Chiral sesquiterpenes

Highly uMtr-diastereofacial selective cycloaddition of isoprene (2) with 4-isopropyl-2-cyclohexenone allowed a short regiocontrolled and stereocon-trolled synthesis [13] of jS-cadinene and (y2-cadinene, Scheme 3.3). High anti-diastereofacial selectivity also occurs in the Diels-Alder reaction of optically active cyclohexenones 6-9 (Figure 3.2), readily available from the chiral pool, with open chain dienes [14-16]. Their cycloadducts are valuable intermediates in the synthesis of optically active sesquiterpenes in view of the easy conversion of the gem-dimethylcyclopropane and gem-dimethylcyclobutane in a variety of substituents. 

Rearrangement of dienynols to vinylallene sulfoxides. A few years ago, Oka-mura et al. (11, 39) reported the rearrangement of a dienynol to an allenyldiene with transfer of chirality of the propargylic alcohol. This rearrangement has now been used for an enantioselective synthesis of a sesquiterpene, (+ )-sterpurene (3).Thus reaction of the optically active propargylic alcohol 1 with C6H,SC1 at 25° results in a vinylallene (a) that cyclizes to the optically active sulfoxide 2. Nickel-... 

Caryophyllenes, as an example of two naturally occurring isomeric sesquiterpenes containing a medium-sized ring, in which the success of the total syntheses lies in the stereoselective control of a chiral centre, in a common synthetic key intermediate, which governs the configuration (JE or Z) of the double bonds present in each one of the two isomers. In this context, a brief reference to Cecropia Juvenile Hormone synthesis by the Syntex group, as well as to Johnson s cationic cyclisation of unsaturated polyolefins to fused polycyclic compounds, is made. 

Semen, reactive oxygen species, 612 Sensorial quaUty appreciation, oxidation stabihty, 664 Semm protein oxidative damage, 614 see also Human seram Sesquiterpenes, stractural chemistry, 133-6 SET see Single electron transfer Sharpless epoxidation, allylic alcohols, 789 Shelf durability, peroxide value, 656 Ship-in-the-bottle strategy, chiral dioxetane synthesis, 1176-7... 

The chiral hydrocarbon germacrene D is a widely spread plant constituent and is considered to be an important intermediate in the biosynthesis of many sesquiterpenes. Schmidt et al. [19, 20] have shown that the plant Solidago canadensis generates both optical antipodes of this compound by enzymatic cyclisation of farnesyl diphosphate using two different enantiospecific synthases. As to be seen in Fig. 17.5, the enantiomeric ratio of germacrene D in Solidago canadensis can vary from individual to individual [21]. 

Schmidt CO, Bouwmeester HJ, Franke S, Konig WA (1999) Mechanisms of the biosynthesis of the sesquiterpene enantiomers (-t) and (-) germacrene D in Solidago canadensis. Chirality 11 353... 

With Tartrate-Derived Chiral 1,4-Diol/Ti Complexes A catalytic asymmetric Diels-Alder reaction is promoted by the use of a chiral titanium catalyst prepared in situ from (Pr O TiC and a tartrate-derived (2.R,3.R)-l,l>4,4-tetraphenyl-2,3-0-(l-phenylethylidene)-l,2,3,4-butanetetrol. This chiral titanium catalyst, developed by Narasaka, has been successfully executed with oxazolidinone derivatives of 3-borylpropenoic acids as P-hydroxy acrylic acid equivalents [40] (Eq. 8A.21). The resulting chiral adduct can be utilized for the first asymmetric total synthesis of a highly oxygenated sesquiterpene, (-i-)-Paniculide. 

In addition, allenes can act as the olefinic part of the reaction [32], Al-lenynes like 12 may react with both double bonds. Brummond established the substitution patterns for the reaction with either the external or the internal bond of the allenic fragment, that give products with different ring sizes (13— 14) [33]. This group has applied these studies to the synthesis of hydroxy-methylfulvalene (17), a potent anticancer agent related with illudines, a natural sesquiterpene family. The key step was the synthesis of 16 from 15 with a PKR mediated by molybdenum carbonyl (Scheme 6) [34,35]. In addition they have developed an asymmetric version of the reaction. They have transferred efficiently chirality from a non-racemic allene to an a-alkylidene and an a-silylidene cyclopentenone in a molybdenum mediated reaction [36-38]. 

Three new chirality centers are formed with high enantio- and complete diastereoselectivity in the course of the reaction of the enol triflate 37 to the bicyclo [3.3.0]octane derivative 38 (Scheme 11) [15]. In this transformation, the intermediate 39, formed by oxidative addition, leads to the cationic palladium-7r-allyl complex 40, which is finally converted to the isolated product 38 by regio- and diastereoselective nucleophilic addition of an acetate anion. The bicyclic product 38 is of interest as a building block for the synthesis of capnellene sesquiterpenes. 

A sesquiterpene of furanodiene structure 60 is chiral because of its conformational rigidity as confirmed by a doubling of resonances in the C NMR spectrum in the presence of Yb(hfc)3/Ag(fod) . The methyl groups at positions 4 and 5 in 3,4,5,6-tetramethylphenanthrene (61) distort out of the plane to minimize steric hindrance, causing the compound to be chiral. Enantiomeric discrimination was observed in the H spectrum in the presence of Ln(hfc)3/Ag(fod) reagents . 

Sesquiterpenes were also separated by chiral chromatography on a Chiracel OD column. 

Uvidins are attractive chiral starting materials for the synthesis of highly oxidized biologically active drimane-like sesquiterpenes as demonstrated by the syntheses of natural (-)-cinnamodial (115) and (-)-cinnamosmolide (116) from uvidin A (3.14),... 

A useful synthesis (ref.ll)of patchouli alcohol, an important fragrant constituent of patchouli oil, from (+)-camphor, that onetime important natural product which was employed as a plasticiser for nitrocellulose (itself a semi-synthetic polymer), was complicated by structural revision of the sesquiterpene alcohol. Dihydrocarvone (14) obtained by saturation of the ring double bond in carvone, a major constituent of oil of spearmint has been employed for two very different sesquiterpenes, the ketone campherenone (15) and the alcohol, occidentalol (16). In the first case an enol acetate was converted to a bicyclic intermediate by earlier established methodology and the route emulated a plausible biogenetic sequence giving racemic campherenone (ref.12) as shown. Any chirality in (14) is apparently lost. 

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