Polyene cyclization, acid-catalyzed

Cyelkation of ( )- and (Z)-4 -dimetfyUiona-3,7-diene-2-one, Acid-catalyzed cyclizalion of polyenes is now a fairly well-known reaction (3, 305). For example cyclization of 6,l0-dimethyl-3.5,10-undecatriene-2-one (1) with concentrated sulfuric acid gives -iononc (2) in 85% yield. ... 

Despite extensive studies on acid-catalyzed diastereoselective polyene-cyclizations, their enantioselective behavior have not yet been reported. The stereochemical implications of polyene-cyclizations can be explained by the Stork-Eschenmoser hypothesis [140], and the most important feature required for an artificial cyclase is asymmetric induction during the initial protonation. Very recently, the author and Yamamoto et al. succeeded in the first enantioselective biomimetic cyclization of polypre-noids catalyzed by LB A [141]. 

Johnson, Corey, and van Tamelen have successfully utilized the biomimetic acid-catalyzed cyclization of polyolehnic substrates for the construction of steroids and some teipenes. Based on the previous work, Ireland and co-workers designed a route to the desired tetracycle 69 utilizing polyene cyclization (Scheme 12), which, although preparatively uncompetitive with the hydrocyanation (Scheme 11), was of some interest. Based on a model study, the symmetric dibromide... 

Andersen and co-workers used an acid-catalyzed polyene cyclization to form the fused ring system of the oncolytic PI3Ka inhibitor liphagal. Treatment of a terpine-based benzofuran with chlorosulfonic acid gave the desired tetracyelic system in 43% yield. 

Selective epoxidation of polyene compound has also achieved with ent 2, the enantiomer of ketone 2. In Morimoto s total synthesis of polyether (+)-aurilol, Shi epoxidation was utilized twice, with ketone 2 and ent-2, respectively." Epoxidation of 79 with ketone 2 gave epoxide 80 with high diastereoselectivity. Epoxide 80 underwent acid catalyzed 5-exo-tet cyclization to produce tetrahydrofuran 81 with the desired stereochemistry. Subsequently, diene 82 was selectively epoxidized with ent-2 only at the trisubstituted olefin to give epoxide 83. Epoxides 80 and 83 played important roles in setting stereocenters in the final product. 

L. E. Overman, M. M. Abelman, D. J. Ricca, and V. D. Tran, in OrganometalUc Reagents in Organic Synthesis, J. H. Batesson and M. B. Mitchell, Eds., Academic Press, London, 1994, 117-128. Eirst Total Synthesis of Scopadulcic Acid B. An Illustration of the Utility of Palladium-Catalyzed Polyene Cyclizations. 

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... 

In 2010, Loh s group reported a highly efficient catalytic enantioselective polyene cyclization [52]. An a-ketoester was demonstrated to initiate enantioselective cationic polyene cyclization catalyzed by a Sc(OTf)3-Pybox Lewis acid catalyst (Scheme 9.39). 

In general, the rearrangements of dienes and polyenes can be both thermal and photochemical reactions (the latters are not included in this chapter), and can be catalyzed by acids, bases, metal complexes and enzymes. They can be degenerate processes or occur with the introduction or elimination of functional groups, be accompanied by shifts of multiple bonds or by migrations of atoms or groups and they may lead to cyclizations. 

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