined, including metal chlorates, HjOa, NMO [181], f-BuOOH [182], and KjFeiCN) [40, 183], The mechanism of 0s04-catalyzed dihydroxylation reactions has been the subject of much debate details of these mechanistic considerations are beyond the focus of this book and are amply discussed elsewhere [40-42,47,49,184-186]. A number of interesting stereoselectivity trends of general importance in substrate-controlled diastereoselective dihydroxylations [40-43] and catalytic asymmetric dihydroxylations are discussed below (Section 9.8) [42, 44-49]. 

In another study of 1,1-disubstituted olefins [200], Evans found that 299 participated in a highly diastereoselective dihydroxylation reaction (Scheme 9.38) [204]. The transformation afforded a 96 % yield of diol 300 as a single isomer, which served as a key intermediate in the synthesis of the macrolide antibiotic cytovaricin (301). 

Asymmetric induction also occurs during osmium tetroxide mediated dihydroxylation of olefinic molecules containing a stereogenic center, especially if this center is near the double bond. In these reactions, the chiral framework of the molecule serves to induce the diastereoselectivity of the oxidation. These diastereoselective reactions are achieved with either stoichiometric or catalytic quantities of osmium tetroxide. The possibility exists for pairing or matching this diastereoselectivity with the face selectivity of asymmetric dihydroxylation to achieve enhanced or double diastereoselectivity [25], as discussed further later in the chapter. 

Permanganese is a common oxidative reagent, the application of which to the asymmetric oxidative cyclization of 1,5-dienes has been reported by Brown (Scheme 3.14). The addition of acetic acid is quite important for the reaction to proceed, and highly functionalized tetrahydrofurans are obtained in a range of 58 to 75% ee, in diastereoselective manner [35]. Another oxidative transformation using KMn04 with a chiral ammonium salt has been investigated. Scheme 3.15 illustrates the asymmetric dihydroxylation of electron-deficient olefins to chiral diols in the... 

Olefin 104 was then subjected to a Sharpless asymmetric dihydroxylation which proceeded with moderate diastereoselectivity, leading to an inseparable mixture of the diols 105a and 105b (d.r. 71 29) (Scheme 29). Fortunately, when the mixture of 105a and 105b was submitted to the following three-step sequence, for example, selective protection of primary alcohol, mesylation, and then cyclization, the major isomer 106a could be isolated in pure form. Two further deprotection steps were necessary to provide the hyacinthacine Bi 3 in 45.5% overall yield and 13 steps from lactam 101. In paraUel,... 

Dihydroxylation of olefins attached to the tricarbonyl(diene)iron system with osmium tetroxide affords the respective diols in good diastereoselectivity. (Methylenecyclohexadienejiron complexes can undergo various stereoselective cyclo-... 

As is apparent from the preceding discussion, a full understanding of the observed diastereoselectivity in dihydroxylation reactions of acyclic allylic alcohols remains elusive. Thus, the use of any one model is insufficient, and a careful analysis of the steric and electronic particulars of a given substrate must be conducted. Nevertheless, an impressive number of diastereoselec-tive dihydroxylations of acyclic olefins in complex molecule synthesis attest to the central role of this transformation [42, 43], Selected examples of stereodivergent dihydroxylations reported by Danishefsky are showcased in Schemes 9.36 and 9.37 [201]. Dihydroxylation of ( )- and (Z)-unsaturated esters 287 and 290, respectively, thus proceeded with excellent diastereoselectivity. Danishefsky has proposed a transition state model based on the ground state conformations of the starting materials as determined by X-ray analysis. The dihydroxylations were thus postulated to occur from the sterically less hindered faces of the olefins, as depicted in 288 and 291. Diol 292 was subsequently converted into N-acetylneuraminic acid (293). 

Next, Pyne et al. successfully utilized the synthetic methodology created for putative uniflorine 75 to construct the analogous castanospermine (84, Scheme 2.19) [38]. In this case, amino-tetraol 81 had to be protected with three different protecting groups resulting in compound 82. Treatment of 82 with [Ru]-II (10mol%) yielded pyrrolo-oxazolone 83 in 88% yield. Diastereoselective syn-dihydroxylation of the olefin, followed by several steps, led then to castanospermine (84). 

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