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Structural Studies on Three Plant Diterpenoids from Leonotis nepetaefolia

This paper describes the interesting structural studies on three new diterpenoids (plant products), namely, hydroxy-dialactone nepetaefolinol (9,13-epoxy-6b-hydroxy-8a-labdane-16,15 :19,20-diolactone), dehydrated nepetaefolinol (9,13-epoxylabd-5-ene-16,15:19,20-diolactone) and isomeric
tetrol (15,16-epoxy-labda-13(16),14-diene-6b,9,17,19-tetrol: which is the reduction product of new diterpenoid leonotinin) isolated from Indian herbal plant Leonotis nepetaefolia (collected in the flowering season from Guindy area, Madras), and are found to be the major compounds in their
extractions, so they are expected to possess the interesting pharmacological properties. In all three structures, the fused ring systems adopt distorted chair-chair, distorted chair and intermediate between 1,2-diplanar and half-chair and chair-chair conformations. In compounds 1 and 2, the twisted form of lactone rings are orthogonal to each other and make dihedral angle of 88.4(1) and 87.5(2)° with each other. The primary interaction between the molecules is van der Waal’s in nature.

Keywords: diterpenoid, leonotis nepetaefolia, hypotensive, antimicrobial, phosphodiesterase inhibitors, pharmacological (Received February 2, 2002; Accepted June 6, 2002)

1. Introduction

Recent chemical and biological research has resulted in much evidence concerning the defensive role of natural products. Natural products, the structurally diverse compounds act as growth inhibitors, antifeedants, antimicrobial, deterrents and insecticides. Leonotis nepetaefolia is one of the 41 species of Labiatae spread all over the country. It is used to treat bronchial asthma, diarrhoea, fever, influenza and malaria, and is also an
analgesic. Phytochemical analysis has revealed that L.nepetaefolia contains, amongst many constituents, labdanic acid (BAGHY et al., 1965), the diterpene methoxynepetaefolin (MACHAND, 1973), the terpenic alcohols nepetaefolinol and leonotinine (PURUSHOTHANAN et al., 1974) and a coumarin, characterized as 4,6,7-trimethoxy -5-methylchromen -2-one (PURUSHOTHAMAN et al., 1976). The pharmacological actions of the crude hydroalcoholic extract and stems of tea extracts obtained from Leonotis nepetaefolia may be purported for anti-asthmatic and anti-diarrhoeal properties. Current anti-asthmatic therapy is based mainly on the use of the drugs which act either via stimulation of b2-adrenoceptors, such as terbutaline and salbutamol, or by inhibition of smooth muscle phosphodiesterase, such as thecophylline. It is suggested that the pharmacological effects of the extracts of L. nepetaefolia may result from potentiation of the cAMP system. Considering that the extracts of L. nepetaefolia could potentially exhibit a profile of activity similar to that of b- adrenoceptor agonists and /or phosphodiesterase inhibitors, their effects on tracheal, ileal and uterine smooth muscle, as well as on cardiac muscle were examined (CALIXTO, YUNES and RAE, 1991).

Leonotis nepetaefolia is one of the important Ayurvedic herbal drug known as Grantiparani in Sanskrit. Phytochemical examination of this plant parts yielded different diterpenoids of labdane type. This plant exhibited various biological activities viz. antifungal, anti-malarial, anti-cancer, hypotensive and has attributed to a variety of salutary physiological effects (DAHL and NORMAN, 1970). The essential oil of L.nepetaefolia (Labiate) was tested for its anti-bacterial activity against both Gram positive and Gram negative bacteria and was found to be moderately active. The essential oil was also found to be inhibitory to dermatophytic fungi and suppressive to other aerial fungi. The oil may be quite useful in skin infections due to dermatophytes even with secondary bacterial infections (GOPAL, SARATHA VASANTH and VASUDEVAN, 1994). The plant is reported to be useful in
skin infections and also as a laxative and narcotic. Certain varieites of the leonotis species are referred to as "Wild Dagga" due to their mild narcotic properties.  It was originally believed that this was due to an alkaloid called leonurine, but there may be other compounds such as the three diterpenoids isolated from Leonotis nepetaefolia. 

A decoction of the leaves is used as tonic and to cure burns. The seeds possess feeble anti-malarial acitivity. The seed extract exhibited toxicity against Gram positive and Gram negative bacteria (GOPAL et al., 1995). Ash from the flowers is treated against burns (Wealth of India, 1962). Many species of the wide-spread family Labiatae have been used in primitive medical treatment of cancer. Thus every component of the plant has useful medicinal properties. Fig.1 shows the chemical diagrams of three diterpenoids..

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