Olefins, cyclization

Proton-catalyzed olefin cyclizations of open-chain educts may give tri- or tetracyclic products but low yields are typical (E.E. van Tamelen, 1968, 1977 see p. 91). More useful are cyclizations of monocyclic educts with appropriate side-chains. The chiral centre to which the chain is attached may direct the steric course of the cyclization, and several asymmetric centres may be formed stereoselectively since the cyclizations usually lead to traas-fused rings. 

The following acid-catalyzed cyclizations leading to steroid hormone precursors exemplify some important facts an acetylenic bond is less nucleophilic than an olelinic bond acetylenic bonds tend to form cyclopentane rather than cyclohexane derivatives, if there is a choice in proton-catalyzed olefin cyclizations the thermodynamically most stable Irons connection of cyclohexane rings is obtained selectively electroneutral nucleophilic agents such as ethylene carbonate can be used to terminate the cationic cyclization process forming stable enol derivatives which can be hydrolyzed to carbonyl compounds without this nucleophile and with trifluoroacetic acid the corresponding enol ester may be obtained (M.B. Gravestock, 1978, A,B P.E. Peterson, 1969). 

Tb allium (ITT) ttifluoroacetate promotes olefin cyclization reactions and intramolecular coupling reactions (32,33). 

The tricyclic fragrances of cedar wood, cedrene and cedrol, were synthesized by several routes involving cation-olefin cyclization. 

The first synthesis of a P-amyrin derivative was accomplished by a convergent route which depended on cation-olefin cyclization to form the critical central ring. The plan of synthesis was largely guided by the selection of SM goals for the A/B and E ring portions of the target. 

Pentacyclosqualene, the symmetrical hydropicene corresponding to squalene, has not been made by acid-induced cation-olefin cyclization of squalene, despite considerable experimental study. A simple, convergent synthesis of pentacyclosqualene using cation-olefin cyclization to generate ring C was developed. The Cjo-framework was constructed by radical coupling to a tetracyclic intermediate that was also used for the synthesis of onoceradiene. 

Recently Johnson and co-workers (84) have taken advantage of triplebond participation in biogeneticlike olefinic cyclizations. Treatment of trienynol 91 with 2% by weight trifluoroacetic acid at -70°C in methylene chloride gave largely the tricyclic triene 92 as product. Similarly, trienynol 93 underwent cyclization under the same conditions to give tricyclic product 94. 

The major product of the keto olefin cyclizations often corresponds to what one would predict, assuming the intermediate ketyl behaves like the corresponding monoradical (Eq. 34). For example, given an option between a 5-exo and a 6-endo-trig cyclization, the former predominates in radical cyclizations [51], and constitutes the exclusive cyclization path in the electrochemical counterpart, 110 - 112 [47]. In addition, the stereochemical outcome parallels that of the radical... 

The Shapiro reaction provides a convenient, easy and straightforward method to convert ketones into a plethora of olefinic substances in high yields. Many of these vinyllithium derivatives are useful for further synthetic manipulations. No attempt is made in this chapter to cover all the applications of the Shapiro reaction and only few representative examples will be described. A variety of polyolefins such as 119, used for cation olefin cyclization, can be stereospecifically formed in a concise and modular approach in a single step from the components shown in equation 42 via the Shapiro reaction . 

Keywords Acetals, Acylals, Allylations, Bismuth(III), 3,4-Dihydro-2//-l -benzopyrans, Ene reactions, Epoxide-olefin cyclizations, Piperidines, Tetrahydro-quinolines, THP ethers... 

Bartlett PA (1984) Olefin cyclization processes that form carbon-heteroatom bonds. In Hassner A (ed) Asymmetric synthesis, vol 3. Academic, Orlando, FL, p 411... 

Epoxides can also serve as effective carbocyclization promotors, either through a polyene cyclization, as in the biomimetic epoxy-olefin cyclization of 100 in the presence of boron trifluoride etherate <99CC325>, or by a Friedel-Crafts approach, as exemplified by the cycli-alkylation of arylalkyl epoxides 102 under the influence of solid acid catalysts <99EJOC837>. 

This is the simplest practical example of an olefin cyclization. The reaction is represented schematically as follows ... 

F. Ketyl-olefin cyclization mediated by samarium(II) iodide 563... 

E. Tl(ll )-Mediated Epoxy-Olefin Cyclization Route to Carbocyclic Compounds from Carbohydrates (see Scheme 9) [31]... 

F. Ketyl-Olefin Cyclization Mediated by Samarium(ll) Iodide [33]... 

Scheme 4.17. Two cation-olefin cyclization events to afford an aromatic indane.

Cationic olefin cyclization of aldehyde enol acetates.1 The enol acetate (2) of citronellal (1) undergoes cyclization to dihydrocyclocitral (3) in the presence of... 

Protonation and hydrolysis of the extended enol ether to release the enone may occur during work-up and the stable enone is the first compound that can be isolated. The 50% yield of this compound represents a much better yield in four steps fragmentation, olefin cyclization, addition of formic acid, and enol ether hydrolysis. 

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