Oxygenated Monoterpenes as Main Constituents

Linalool

Linalool is one of the most common compounds in basil oil. It is characteristic of the essential oil of European or sweet basil (O.basilicum), but it is also common in oils of O.canum (e.g. Günther 1949, Gulati et al., 1977b, Sobti et al., 1976, Ntezurubanza et al., 1985, Gupta and Sobti 1993, Gupta and Tawa 1997), in O.menthaefolium and in

O.sanctum (Hegnauer 1966). Its percentage proportion in the oils ranges from traces up to almost 90%. linalool may co-exist with methyl chavicol, eugenol, methyl cinnamate, 1,8-cineole and with some other monoterpenes such as geraniol, citronellol, camphor, p-cymene, myrcene, and many others. Table 4.3 depicts the origin of the studied basil plants, the content of linalool and references in which linalool has been detected as the main compound or at least one of the major constituents.

Camphor

Hegnauer (1966) reported a camphor type essential oil of O.basilicum originating from Reunion, the Comoros and the Seychelles. Sobti and Pushpangadan (1982) reported that the camphor chemotype of O.basilicum contains 10-15% of camphor. Otherwise the reports of the chemical composition of O.basilicum essential oils, published during the last 20-30 years, do not include oils rich in camphor. For instance Fleisher and Fleisher (1992) found only 1.3% of camphor in the oil of O. basilicum grown in Israel.

Also the oils of O.gratissimum grown in Russia (Zamureenko et al., 1986) and in India (Khanna et al., 1988) have low levels of camphor (traces-6.7%). However, the oils of some O. gratissimum varieties such as O.gratissimum var. glabratum (Gupta 1994), O.gratissimum var. camphorata (Hegnauer 1966) and O. gratissimum var. intermedia (Gildemeister and Hoffmann 1961), as well as the oil of O.canum (Hegnauer 1966, Hoppe 1975, Xaasan et al., 1981) and O.kilimandscharicum (Hegnauer 1966, Gildemeister and Hoffmann 1961) have been reported to be rich in camphor. The camphor content of these oils varied from 23 to 70%, being generally 50 to 70% in the oils of O. kilimandscharicum and O.canum.

1,8-Cineole

1,8-Cineole is one of the key compounds in the classification of the chemical composition of essential oils of the Ocimum species. Among Ocimum oils the amount of 1,8cineole varies from traces to more than 60%. In the oil of O.kilimandscharicum and O. keniense 1,8-cineole was the main compound. Ntezurubanza et al. (1984) reported a new chemotype, 1,8-cineole type, of the essential oil of O.kilimandscharicum grown in Rwanda. This oil contained 62.2% of 1,8-cineole. 1,8-Cineole (38.4%) was reported as the main compound in the essential oil of O.keniense collected in Kenya (Mwangi et al., 1994).

The oil of O.gratissimum is either rich or poor in 1,8-cineole. Cheng and Liu (1983) examined an essential oil of O.gratissimum originating from Taiwan and found that the stem oil contained 40.2% and the flower oil 23.0% 1,8-cineole. In the leaf oil the percentage of 1,8-cineole was only 2.9%. Also an oil of O.gratissimum from Madagascar was rather rich in 1,8-cineole (12.0%) (De Medici et al., 1992). Lower levels of 1,8-cineole in O.gratissimum essential oils have been reported by Ntezurubanza et al. 1987), from the Amazon 3.3% (Vostrowsky 1990) and from Aruba 6.2% (Fun and from plants grown in Rwanda contained 0.3-3.5% 1,8-cineole (Ntezurubanza et al., (1987), Vostrowsky (1990) and Fun and Baerheim Svendsen (1990). The oils obtained Baerheim Svendsen 1990).

Table 4.3 Linalool as a prédominant constituent in the essential oils of basil

Origin

Linalool Content (%)

References

Calabria

40.0

Günther 1949

India

45

Egypt

not mentioned

Hörhammer et al., 1964

Bulgaria

50-63

Georgiev and Genov 1973

France

39.1

Zola and Garnero 1973

Italy

43.8

Zola and Garnero 1973

Morocco

41.9

Zola and Garnero 1973

Egypt

38

Karawya et al, 1974

Bulgaria

not mentioned

Hoppe 1975

80.7-87.3

Skrubis and Markakis 1976

not mentioned

Lang and Hörster 1977

South Africa, Egypt,

66-86

Peter and Rémy 1978

India, Yugoslavia,

France, Israel

Israel

37.5-55.4

Fleischer 1981

30-35

Sobti and Pushpangadan 1982

Bulgaria, Egypt, France,

56.0-70.0

Vernin et aL, 1984

Yugoslavia

Austria

40.7-75.5

Kartnig and Simon 1986

62.3-62.4

Srinivas 1986

commercial oil (group 2)

43.8-52.0

Srinivas 1986

commercial oil (group 3)

41.9-54.4

Srinivas 1986

experimental oil (group 2)

23.0-75.4

Srinivas 1986

experimental oil (group 3)

