Terpenes rearrangement

Hydrogen fluoride adds to more complex molecules, such as unsaturated steroids, to give fluorinated derivatives [/, 8] Low temperatures and inert diluents, such as tetrahydrofuran or methylene chloride, are generally employed. With bicyclic unsaturated terpenes, rearrangements often accompany addition to the double bond [/]. 

An early example is the fascinating rearrangement802 of acyclic monoterpenes geraniol 195 and nerol 196 to a stable observable oxonium ion in HSO3F—SO2—CS2 at —78°C, which represents yet another dramatic example of altered course of terpene rearrangement in superacid medium. Careful quenching of this ion led to the isolation of 3/3,6a,6a/3-trimethyl- s-perhydrocyclopenta[b]furan 197 in excellent yield (Scheme 5.76). 

This c20 pyrophosphate (122), C20H36O7P2, is thought to provide the carbon framework of the diterpene alkaloids such as veatchine (123), atisine (124), and aconitine (125) (Fig. 5). It is not known at what stage the nitrogen is incorporated into the framework established by the skeleton. The potential for terpene rearrangements and the observation that the alkaloids are frequendy found esterified, often by acetic or benzoic acid, as well as free, has led to permutations and combinations producing over 100 such compounds. 

Heterogeneous solid-acid catalysis has the potential to make a major contribution towards improving the environmental acceptability of terpene rearrangement and isomerization processes. There are potential heterogeneously catalyzed replacement processes for the production of campholenic aldehyde, an important intermediate for many fragrance compounds. With continuing advances in the field of solid-acid catalysis it is likely that other industrially useful heterogeneous catalysts will be discovered. 

Esterification of Terpenes - Under standard conditions and temperatures from 40 to 100°C limonene and a-pinene were esterificated with acetic and acrylic acid, respectively, over the Amberlyst resin as well as the Nation composite. The conversion is high, however, the reaction is very unselective and no distinct main product is obtained. It seems that both terpenes rearrange to several different products before the final esterification takes place. Using camphene (2) as a starting material, much better results are obtained (Scheme 3). 

Reviews this year include the collation of evidence in favour of the o-bridged 2-norbornyl cation,the use of aliphatic diazonium ions as means for the production of carbocations, terpene rearrangements in superacid media,and the formation of carbocations by the silver-assisted reactions of chloroformates. A convenient procedure, using syringe techniques, has been described for the preparation of ca. IM solutions of alkyl cations in SbF5-S02ClF from suitable precursors that are soluble in SO2CIF at —78 Since conversions are essentially quantitative, the... 

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