Sharpless epoxidation stereochemical outcome

Reactions have been carried out adjacent to the epoxide moiety in order to examine the effects, if any, that the epoxide has on subsequent reactions with respect to the regio- and stereochemical outcome. Dihydroxylation using osmium tetraoxide and Sharpless asymmetric dihydroxylation reactions have been extensively studied using substrates 29 and 31. Initial studies centred on the standard dihydroxylation conditions using AT-methylmorpholine-AT-oxide and catalytic osmium tetraoxide. The diastereomeric ratios were at best 3 2 for 29 and 2 1 for 31, indicating that the epoxide unit had very little influence on the stereochemical outcome of the reaction. This observation was not unexpected, since the epoxide moiety poses minimal steric demands (Scheme 21). 

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. 

The unusual nucleophilic epoxidation of /i-hydroxyenones under Sharpless conditions (see Section 4.5.1.3.2.1.) is also applicable to compounds 1 with endocyclic double bonds. The. sj H-epoxides are produced with complete selectivity. The stereochemical outcome of the reaction under Weitz-Scheffer conditions significantly differs from that observed for acyclic compounds. While the acyclic enones afforded preferentially the moderate ratio, cyclic ones gave predominantly s>7i-epoxides32. 

One of the major advantages of the Sharpless [3,4] titanium asymmetric epoxidation is the simple method by which the stereochemical outcome of the reaction can be predicted [5]. The other powerful feature is the ability to change this selectivity to the other isomer by simple means (Figure 1) [6-8]. 

Despite the complexity of the active catalyst, the sense of asymmetric induction in Sharpless asymmetric epoxidation reactions can be rehably predicted using the model shown in Figure 4.2. In order for the model to predict the stereochemical outcome correctly, only two points need to be remembered. The allyhc hydroxy group resides in the bottom right corner and D-(-)-diethyl tartrate (which has the (S,S)-configuration) attacks from above the plane. 

Without doubt the most general method for the synthesis of chiral epoxides to date is that of Sharpless and co-workers such that the stereochemical outcome of the epoxidation of the allylie alcohol (6) holds true almost no matter the nature of to R. However, recent studies have shown that if t-butyl groups are incorporated at either the carbinol carbon or at the P- -position then the e.e.s of the a-hydroxy-epoxides are much reduced. The synthesis of a-keto-epoxides by the Darzens condensation is well known, but this year has seen the first example of chiral, aqueous-catalysed Darzens condensation to give the epoxy-ketones (7) albeit in only moderate optical and chemical yields (2-62% and 5- 3% respectively). ... 

Directed epoxidations of acyclic, allylic substrates may also proceed with high stereoselectivity. Sharpless proposed a transition state model to assist understanding of the stereochemical outcome in such transformations (Figure 9.2) [65, 66]. These indicated that the optimum 0-C-C=C dihedral angles in peracid- (26/27) and in vanadium-catalyzed (28/29) epoxidations were 120° and 50°, respectively. When these empirical guidelines are fol-... 

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