Bicyclic olefins, asymmetric

The ring-opening polymerization of a simple cyclic olefin such as cyclooctene yields two structures of maximum order, which are distinguished by the configuration (cis or irans) of their main-chain olefins. In contrast, polymers made from bicyclic olefins such as norbornene are inherently more complicated and have four structures of maximum order (Scheme 24). In addition to cis- and trans-olefins, the polymers can also be isotactic or syndiotactic. The stereochemistry of these polymers becomes even more complicated when the monomer is asymmetric, since head—head, head—tail, and tail—tail regioisomers are possible. Nevertheless, single-site metathesis catalysts have been developed that can control polymer stereochemistry to an impressive degree by both chain-end and site-control mechanisms. ° ° ... 

Huang JK, Bunel E, AUgeier A, Tedrow J, Storz T, Preston J, CorreU T, Manley D, Soukup T, Jensen R, Syed R, Moniz G, Larsen R, MartineUi M, Relder PJ. A highly enantioselective catalyst for asymmetric hydroformylation of [2.2.1]-bicyclic olefins. Tetrahedron Lett. 2005 46 7831-7834. 

Following Uskokovic s seminal quinine synthesis [40], Jacobsen has very recently reported the first catalytic asymmetric synthesis of quinine and quinidine. The stereospecific construction of the bicyclic framework, introducing the relative and absolute stereochemistry at the Cg- and expositions, was achieved by way of the enantiomerically enriched trans epoxide 87, prepared from olefin 86 by SAD (AD-mix (3) and subsequent one-pot cyclization of the corresponding diol [2b], The key intramolecular SN2 reaction between the Ni- and the Cg-positions was accomplished by removal of the benzyl carbamate with Et2AlCl/thioanisole and subsequent thermal cyclization to give the desired quinudidine skeleton (Scheme 8.22) [41],... 

Asymmetric cyclopropanol formation has been achieved with olefinic acylsulfonamides, which act like olefmic esters. Thus, their reaction with 1 provides a method for synthesizing cyclopropanols in an optically active form. As represented by Eq. 9.41, alkylation of Oppolzer s camphor sultam and reaction of the resulting unsaturated acylsulfonamides with 1 provides optically active bicyclic cyclopropanols having exclusively the structure shown in the equation [76],... 

Vishnumaya and Singh developed a highly efficient water-compatible binaphthyl derived diamine organocatalyst Ih and successfully used it in asymmetric Michael reactions. The treatment of ketones with both aryl and allgrl nitro-olefins afforded the desired products with high to excellent stereoselectivities (Scheme 9.16). The Chen group used the bicyclic... 

Hoveyda et al. reported the enantioselective synthesis of (+)-africanol through asymmetric olefin metathesis followed by one-carbon contraction via decarbonylation (Scheme 8.16) [65]. When symmetrical norbornene 84 was treated with a chiral molybdenum catalyst, desymmetrization took place to afford the chiral bicycle 85 in an enantioselective fashion. The resulting vinyl terminus was subsequently transformed into a methyl group via hydroboration, oxidation to aldehydes, and decarbonylation. Further manipulations of the functional groups gave (+)-africanol. 

Scheme 7.33). The dehydration proceeds according to an El-mechanism, and the carbenium ions exist as contact ion pairs with the base [OC(Ph)(CF3)2] as the counter ion the H-atom, the empty p-orbital, and the p-orbital of the carbonyl are most likely aligned in a mutually periplanar fashion, as in 21Sb. Attack of the base on the pro-R H atom leads to the 2-alkene from the transition state that is hindered neither by the auxiliary nor by the bicyclic ring system. This successfiil extension of the two-step asymmetric olefination of prochiral cydoalkanone derivatives proved to offer an additional route to axially chiral alkenes. 

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