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FATTY ALCOHOLS

 

Aliphatic alcohols occur naturally in free form (component of the cuticular lipids) but more usually in esterified (wax esters) or etherified form (glyceryl ethers). Several alcohols belong to aroma compounds which are found in environmental or food systems (see the website: Flavornet).
They are found with normal, branched (mono- or isoprenoid), saturated or unsaturated of various chain length and sometimes with secondary or even tertiary alcoholic function. An unusual phenolic alcohol is found as a component of glycolipids in Mycobacteria. Some cyclic alcohols have been described in plants.

A classification according to the carbon-chain structure is given below.

  1. Normal-chain alcohols

  2. Branched-chain alcohols

  3. Phenolic alcohols

  4. Cyclic alcohols



1 – Normal-chain alcohols

The carbon chain may be fully saturated or unsaturated (with double and/or triple bonds), it may also be  substituted with chlorine, bromine or sulfate groups. Some acetylenic alcohols have been also described.

– Saturated alcohols

Among the most common, some are listed below

Formula

Normal alcohols

Iso-alcohols

Anteiso-alcohols

C12H25OH

1-dodecanol
(lauryl alcohol)

10-methyl-1-hendecanol
(isolauryl alcohol)

9-methyl-1-hendecanol (anteisolauryl alcohol)

C14H29OH

1-tetradecanol
(myristyl alcohol)

12-methyl-1-tridecanol
(isomyristyl alcohol)

11-methyl-1-tridecanol (anteisomyristyl alcohol)

C16H33OH

1-hexadecanol
(cetyl alcohol)

14-methyl-1-pentadecanol (isopalmityl alcohol)

13-methyl-1-pentadecanol (anteisopalmityl alcohol)

C18H37OH

1-octadecanol
(stearyl alcohol)

16-methyl-1-heptadecanol (isostearyl alcohol)

15-methyl-1-pentadecanol (anteisostearyl alcohol)

Free fatty alcohols are not commonly found in epicuticular lipids of insects, although high molecular weight alcohols have been reported in honeybees (Blomquist GJ et al., Insect Biochem 1980, 10, 313). Long-chain alcohols also have been reported in the defensive secretions of scale insects (Byrne DN et al., Physiol Entomol 1988, 13,267). Typically, insects more commonly produce lower molecular weight alcohols. Honeybees produce alcohols of 17–22 carbons, which induce arrestment in parasitic varroa mites (Donze G et al., Arch Insect Biochem Physiol 1998, 37, 129). Two female-specific fatty alcohols, docosanol (C22) and eicosanol (C20), which have been found in epicuticle of Triatoma infestans (a vector of Chagas disease in South America), are able to trigger copulation in males (Cocchiararo-Bastias L et al., J Chem Ecol 2011, 37, 246). Hexadecyl acetate is found in the web of some spiders (Pholcidae) to attract females (Schulz S, J Chem Ecol 2013, 39, 1).
Long-chain alcohols (C18, C24, C28) from the femoral glands in the male lizard Acanthodactylus boskianus play a role in chemical communication as a scent marking pheromone (Khannoon ER et al., Chemoecology 2011, 21, 143).

