Asymmetric synthesis, medium-sized

To extend the scope of asymmetric transannular C-H insertions, more highly functionalized medium-sized cyclic epoxides have been investigated. A triad of cydooctene oxides 34, 36, and 38, possessing protected diol units, gave the expected alcohols 35, 37, and 39 (Scheme 5.10) [17, 18] an asymmetric synthesis of (-)-xialenon A has been achieved starting from alcohol 39 [19]. In comparison,... 

Through the efforts of the Overman research group, the acetal-vinylsilane cyclization reaction has been shown to be a useful strategy for the stereoselective preparation of a variety of medium-sized oxygen heterocycles. This cyclization strategy has been used successfully in the asymmetric synthesis of the marine natural product (-)-laurenyne (40a), an eight-membered cyclic ether. These molecules are members of an unusual class of C-15 nonisoprenoid metabolites. 

In addition, silver-catalyzed asymmetric aza-Diels-Alder reactions provide a useful route to optically active nitrogen-heterocyclic compounds such as piperidines or pyrid-azines. Substituted dihydrobenzofurans can also be enantioselectively prepared through silver-promoted allylation of aldehydes. Other types of silver-mediated cyclizations can also be used in the synthesis of tetrahydrofnrans, tetrahydropyrans, 1,2-dioxetanes, 1,2-dioxolanes, medium-sized lactones, dihydroisoqninolines, and so on. Silver salts can also be used as cocatalysts with other transition metals. Unique activity was observed for these silver-based systems in several cases. Conseqnently, the use of silver can enrich several available heterocyclization methods, and fnrther developments in the application of chiral silver complexes will hopefnlly appear in the near future. 

Watson, L, Ritter, S., Toste, R (2009). Asymmetric synthesis of medium-sized rings by intramolecular Au(I)-catalyzed cyclopropanation. Journal of the American Chemical Society, 131, 2056-2057. 

The most prominent and most successful compound classes in this respect are arguably VCPs and MCPs. The ring-expanding annulations of these substrates provide an opportunity for the synthesis of various medium-sized carbocyclic ring systems. However, the number of examples of asymmetric reactions remains relatively low. Their application in asymmetric catalytic processes is the next challenge to render cyclopropane ring opening more useful and fruitful. 

Shiina, I., Hashizume, M., Yamai, Y., Oshiutni, H., Shimazaki, T, Takasuna, Y, and Ibuka, R. (2005) Enantioselective total synthesis of octalactin A using asymmetric aldol reactions and a rapid lactonization to form a medium-sized ring. Chem, Ear, J 11, 6601-6608. 

Asymmetric Synthesis with Medium-Sized Rings... 

The catalyzed version of this rearrangement is useful for the synthesis of medium and large-sized carbocycles [84]. For example, the enantioselective synthesis of carbocyclic natural product, (-)-9,10-dihydroecklonialactone B 121 was done successfully by catalytic asymmetric Claisen rearrangement of a GosteU-type aUyl vinyl ether 122 in the presence of (S,S)-Cu (box)-catalyst A to produce a chiral a-ketoester 123, as a building block unit [85]. 

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