Monoterpenoids acyclic

6 Dimethyk ctanes.—Dembitskii et al. have reported the isolation of a rather unusual hydrocarbon, cis-2,6-dimethylocta-l,4,7-triene (4), from the plant Achilla filipendulina, the configuration about the cis-double bond being ascribed mainly on the basis of i.r. spectra. This recalls hymentherene that had been 

Dehydrolinalool (7), one of the intermediates in the synthesis of linalool and vitamin A, does not react with formic acid to give the expected a)9-unsaturated aldehyde or ketone, but forms instead the tetrahydropyran (8), together with the acetylcyclohexene (9).  

7-Dimethyl-l,5,7-octatrien-3-ol [(15) = R-isomer)] has been found naturally in both chiral forms. The 3S-(+)-enantiomorph occurs in Japanese Ho leaf oil (whence its trivial name, hbtrienol), while the R-isomer has been isolated from black tea and green tea. The R-isomer (15) was synthesised from R-linalyl acetate (10) by bromination with iV-bromosuccinimide, giving three allylically brominated acetates (11), (12), and (13), which all lead to the acetate 

Yamanishi, M. Nose, and Y. Nakatani, Agric. and Biol. Chem. (Japan), 1970, 34, 599. 

A certain number of well-known reactions in this series have been reinvestigated. Sasaki et al. have discussed the Diels-Alder 1,4-cycloaddition reactions of myrcene (27), comparing its activity with other dienes, in particular isoprene.  

The alcohols geraniol/nerol, linalool, citronellol and their esters are the largest tonnage materials of this class. (People in the fragrance industry tend to use nomenclature rather loosely. For example, to the chemist, 

A number of hydrocarbons of this family add oily, green or herbaceous notes to essential oils. Two hydrocarbons, myrcene and dihydromyrcene (also known as citronellene), deserve mention as feedstocks for other fragrance ingredients. 

There are several cyclic ethers derived from acyclic monoterpenoids which are of importance at lower levels in fragrances. 

---

G.l.c. papers of interest include the classification of 22 acyclic monoterpenoid alcohols according to retention indexes, resolution of cyclic ketones [e.g. ( )-menthone, ( )-isomenthone] as diethyl (+)-tartrate acetals, and the use of lanthanide shift reagents to resolve non-terpenoid racemic epoxides.The occurrence and prevention of monoterpenoid hydrocarbon isomerization during silica gel chromatography has been examined and the separation of monoterpenoids and sesquiterpenoids by gel permeation chromatography is reported. Monoterpenoid hydrocarbons have been selectively extracted from essential oils using dimethylsilicone. ... 

Monoterpenoids are responsible for fragrances and flavors of many plants and thus their products are used in perfumery and as spices. To date over 1,500 monoterpenoids are known, and these constitute acyclic, monocyclic, and bicyclic monoterpenoids (32), which occur in nature as hydrocarbons, alcohols, aldehydes, and carboxylic acids and their esters. Several acyclic monoterpenoid hydrocarbons are known, and these include trienes such as 3-myrcene (Cl), a-myrcene (C2), (Z)-a-ocimene (C3), ( )-a-ocimene (C4), (Z)-3-ocimene (C5), and ( )-P-ocimene (C6). 3-Myrcene and 3-ocimene are constituents of basil (Oci-mum basilicum, Labiatae) and bay (Pimenta acris, Myrtaceae), pettitgrain (Citrus vulgaris, Rutaceae) leaves, strobiles of hops (Humulus lupulus, Cannabaceae), and several other essential oils. Unsaturated acyclic monoterpene alcohol constituents of plants and their derived aldehydes play a signihcant role in the perfume industry. Some common acyclic monoterpene alcohols and aldehydes include geraniol (C7), linalool (C8) (a... 

References 275 vol. Ill c, d, 379 and 396 give more complete listings of acyclic monoterpenoids occurring in woody tissues. 

Clark et al. (1959) proposed citronellal (V) as a precursor, and were able to synthesize isoiridomyrmecin (II) from this acyclic monoterpenoid as shown below some iridomyrmecin (I) was undoubtedly formed as well, although it was not isolated. In the laboratory synthesis, the aldehyde grouping was protected during the allylic oxidation this would be unnecessary in an enzymatic oxidation. The second step is an intramolecular Michael addition, yielding iridodial (III), the immediate precursor to iridomyrmecins (I and II) in all three schemes. The final step is an intramolecular Cannizzaro reaction. 

---