Sharpless facial selectivity

Consider the reactions A-F. Assume that the Sharpless epoxidations proceed with complete a-facial selectivity regardless of substrate. Select the best answer among the following choices regarding the stereochemical outcome of each of the reactions. 

Reactions B, C, E and F lead to mixtures of diastereomers. Reaction D affords a single enantiomer since the Sharpless epoxidation proceeds with complete n-facial selectivity. 

Schreiber and co-workers48 have developed a mathematical model that allows calculation of the enantiomeric purity of products of reactions exhibiting enantiocontrol and diastereoselection. Application of this model to the Sharpless epoxidation of 10 using the relative facial selectivities obtained34 for 9 leads to the expectation that the enantiomeric excess of the epoxide products 11 should increase with the progress of the reaction. Within the limits of detection, this was experimentally observed (Table 6)48. 

In order to predict facial selectivity, Sharpless and co-workers invoke a mnemonic device.25 To an approaching olefin, the greatest steric constraints are presented by the NW, and to an even greater extent, the SE quadrants. The SW and NE quadrants are more open and, in addition, the SW quadrant contains what is described as an attractive area . The attractive area is particularly well suited to accommodate flat aromatic groups. The olefin positions itself according to the constraints imposed by the ligand and is dihydroxylated from above (p-facc), in the case of dihydroquinidine derivative, or from below (a-face) in the case of dihydroquinine derivatives. The commercially available AD-mix-a and AD-mix-P are chosen according to this mnemonic. 

The Sharpless asymmetric epoxidation (sec. 3.4.D.i) exploits this chelation effect because its selectivity arises from coordination of the allylic alcohol to a titanium complex in the presence of a chiral agent. The most effective additive was a tartaric acid ester (tartrate), and its presence led to high enantioselectivity in the epoxidation.23 An example is the conversion of allylic alcohol 40 to epoxy-alcohol 41, in Miyashita s synthesis of the Cg-Ci5 segment of (-t-)-discodermolide.24 in this reaction, the tartrate, the alkenyl alcohol, and the peroxide bind to titanium and provide facial selectivity for the transfer of oxygen from the peroxide to the alkene. Binding of the allylic alcohol to the metal is important for delivery of the electrophilic oxygen and... 

The second key contribution that culminated in his share of the 2001 Nobel Prize in chemistry was Sharpless catalytic asymmetric dihydrorylation (SAD). ° The original catal34ic asymmetric work, published in 1987, ultimately resulted in the commercialisation of ADmix-a and ADmix-p, which represent very reliable pre-packaged means for predictably accessing enantioenriched diols. The facial selectivities of these catalyst systems are well-documented and will not be explored in depth for the purpose of this review (Figure 14.16). Rather, the focus will be on the application of Sharpless chemistry in the industrial environment. Specifically, SAD has... 

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