Various fatty alcohols are found in the waxy film that plants have over their leaves and fruits. Among them, octacosanol (C28:0) is the most frequently cited.
Policosanol is a natural mixture of higher primary aliphatic alcohols isolated and purified from sugar cane (Saccharum officinarum, L.) wax, whose main component is octacosanol  but contains also hexacosanol (C26:0) and triacontanol or melissyl alcohol (C30:0). Policosanol is also extracted from a diversity of other natural sources such as beeswax, rice bran, and wheat germ (Irmak S et al., Food Chem 2006, 95, 312) but is also present in the fruits, leaves, and surfaces of plants and whole seeds. A complex policosanol mixture has been identified in peanut (Cherif AO et al., J Agric Food Chem 2010, 58, 12143). More than 20 aliphatic alcohols were identified (C14-C30) and four unsaturated alcohols (C20-24). The total policosanol content of the whole peanut samples varied from 11 to 54 mg/100 g of oil.
This mixture was shown to have cholesterol-lowering effects in rabbits
(Arruzazabala ML et al., Biol Res 1994, 27, 205). Octacosanol was also able to suppress lipid accumulation in rats fed on a high-fat diet (Kato S et al., Br J Nutr 1995, 73, 433) and to inhibit platelet aggregation (Arruzazabala ML et al., Thromb Res 1993, 69, 321). The effectiveness of policosanol is still questionable but it has been approved as a cholesterol-lowering drug in over 25 countries (Carbajal D et al., Prostaglandins Leukotrienes Essent Fatty Acids 1998, 58, 61), and it is sold as a lipid-lowering supplement in more than 40 countries. More recent studies in mice question about any action on improvement of lipoprotein profiles (Dullens SPJ et al., J Lipid Res 2008, 49, 790). The authors conclude that individual policosanols, as well as natural policosanol mixtures, have no potential for reducing coronary heart disease risk through effects on serum lipoprotein concentrations. Furthermore, sugar cane policosanol at doses of 20 mg daily has shown no lipid lowering effects in subjects with primary hypercholesterolemia (Francini-Pesenti F et al., Phytother Res 2008, 22, 318). It must be noticed that, for the most part, positive results have been obtained by only one research group in Cuba. Outside Cuba, all groups have failed to validate the cholesterol-lowering efficacy of policosanols (Marinangeli C et al., Crit Rev Food Sci Nutr 2010, 50, 259). Independent studies are required before evaluating the exact value of the therapeutic benefits of that mixture.
An unsaturated analogue of octacosanol, octacosa-10, 19-dien-1-ol was synthesized and was as effective as policosanol in inhibiting the upregulation of HMGCoA reductase (Oliaro-Bosso S et al., Lipids 2009, 44, 907). This work opens promising perspectives for the design of new antiangiogenic compounds (Thippeswamy G et al., Eur J Pharmacol 2008, 588, 141).
An unsaturated analogue of octacosanol, octacosa-10, 19-dien-1-ol was synthesized and was as effective as policosanol in inhibiting the upregulation of HMGCoA reductase (Oliaro-Bosso S et al., Lipids 2009, 44, 907). This work opens promising perspectives for the design of new antiangiogenic compounds.

1-Octanol and 3-octanol are components of the mushroom flavor (Maga JA, J Agric Food Chem 1981, 29, 1). 3-Octanol is a volatile infochemical present in fungi and recognisable by fungivores (Holighaus G et al., Chemoecology 2014, 24, 57).
Many alcohols in the C10 to C18 range, and their short-chain acid esters are potent sex or aggregation pheromones. They are mainly found as components of specialized defensive glands, pheromone glands or glands of the reproductive system.

A series of C22 up to C28 saturated n-alcohols, with even carbon numbers predominating, and a maximum at C26 and C28, has been identified in the cyanobacterium Anabaena cylindrica (Abreu-Grobois FA et al., Phytochemistry 1977, 16, 351). Several authors have reported high contents of the 22:0 alcohol in sediments where an algal origin is plausible. For example, the major alcohol in a sample of the lacustrine Green River Shale of Eocene age is also 22:0 which comprises over 50% of the alcohols present (Sever JR et al., Science 1969, 164, 1052)

Long-chain alcohols are known as major surface lipid components (waxes) with chains from C20 up to C34 carbon atoms, odd carbon-chain alcohols being found in only low amounts. Very long-chain methyl-branched alcohols (C38 to C44) and their esters with short-chain acids were shown to be present in insects, mainly during metamorphosis. A series of long-chain alkanols (more than 23 carbon atoms) were identified in settling particles and surface sediments from Japanese lakes and were shown to be produced by planktonic bacteria being thus useful molecular markers (Fukushima K et al., Org Geochem 2005, 36, 311).
Cutin and suberin contain as monomer saturated alcohols from C16 to C22 up to 8% of the total polymers. C18:1 alcohol
(oleyl alcohol) is also present.