63.8-74.2

Srinivas 1986

Nigeria

30.1

Ekundayo et al., 1987

Hungary

33.8-46.7

Hälvä 1987

U.SA

7-62

Simon et aL, 1990

Portugal

32.2

Carmo et aL, 1989

Italy

46.0-50.0

Tateo et aL, 1989

Italy

45.7-63.1

Mariani et aL, 1991

Poland

54.1

Kostrzewa and Karwowska 1991

Bernreuter and Schreier 19911

Portugal

52.3-64.1

Roque

Portugal

41.7

Fleischer and Fleischer 1992

France

39.9

Baritaux et aL, 1992

Portugal

38.2

Baritaux et aL, 1992

Bulgaria

59.6

Baritaux et aL, 1992

Egypt

41.1

Baritaux et aL, 1992

The Netherlands

not mentioned

Suchorska and Osinska 1992

Egypt

51.6

Staath and Azzo 1993

Turkey

43.7

Pérez-Alonso et aL, 1995

Turkey

17.7-24.3

Özek etaL, 1995

1 enantiomeric distribution of linalool: R-(-)(90.7-100%), S-(-) (0-9.3%).

1 enantiomeric distribution of linalool: R-(-)(90.7-100%), S-(-) (0-9.3%).

O.canum originating from Aruba contained 7.0% of 1,8-cineole (Fun and Baerheim Svendsen 1990).

Charles et al. (1990) found 1,8-cineole to be the main compound together with eugenol in an essential oil of a Peruvian O.micranthum. The 1,8-cineole content was highest in the leaf oil (20%). The flower oil contained 7.0% and the stem oil 10.9% 1,8-cineole. An Egyptian O.rubrum oil contained rather high amounts of 1,8-cineole. In the leaf oil the 1,8-cineole concentration was 16.5% and in the flower oil it was 14.5% (Karawya et al., 1974).

The content of 1,8-cineole in the essential oil of O.basilicum varies from 2 to 16%, being generally less than 10%. The oil of O.basilicum grown in Madagascar (Randrimiharisoa et al., 1986), Nigeria (Ekundayo et al., 1987), Italy (Tateo 1989), Taiwan (O.basilicum var. minimum) (Cheng and Liu 1983) and in Turkey (Akgul 1981) contained less than 5% of 1,8-cineole. In addition, O.basilicum oils originating from Haiti (Srinivas 1986), Taiwan (Sheen et al., 1991), Israel (Fleisher 1981, Fleisher and Fleisher 1992), Turkey (Perez-Alonso 1995), Egypt (Karawya et al., 1974) contained less than 10% 1,8-cineole.

Among the major constituents of basil oils the content of (Z)-^-ocimene+1,8cineole varied from traces to 13.6% (Lawrence 1980). Ozek et al. (1995) compared the chemical composition of O.basilicum essential oils of Turkish origin obtained either by steam distillation or by hydrodistillation. They found that the oil obtained by steam distillation contained only about half the amount of 1,8-cineole compared to the oil obtained by hydrodistillation. The content of 1,8-cineole was 7.0% and 13.6%, respectively. Also Laakso et al. (1990) found the same phenomenon when studying the difference in chemical composition of essential oils obtained either by steam distillation or by hydrodistillation. They reported that the steam-distilled oil from O. sanctum of German origin contained 5.6% 1,8-cineole and the oil obtained by hydrodistillation from the same material contained 11.0% 1,8-cineole.

Modawi et al. (1983) reported 1,8-cineole to be one of the main compounds in the essential oil of O.basilicum var. thyrsiflorum grown in Sudan and Holm et al. (1989) reported a new chemotype of O.basilicum. They found 1,8-cineole to be the main compound in the material originating from Hungary and cultivated in Finland.

Citral, citronellal and geraniol

Citral, citronellal and geraniol are acyclic monoterpenes which generally co-exist in plant volatiles. Citral is a mixture of two acyclic monoterpene aldehydes, geranial [(E)-citral] and neral [(Z)-citral]. According to Hegnauer (1966) the citral-type essential oil of O.canum contains about 75% citral, O.gratissimum about 65%, O.menthaefolium Hochts. var. citrata about 56%, O.canum about 70% and the oil of O.pilosum Roxb. 34% of citral. Co-occurence of geraniol with citral has been detected in oils of O.gratissimum (geraniol 25%) and O.menthaefolium var. citrata (geraniol 10%). Hegnauer (1966) and Hoppe (1975) reported a high content of citronellal (41%) together with citral in an oil of O.pilosum.

Five chemotypes of O.basilicum have been reported. One of these was a geraniol rich chemotype (40-50%) and another chemotype contained 20-30% geraniol together with linalool and eugenol (Sobti and Pushpangadan 1982).

Thymol

The isolation of thymol from O.basilicum was reported for the first time in 1988 by Fatope and Takeda. Thymol is more characteristic of the essential oil of O.gratissimum where it is found in amounts of 19.3-47.6% (Hegnauer 1966, Sofowora 1970, Sainsbury and Sofowora 1971, Ntezurubanza et al., 1987, Dro 1974, Pino et al., 1996) and O.viride (40-65%) (Hegnauer 1966, Hoppe 1975, Simon et al., 1990).

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