Citronellal monoterpene

Isoprene metabolism in plants is very complex. Plants can synthesize many types of aromatic substances and volatile oils from isoprenoids. Examples include menthol (1= 2 ), camphor (1 = 2), and citronellal (1 = 2). These Cio compounds are also called monoterpenes. Similarly, compounds consisting of three isoprene units (1 = 3) are termed sesquiterpenes, and the steroids (1 = 6) are called triterpenes. 

Figure 15.7 Response of the molecular imprinted polymer quartz crystal microbalance (MIP-QCM) sensor to monoterpene analogues ( ) L-menthol, (A) D-menthol, ( ) citronel-lol, (A) citronellal, and (O) menthone. Reprinted from Percival et al. (2001). Copyright 2001 American Chemical Society.

In a similar approach, Shishido et al. (241) used oxime 215 [derived from the monoterpene (+)-citronellal (214)] for the synthesis of (—)-mintlactone (218) and (+)-isomintlactone (219), sweet compounds isolated from some Mentha species (Scheme 6.89). Bicyclic isoxazoline 216 was obtained in good yield from the cycloaddition. As expected, the product possessing tra i-l,4-substimtion prevailed. Reductive hydrolysis of the major isomer of 216 using hydrogen-Raney Ni-trimethyl borate provided p-hydroxyketone 217. This compound was dehydrated to give an enone and this was followed by carbonyl reduction-lactonization to complete the synthesis of both lactones 218 and 219 (241). 

Many of the monoterpenes found in essential oils of plants also occur as pheromonal substances in insects (45,57-60) and are often involved in plant-insect interactions. Some compounds found both in plants and insects are the monoterpenes citronellal, cit-ronellol, geraniol, myrcene, citral, 6-phellandrene, limonene, 2-terpinolene, a-pinene, 6-pinene, 1,8-cineole, and verbenone. 

These regular monoterpenes constitute a small class which includes the trienes myrcene 4 and ocimenes (5-7) and the alcohols geraniol 15, nerol 16, citronel-lol 17, linalool 18, etc (Structure 4.5). 

Fig. 8 Monoterpene ketones carvone, geranial, neral, and citronellal together geranial and neral make up citral...

Simple transformations (hydrolysis of the pyrophosphate group to a hydroxy group, elimination, oxidation, etc.) convert geranyl pyrophosphate to monoterpenes such as geraniol, myrcene, and citronellal. 

Figure 3.8 Aldehydes. Citronellal (C H180), monoterpene aldehyde. Courtesy Spiring Enterprises Ltd.

Non-cyclic monoterpenes are unsaturated, pleasant-smelling, C]q aliphatic compounds including aldehydes such as citronellal and citral (lemon-scented) the sweet-rose... 

D-Bornyl acetate (= Borneol acetate) (monoterpene) Cardenolides (triterpene glycosides aglycones) see Table 4.1 for cardenolide Na+, K+-ATPase inhibitors Citral (= mixture of a-Citral (Geranial) (3-Citral (Neral) = trans-St av-3,7-Dimethyl-2,6-octadienal) (monoterpene) Citronellal (= 3,7-Dimethyloct-6-enal) (monoterpene) a-Farnesene (sesquiterpene)... 

The molecules we have seen so far have usually been incorporated into the target molecule complete. There are two further and most important groups of larger molecules, the sugars and the terpenes, from which pieces are usually snipped out for incorporation. The simple monoterpenes (C10 compounds) citronellol 75, citronellal 76, and citronellic acid 77 are good examples. They are not cheap but both enantiomers are available, not always in excellent ee. 

N,H-D1ethylgeranylamine is a key intermediate for the synthesis of industrially important acyclic monoterpenes such as geranyl acetate, linalooi, 8 citral and citronellal... 

CHronellols (3,7-dimethyloct-6- and -7-en-l-ol). CioHjoO, Mr 156.27, formula, see citronellals. Acyclic monoterpene alcohols, of which the /8-forms of both enantiomers occur in nature (3/ )(+)-/S- and (35)(-)-/S-C., bp. 224-224.5 °C, [a]o 5 (neat). Both have strong, rose-like odors. 

Enantioselective GC has found a wide variety of applications, for instance, studies of citronellol (Ravid et al., 1992) and a-terpineol (Ravid et al., 1995) in a variety of species and verbenone in rosemary oils (Ravid et al., 1997). Chiral phases were applied to the separation of linalool and linalyl acetate in a variety of plant species (Konig et al., 1992 Cassabianca et al., 1998), extracts of Angelica seeds and roots (Holm et al., 1997) and other monoterpenoids in geranium oils (Kreis Mosandl, 1993). Separation of monoterpenes in Scots pine and juniper oils (Hiltunen Laakso, 1995), Abies (Holm et al., 1994) and Picea (Persson et al., 1996) oils and limonene, linalool, citronellal and P-citronellol Cymbopogon winterianus)... 

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