## Introduction
### Chemical constituents  
The chemistry of _Withania_ species has been extensively studied and several groups of chemical constituents such as steroidal lactones, alkaloids, flavonoids, tannin etc. have been identified, extracted, and isolated [14,21-25]. At present, more than 12 alkaloids, 40 withanolides, and several sitoindosides (a withanolide containing a glucose molecule at carbon 27) have been isolated and reported from aerial parts, roots and berries of _Withania_ species. The major chemical constituents of these plants, withanolides, are mainly localized in leaves, and their concentration usually ranges from 0.001 to 0.5% dry weight (DW) [14,26,27]. The withanolides are a group of naturally occurring C\({}_{28}\)-steroidal lactones built on an intact or rearranged ergostane framework, in which C-22 and C-26 are appropriately oxidized to form a six-membered lactone ring. The basic structure (Figure 1) is designated as the withanolide skeleton [28-32]. The withanolide skeleton may be defined as a 22-hydroxygostan-26-oic acid-26,22-lactone. There are many novel structural variants of withanolides with modifications either of the carbocyclic skeleton or the side chain and these have often been described as modified withanolides or ergostan-type steroids related to withanolides. These compounds are generally polyoxygenated and it is believed that plants elaborating them possess an enzyme system capable of oxidizing all carbon atoms  
Figure 1: The basic structure of withanolides. in a steroid nucleus. The characteristic feature of withanolides and ergosane-type steroids is one C\({}_{8}\) or C\({}_{9}\)-side chain with a lactone or lactol ring but the lactone ring may be either six-membered or five-membered and may be fused with the carbocyclic part of the molecule through a carbon-carbon bond or through an oxygen bridge. Appropriate oxygen substituents may lead to bond scission, formation of new bonds, aromatization of rings and many other kinds of rearrangements resulting in compounds with novel structures [28,30,33]. Withanolides are present in fifteen Solanaceous genera i.e. _Acnistus_, _Datura_, _Deprea_, _Dunalis_, _Iochroma_, _Jaborosa_, _Lycium_, _Nicandra_, _Physalis_, _Salpichroa_, _Tubocapsicum_, _Discopodium_, _Trechonaetes_, _Withania_ and _Witheringia_. The occurrence of withanolides is not completely restricted to Solanaceous plants and reports of their isolation from marine organisms, and from members of the Taccaceae [34,35], Fabaceae (Leguminosae) [36] and Lamiaceae (Labiatae) [37] families suggest that they are much more widely distributed. **Biosynthesis of withanolides**  
The biosynthetic pathways of withanolides and other chemical constituents of \(W\). _somnifera_ are not fully known, and there is very little information about their biogenic aspects [38-40]. It has been reported that, except for a very few exceptions, the plants that synthesize the 20-H withanolides are unable to produce the 20-OH counterparts and vice versa [38]. Since withanolides are probably derived from cholesterol, this is a pertinent starting point to discuss their biosynthesis. The first step in the biosynthesis of cholesterol is the activation of acetate by its conversion to acetyl Co-enzyme A, abbreviated as acetylCoA. Two units of acetylCoA are combined and metabolized to mevalonic acid. Only the _R_- form of mevalonic acid is used by the living system to produce terpen, while the _S_- form is metabolically inert. The (_R_)-mevalonic acid is converted into isopentenyl pyrophosphate (IPP) through the loss of one carbon atom. The molecule of 3-isopentenyl pyrophosphate (IPP) can condense in a head-to-tail manner with its isomer, 3,3-dimethyl allyl pyrophosphate (DMAPP), to give geranyl pyrophosphate (GPP). A condensation reaction of _trans_ geranyl pyrophosphate with another molecule of IPP yields farnesyl pyrophosphate (FPP). The enzyme squalene synthase catalyses the condensation of two molecules of farnesyl pyrophosphate in a head-to-head manner in the presence of NADPH to produce squalene.