Sharpless epoxidation capacity

Sesquiterpenoid. 203, 1071 Sex hormone, 1082-1083 Sharpless, K. Barry. 734 Sharpless epoxidation, 735 Shell (electron), 5 capacity of, 5 Shielding (NMR). 442 Si prochirality, 315-316 Sialic acid. 997 Side chain (amino acid), 1020 Sigma (cr) bond, 11 symmetry of, 11 Sigmatropic rearrangement, 1191-1195... 

Sharpless epoxidation is considered highly valuable because it combines the powerful nature of the reaction with the capacity of the resultant epoxy alco-... 

Although the Sharpless catalyst was extremely useful and efficient for allylic alcohols, the results with ordinary alkenes were very poor. Therefore the search for catalysts that would be enantioselective for non-alcoholic substrates continued. In 1990, the groups of Jacobsen and Katsuki reported on the enantioselective epoxidation of simple alkenes both using catalysts based on chiral manganese salen complexes [8,9], Since then the use of chiral salen complexes has been explored in a large number of reactions, which all utilise the Lewis acid character or the capacity of oxene, nitrene, or carbene transfer of the salen complexes (for a review see [10]). 

Sharpless advanced this notion through using a metal-based epoxidation catalyst (Scheme 4.18) [44]. This approach took advantage of the capacity of VO(acac)j to coordinate simultaneously to alcohols and peroxides and promote regioselective epoxidations of allylic and homoal-lylic alcohols with iBuOOH as the oxidant, as shown by the conversion of 87 to 88. This approach also provides stereoselectivity, as shown by the transformation of 89 to 90 in which the hydroxyl group directs a yn-oxidation of the alkene. This procedure is selective because alkyl peroxides generally do not epoxidize alkenes but can be activated by coordination to the alcohol-coordinated vanadium Lewis acid. 

Asymmetric Epoxidation Reactions. While Ti(0-i-Pr)4 clearly has the capacity to bring about the nucleophilic ring-cleavage of 2,3-epoxy alcohols (see above), it remains the preferred species for the preparation of the titanium tartrate complex central to the Sharpless asymmetric epoxidation process (see, for example, eq 7). Since f-butoxide-mediated ring-opening of 2-substituted 2,3-epoxy alcohols (a subclass of epoxy alcohols particularly sensitive to nucleophilic ring-cleavage) is much slower than by isopropoxide, the use of Ti(0-f-Bu)4 is sometimes recommended in place of Ti(0-i-Pr)4. However, with the reduced amount of catalyst that is now needed for all asymmetric epoxidations, this precaution appears unnecessary in most instances. 

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