Long-chain di-alcohols (1,3-alkanediols) have been described in the waxes which impregnate the matrix covering all organs of plants (Vermeer CP et al., Phytochemistry 2003, 62, 433). These compounds forming about 11% of the leaf cuticular waxes of Ricinus communis were identified as homologous unbranched alcohols ranging from C22 to C28 with hydroxyl group at the carbon atoms 1 and 3. Very-long-chain compounds were identified and quantified in the petal wax of Cosmos bipinnatus (Asteraceae). The most important were homologous series of alkane 1,2-diols and 1,3-diols, both ranging from C20 to C26 (Buschhaus C et al., Phytochemistry 2013, 91, 249). Relatively little is known about the functions of these compounds in the ecological and physiological fields.
In the leaf cuticular waxes of Myricaria germanica (Tamaricaceae) several alkanediols were identified (Jetter R, Phytochemistry 2000, 55, 169). Hentriacontanediol (C31) with one hydroxyl group in the 12-position and the second one in positions from 2 to 18 is the most abundant diol (9% of the wax). Others were far less abundant : C30-C34 alkanediols with one hydroxyl group on a primary and one on a secondary carbon atom, C25-C43
b-diols and C39-C43 g-diols. Very-long-chain 1,5-alkanediols ranging from C28 to C38, with strong predominance of even carbon numbers, were identified in the cuticular wax of Taxus baccata (Wen M et al., Phytochemistry 2007, 68, 2563). The predominant diol had 32 carbon atoms (29% of the total).
Long-chain saturated C30-C32
diols occur in most marine sediments and in a few instances, such as in Black Sea sediments, they can be the major lipids (de Leeuw JW et al., Geochim Cosmochim Acta 1981, 45, 2281). A microalgal source for these compounds was discovered when Volkman JK et al. (Org Geochem 1992, 18, 131) identified C30-C32 diols in marine eustigmatophytes from the genus Nannochloropsis
Two nonacosanetriols (7,8,11-nonacosanetriol and 10,12,15-nonacosanetriol) have been isolated from the outer fleshy layer (sarcotesta) of the Ginkgo biloba "fruit" (Zhou G et al., Chem Phys Lipids 2012, 165, 731). They exhibited slight activity of antithrombin and moderate activities of platelet aggregation in vitro.

The chief lipid fraction in the uropygial gland excretion of the domestic hen is a diester wax.  The unsaponifiable fraction consists of a series of three homologous compounds, which have been named the uropygiols and identified as 2,3-alkanediols containing 22-24 carbon atoms. These fatty alcohols are esterified by saturated normal C22-C24 fatty acids (Haahti E et al., J Lipid Res 1967, 8, 131).

Unsaturated alcohols

Some fatty alcohols have one double bond (monounsaturated). Their general formula is:

CH3(CH2)xCH=CH(CH2)y-CH2OH

The unique double bond may be found in different positions: at the C6: i.e. cis-6-octadecen-1-ol (petroselenyl alcohol), C9 i.e cis-9-octadecen-1-ol (oleyl alcohol) and C11 i.e cis-11-octadecen-1-ol (vaccenyl alcohol). Some of these alcohols have insect pheromone activity. As an example, 11-eicosen-1-ol is a major component of the alarm pheromone secreted by the sting apparatus of the worker honeybee. In zooplankton, the cis-11-docosen-1-ol (22:1 (n-11) alcohol) is not only present in high proportion in wax esters (54 to 83%) but may be also predominant in free form (75-94% of free alcohols) in ctenophores (Graeve M et al., Mar Biol 2008, 153, 643). This presence is unexplained because pathways for conversion and catabolism of fatty alcohols in ctenophores are still unknown.
Some short-chain unsaturated alcohols are components of mushroom flavor, such as
1-octen-3-ol, t2-octen-1-ol, and c2-octen-1-ol (Maga JA, J Agric Food Chem 1981, 29, 1). 1-Octen-3-ol is a volatile infochemical present in fungi and recognisable by fungivores (Holighaus G et al., Chemoecology 2014, 24, 57). The liverwort Marchantia polymorpha produces C8 volatiles mainly consisting of (R)-1-octen-3-ol (and octan-3-one) from arachidonic acid upon mechanical wounding (Kihara H et al., Phytochemistry 2014, 107, 42).
An acetoxy derivative of a 16-carbon alcohol with one double bond, gyptol (10-acetoxy cis-7-hexadecen-1-ol), was described to be a strong attractive substance secreted by a female moth (Porthetria dispar, "gypsy moth").

A fatty alcohol with two double bonds, bombykol (t