Terpenes, cyclic olefination

Cyclic olefins and diolefins form much more aerosol than 1-alkenes that have the same number of carbon atoms (for example, cyclohexene 1-hexene, and 1,7-octadiene 1-octene). The same effect of chain length and double-bond position is observed for diolefins (1,7-octadiene > 1,6-heptadiene > 1,5-hexadiene, and 1,7-octadiene 2,6-octadiene). Heavier unsaturated cyclic compounds, such as indene and terpenes, form even more aerosol. 

The esterification of cyclic olefins such as dicyclopentadiene or naturally occurring terpenes with carboxylic acids yields compounds of industrial value (74). For instance, the esterification of dicyclopentadiene 47 (Eq. 15.5.1) with saturated carboxylic acids, such as acetic acid, leads to a starting material 48 for the flavour and fragrance industry, as well as the resulting alcohols in consecutive hydrolysis (75). 

Acetoxylation is a valuable method for the introduction of an OH group into organic compounds, which can be used for further syntheses. In Section 3.3.14.2 it has been mentioned that acetoxylation of higher and cyclic olefins with palladium salts, or catalyzed by palladium salts or metal, mostly leads to allylic derivatives. This also takes place in the catalytic acetoxylation of terpenic olefins [79, 80]. 

Other Peroxyacids. Benzeneperoxyseleninic acid has been prepared in situ from benzeneseleninic acid and hydrogen peroxide and is used to epoxidize terpenic olefins and Baeyer-Villiger oxidation of cyclic ketones. 

Terpenes are a class of naturally occurring olefins found in many plants. They are biosynthetically derived from isoprene, CsHg, and therefore consist of Cs-building blocks and are found in both cyclic and acyclic forms. An important subgroup is the monoterpenes, which consist of two isoprene units and are of the molecular formula Ci0H16. Of the monocyclic monoterpenes, by far the most industrially relevant is limonene, which is abundantly present as a fragrance in various household products. From the bicyclic monoterpenes, a-pinene, p-pinene, camphene, and carene are of industrial relevance and are also mostly used in the fragrance industry [44]. 

It is therefore that in recent articles the focus has shifted to Ti-Beta as the catalyst. Allthough differences in activity between Ti-Beta and TS-1 exist for small molecules, the latter usually being slightly more active, the real difference between the two types of catalysts is in the epoxidation of bulky molecules. Whereas TS-1 is not capable of epoxidizing cyclohexene, Ti-Beta has no problem with a wide range of cyclic alkenes, bulky olefins or terpenes.85... 

Terpene synthases, also known as terpene cyclases because most of their products are cyclic, utilize a carbocationic reaction mechanism very similar to that employed by the prenyltransferases. Numerous experiments with inhibitors, substrate analogues and chemical model systems (Croteau, 1987 Cane, 1990, 1998) have revealed that the reaction usually begins with the divalent metal ion-assisted cleavage of the diphosphate moiety (Fig. 5.6). The resulting allylic carbocation may then cyclize by addition of the resonance-stabilized cationic centre to one of the other carbon-carbon double bonds in the substrate. The cyclization is followed by a series of rearrangements that may include hydride shifts, alkyl shifts, deprotonation, reprotonation and additional cyclizations, all mediated through enzyme-bound carbocationic intermed iates. The reaction cascade terminates by deprotonation of the cation to an olefin or capture by a nucleophile, such as water. Since the native substrates of terpene synthases are all configured with trans (E) double bonds, they are unable to cyclize directly to many of the carbon skeletons found in nature. In such cases, the cyclization process is preceded by isomerization of the initial carbocation to an intermediate capable of cyclization. 

In the epoxidation of olefins not bearing a complexing group, there is not a great difference in stereoselectivity in comparison with the peracids. The c/s-oxirane is formed from cis-butene-2 and the trans-oxirane from trans-butene-2. Due to steric hindrance, regioselectivity may be observed for nonconjugated dienes. °° ° In terpenes, the attack takes place from the less hindered side. Double bonds in bridged cyclic systems have an effect on the reaction rate. ... 

Olefine and Cyclic Terpenes.—Two distinct groups are known which have entirely different structure. The first and smaller group includes strictly aliphatic hydrocarbons belonging to the olefine or ethylene unsaturated series. The second group, which is much larger, includes cyclo-aliphatic hydrocarbons or as we have previously described them the hydro-aromatic hydrocarbons. Thus we have ... 

The hydrocarbons of the various groups which we have just discussed are the true terpenes. On oxidation these yield alcohol and aldehyde or ketone derivatives. The olefine terpenes, only, yield aldehydes that occur as constituents of natural products known as essential oils (p. 840). The derivatives of both groups of cyclic terpenes which are present in essential oils and plant gums and resins are either secondary alcohols or ketones. Among these latter are the camphors of which common camphor is the most important and best known example. In a general sense all of the oxidation products of the cyclic terpenes are termed camphors. 

The signs of the Cotton effects near 255 and 315 nm observed in the c.d. spectra of the IV-salicylidene derivatives of a number of cyclic terpene amines correlate with the absolute configurations of the amines. The assignment of structure (lO)," to one of a pair of diastereisomeric olefins formed in the reaction of d-( + )-camphor with TiCl3-LiAlH4, is consistent with the spectroscopic and chiroptical data and is in agreement with the olefin oetant rule of Seott and Wrixon. 

The ozonolysis of acyclic alkenes including terpenes is reviewed in Ref. [85]. The role of ozonolysis as ecological process for the selective and specialized oxidation of petrochemical olefins and cyclic alkenes, for the manufacture of biologically active substances and normal organic compounds is thoroughly discussed in Ref. [86],... 

One of the early approaches to cyclic halogenated terpenes used the Diels-Alder reactions of (3-halogenated dienophiles, which enabled predetermined installation of the halogen functionality into the target architecture. This concept was first realized by Williard and de Laszlo who reported on a route to c/i/-plocamine 65 (Scheme 43.8). It has been demonstrated that the olefinic geometry of P-chloro dienophiles considerably affects the efficiency of the Diels-Alder reaction. Fortunately, (Z)-3-chloro-2-meth-ylpropenal 68 proved to be more reactive than the corresponding ( )-isomer toward various dienes to provide... 

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