Sesquiterpenes interconversions

While a dynamic NMR study38 was carried out to reveal rather complicated patterns of conformational interconversion in some eleven-membered sesquiterpenes including humulene (40) and zermbone, an interesting controversy concerning conformational stability in the germacrone family of sesquiterpenes is worth mentioning here. 

Structure elucidation of many Lactarius sesquiterpenes often requires interconversions of y-hydroxybutenolide, butenolide, and furan rings for confirming spectroscopic assignments. Examples of DIBAL reductions of y-lactones to the corresponding furans include the conversions of blennin C (14.5) to lactarol (19.3) (Scheme 12) (60) and of compound 11.20 to 3-dcoxy-3-cp/-furandiol (87) (Scheme 10). 

In general, the sesquiterpenes described to date have a stable configuration and conformation free from interconversions and normally determined by the configuration of the chiral centres C-1, C-4, C-5, C-7 and C-10. This structural rigidity is also apparent in the stereochemistry and functionality of some stereocentres ... 

The lactarane sesquiterpenes 10.2, 10.10, 10.13, containing the 2(9),7(8)-cycloheptadiene ring were submitted to conformational analysis by molecular mechanics and HNMR spectroscopy (9). It was observed that the conformational mobility of each compound is practically restricted to the interconversion of the envelope forms of the cyclopentene ring by contrast, essentially only a single conformation of the seven membered ring, with the 3-methyl group pseudo-equatorially oriented, is populated. This is due to the planarity of either the diene in 10.13 or the diene-carbonyl double bonds in 10.2, 10.10. 

Lactaranolide derivatives included in Part 12 are produced by silica gel degradation of velutinal esters (7.28, 7.30), methylvelutinal (7.17) or free velutinal itself (for a discussion of these transformations see the chapter on chemical interconversions of Lactarius sesquiterpenes). The compounds upon acidification underwent aromatization to form furanoid derivatives (see Part 18). Under these conditions dehydration reactions took place and also dienes were formed (72). 

Biosynthesis of ( , )-Famesyl Pyrophosphate Interconversion of 2-E-6-E- and 2-Z-6- -Famesyl Pyrophosphate Acyclic Sesquiterpenes Cyclization of Acyclic Precursors The Origin of Major Groups of Cyclic Sesquiterpenes Biological Activity Allelopathy Medicinal Properties... 

Wallach proposed that one isoprene (2-methyl-l, 3-butadiene) unit of five carbon atoms could serve as this function. This idea was eventually elaborated on by Ruzicka and Bloch into the isoprene rule. Eventually, as we will see, the family of terpenes (Cio), sesquiterpenes (C15), diterpenes (C20), sesterpenes (C25), triter-penes (C30), and truly large compounds (polymers) found in latex and gutta-percha comprises tens of thousands of individual, unique compounds all of which may be considered as derived from isoprene (2-methyl-l,3-butadiene) and that the pair of C5 isomers, isopentenyl diphosphate and dimethylallyl diphosphate, compounds linked by interconversion (Scheme 11.41) under the influence of the isopentenyl isomerase (EC 5.3.3.1) serve as the biochemical parents to the family. ... 

Hayashi, Y., M. Nishizawa, and T. Sakan Interconversion of Pterosins, Illudoid Sesquiterpenes Synthesis of Onitin (4-Hydroxypterosin Z) and ( + )-Pterosin D. Chemistry Letters 1974, 945. 

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