Fenchones enantiomers

The disruption of a dimeric form of cyclodextrins containing an appended pyrene unit (Fig. 14a) as a result of the inclusion of a guest compound was the rationale of the observed quenching of the pyrene excimer band observed by Ueno and coworkers [122]. Small differences in the inclusion of the two fenchone enantiomers (+)- and (—)-46 were reported. 

Figure 14. The C li core region XPS of fenchone recorded with a photon energy hv = 308.5eV. Included in the figure are bars indicating calculated AExxCPWSh — PW9I) + Qa core-binding energies. Data taken from Ref. [38]. The inset shows the structure of the (1S,4/ )-enantiomer.

The well-resolved C=0 li peak in the fenchone XPS provides an excellent opportunity to examine PECO from a single, well-characterized initial orbital. As has been previously mentioned, it might be thought that such a localized, spherically symmetric initial orbital would not be sensitive to the molecular enantiomer s handedness, but as can be seen in Fig. 15 (a) the dichroism in the electron yield recorded at the magic angle is sufficiently large to be easily visible by eye as a difference in the intensity of the Icp and rep spectra. 

Figure 15. Circular dichroism of the C=0 C li peak (BE = 292.7 eV) in fenchone at three different photon energies, indicated, (a) Photoelectron spectrum of the carbonyl peak of the (1S,4R) enantiomer, recorded with right (solid line) and left (broken line) circularly polarized radiation at the magic angle, 54.7° to the beam direction, (b) The circular dichroism signal for fenchone for (1R,4A)-fenchone (x) and the (lS,41 )-fenchone (+) plotted as the raw difference / p — /rep of the 54.7° spectra, for example, as in the row above, (c) The asymmetry factor, F, obtained by normalizing the raw difference. In the lower rows, error bars are included, but are often comparable to size of plotting symbol (l/ ,4S)-fenchone (x), (lS,4R)-fenchone (+). Data are taken from Ref. [38],...

In this instance, the (5)-enantiomer data have been negated prior to plotting. From previous discussion of the antisymmetry of the parameters under enantiomer exchange (e.g., Section III.A) it is recognized that it is then to be expected that the (R)- and (5)-enantiomer data should fall on the same experimental trend line. That they do indeed do so shows, as was argued in the Section IV.A for fenchone, that the behavior is at least qualitatively in accord with a pure electric dipole model. Furthermore, combining two distinct data sets [(/ )- and (5)-enantiomers] in this manner provides a consistency check on the reproducibility of the PECD data. It seems good practice to include measurement of both enantiomers, where this is feasible, in an experimental study. 

The two enantiomers of fenchone occur in a number of essential oils. Optically pure (15) (-f)-fenchone has been detected in bitter fennel oil Foeniculum vulgare var. vulgare) and in sweet fennel oil F. vulgare var. dulce) from various sources.It has also been reported to exist in... 

Cyclopentane bicyclic monoterpenoids that occur in the plant kingdom belong to three major skeletal types camphane, iso-camphane, and fenchane (Fig. 7). Camphane-type terpenoid alcohols, (+)-bomeol (Gl) and (—)-isobomeol (G2), have been isolated from Cinnamomum camphora (Lauraceae) and Achillea filipendulina (Asteraceae). A ketone derived from these, (-h)-camphor (G3), is found in the camphor tree Cinnamomum camphora) and in the leaves of rosemary Rosmarinus officinalis) and sage Salvia officinalis, Labiatae). Camphene (G4) and its enantiomer with the isocamphane carbon skeleton are known to occur in the oils of citronella and turpentine. Fenchane-type bicyclic cyclopentane monoterpenoids are commonly found in plants as their ketone derivatives. (—)-Fenchone (G5) occurs in the tree of life Thuja occidentalis, Cupres-saceae). Its enantiomer, (+)-fenchone (G6), has been isolated from the oil of fennel Foeniculum vulgare, Umbelliferae). 

A vast array of chiral auxiliaries have been derived from naturally occurring compounds containing the bicyclo[2.2.1]heptane unit (for review articles, see refs 1 -3). In all cases, the ultimate sources of these auxiliaries are the ketones camphor and fenchone, and the alcohols borneol and fenchol, as at least one enantiomer of each compound is provided in enantiomericaUy pure form by nature. Thus. ( + )-camphor [( + )-2], (-)-borneol [(-)- ], and (+)-fenchonc [( + )-5] are enan-tiomerically pure, convenient and inexpensive starting materials for organic synthesis and deriva-tization to give chiral auxiliaries. Most other compounds of this series are also commercially available, but can be prepared by oxidation or reduction of inexpensive precursors by standard methods. The evo-alcohols, such as the enantiomeric isoborneols, are accessible by standard complex hydride reductions of the ketones. The interconnection between these compounds is shown diagrammatically. 

As discussed in the next section, the magnitude of the stability constant does not determine the enantioselectivity. For example the enantiomers of fenchone and isomenthone form more stable complexes with than with a-CyD while the observed enantioselectivity is much higher for a- than for fi-CyD (see Table 5.1) [61]. 

Fenchane derivatives occur as fenchones and fenchols in several ethereal oils. Oil of fennel, obtained from the dried fruit of Foeniculum vulgare (Umbelliferae), contains up to 20 % (+)-fenchone, and is associated with limonene, phellandrene and a-pinene. (-)-Fenchone is isolated from the tree of life Thuja occidentalis (Cu-pressaceae), which is cultivated as hedges. The dextrorotatory enantiomer of a-fenchol with an endo OH, requested in perfumery, as well as its stereoisomers are found in fresh lemon juice, in oil of turpentine obtained from Pinus palustris (Pina-ceae), in ethereal oils originating from the Lawson white cedar Chamaecyparis lawsoniana (Cupressaceae) and other plant families such as Ferula, Juniperus, and Clausena species... 

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