Cyclization poly olefinic

Olefin metathesis does not generate stereogenic centers, however, the reaction may be employed in the desymmetrization of prochiral (poly)olefins of the kinetic resolution of racemates. In the example depicted in Scheme 17, a trialkene is desymmetrized, and the preference for the cyclization reaction with one of the two symmetry-equivalent C = C double bonds leads to the enantioselective formation of the reaction product, a chiral dihydrofuran. The following principal conclusions can be drawn from this study ... 

In Figure 8 the NMR spectrum of a completely cyclized poly-isoprene (L2-11) is shown. The olefinic cadxm region consisted mainly of tetrasvibstituted carbons and these were not assigned to Individual structures. 

Polyketide and fatty acid biosyntheses begin with condensation of the coenzyme A thioester of a short-chain carboxylic acid starter unit such as acetate or propionate with the coenzyme A thioester of a dicarboxylic acid extender unit such as malonate or methyl malonate. The driving force for the condensation is provided by the decarboxylation of the extender unit. In the case of fetty acid synthesis, the resulting -carbonyl is completely reduced to a methylene however, during the synthesis of complex poly-ketides, the -carbonyl may be left untouched or variably reduced to alcohol, olefinic, or methylene functionalities depending on the position that the extender unit will occupy in the final product. This cycle is repeated, and the number of elongation cycles is a characteristic of the enzyme catalyst. In polyketide biosynthesis, the full-length polyketide chain cyclizes in a specific manner, and is tailored by the action of additional enzymes in the pathway. 

Whereas poly(a-olefins) have only two microstructures of maximum order (isotactic, syndiotactic). cyclopolymers ° have /bur microstructures due to the rings present in their main chain which can be either cis or trans in configuration (Scheme 17). While the key issues concerning selectivity in the polymerization of a-olefins are regioselectivity (head-to-tail monomer incorporation) and enantioface selectivity (tacticity). cyclopolymerization of a.co-diolefins has added concerns. First, since the monomer has two olefins, either cyclization or cross-linking of the... 

Triflic acid was also used in the synthesis of dixanthones and poly(dixanthones) by cyclization of 2-aryloxylbenzonitriles at room temperature. Addition of dialkyl disulfides to terminal alkynes is catalyzed by a rhodium-phosphine complex and triflic acid giving (Z)-bis(alkylthio)olefins stereoselectively (eq 50). ... 

The chemical simulation of this biosynthetic process has been developed as an important methodology in organic synthesis. Van Tamelen reported the add-catalyzed cyclization of chiral terminal epoxides of polyprenoids [11]. In contrast, Johnson adopted the acid-catalyzed cyclization of poly-prenic acetals derived from chiral diols [12]. In addition to these pioneering studies, the biomimetic polyene cycliza-tions of polyprenoids, which are induced by a variety of electrophiles such as proton, oxonium ion, halonium ion [13], or metal ion [14], have also been developed. Despite extensive studies on these diastereoselective olefin cyclizations, enantioselective processes using synthetic chiral catalysts had not been developed for a long time. In 1999, Yamamoto s... 

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