Terpene polyenes

This template directed functionalization of double bonds within reach was also used to determine the conformational preferences of some terpene polyenes [62], As in the previously described work with attached benzophenone reagents, the selective template-directed epoxidations revealed the details of chain folding in flexible molecules. 

Polyene cyclizations are of substantial value in the synthesis of polycyclic terpene natural products. These syntheses resemble the processes by which the polycyclic compounds are assembled in nature. The most dramatic example of biosynthesis of a polycyclic skeleton from a polyene intermediate is the conversion of squalene oxide to the steroid lanosterol. In the biological reaction, an enzyme not only to induces the cationic cyclization but also holds the substrate in a conformation corresponding to stereochemistry of the polycyclic product.17 In this case, the cyclization is terminated by a series of rearrangements. 

Polyene cyclization in terpene and steroid synthesis is critically dependent on the terminator in order to generate useful functionalities for further modification of the products. Allyl- and propargylsilanes have proven their value in facilitation of the cyclization and generation of an exocyclic methylene and allene, respectively. Thus, a concise approach to albicanyl acetate [126] and the rapid construction of a tetracyclic precursor of steroids [127] are sufficient to demonstrate the concept. Again, a comparison of the substrates with a silyl group with those having a simple alkyl moiety is very enlightening. 

Allylic alcohol (166) is the product of right-to-left linear iteration by this process. Not only is the tri-substituted alkene accessible with high stereochemical control, but also the ( )-disubstituted alkene is readily prepared. These linear polyenes play an important role in the biomimetic synthesis of steroids and higher terpenes. ... 

Paschall CM, Hasserodt J, Jones T, Lemer RA, Janda KD, Christianson DW. Convergence of catalytic antibody and terpene cyclase mechanisms polyene cyclization directed by carbocation-p interactions. Angew. Chem. Int. Ed. Engl. 1999 38(12) 1743-1747. 

Terpenes are formed by coupling double-bond isomers of the same momoner, isopentenyl pyrophosphate (IPP), with loss of a pyrophosphate ion (15, 16). Three such coupling reactions join four IPPs, each with hve carbon atoms, to make geranylgeranyl-pyrophosphate, the C2o-precursor of diterpenes such as taxol (Fig. 3). Linear terpene precursors, which differ only in the number of IPP units joined, can be assembled by a few polyprenyl synthase enzymes, as the monomers are identical and the only variable is how many monomers to link together. In subsequent steps, these linear polyenes undergo cation-induced cyclizations to create the starting frameworks for terpenes and their derivatives. 

Alkenes and polyenes (alkenes with several carbon-carbon double bonds) are common in nature. Ethene, the simplest alkene, is a plant growth substance involved in fruit ripening, senescence and leaf fall, and responses to environmental stresses. Isoprenoids, or terpenes, are polyenes built from one or more isoprene units. Isoprenoids include steroids, chlorophyll and other photos)mthetic pigments, and vitamins A, D, and K. 

Empirical rules based on thousands of laboratory observations have been developed over the years relating the wavelengths of the UV absorption maxima to the structures of molecules. R.B. Woodward in 1941 and then L. Fieser and M. Fieser developed rules for predicting the absorption maxima of dienes, polyenes, and conjugated ketones by studying terpenes, steroids, and other natural products. The rules are known as Woodward s Rules or the Woodward-Fieser Rules. 

Cation-JT interactions are also prominent at the active sites of enzymes involving cationic substrates. Key examples include the blood coagulation serine proteases Factor Xa and thrombin, and a number of enzymes that use S-adenosyhnethionine. a sulfonium ion that serves as nature s ubiquitous methyl transfer agent. A spectacular series of examples is the array of enzymes that catalyze the cationic cyclizations of polyenes in a key step of terpene and steroid biosynthesis. It is now clear that... 

One final class of compounds that is worthy of mention is the polyenes because it is quite possible that it could make some contribution to the coal precursor. These compounds comprise bicy-clic alkanes, the terpene hydrocarbons, and the carotenoid pigments (Figure 3.18) (Meinschein, 1969 Schwendinger, 1969 Luo et al., 1991). 

Metabolic cyclization routes of polyenes leading to terpenes [1] inspired chemists to synthesize steroids and polycyclic structures [2]. Metal catalysis, chiral auxiliaries, substrate control, and other stoichiometric methods reach some level of success [3], but only recently Rendler and MacMillan published independently two approaches using organo-SOMO catalysis to synthesize these features (Scheme 10.1) [4], as a continuation of their work on the asymmetric cyclization of aldehydes [5]. 

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