4-Hydroxy-4,6a,6b,9,9,12a,14b-heptamethylperhydropicen-3-one hemihydrate isolated from Adiantum incisum

The title compound, C29H48O2·0.5H2O, is a triterpenoid isolated from the stems and rhizomes of Adiantum incisum. The basic skeleton of the molecule contains five six-membered rings, all adopting chair conformations, bearing a total of seven methyl, one hydroxyl and a keto group. There are two molecules of the triterpene and one water molecule of crystallization in the asymmetric unit. The two unique triterpenoid molecules hydrogen-bond directly via an O—H⋯O=C interaction, and are also bridged by the water molecule. The water also bridges to another pair of hydrogen-bonded triterpenoid molecules.

The title compound, C 29 H 48 O 2 Á0.5H 2 O, is a triterpenoid isolated from the stems and rhizomes of Adiantum incisum. The basic skeleton of the molecule contains five six-membered rings, all adopting chair conformations, bearing a total of seven methyl, one hydroxyl and a keto group. There are two molecules of the triterpene and one water molecule of crystallization in the asymmetric unit. The two unique triterpenoid molecules hydrogen-bond directly via an O-HÁ Á ÁO C interaction, and are also bridged by the water molecule. The water also bridges to another pair of hydrogenbonded triterpenoid molecules.  Table 1 Hydrogen-bond geometry (Å , ). Data collection: COLLECT (Hooft, 1998); cell refinement: DENZO (Otwinowski & Minor, 1997) and COLLECT; data reduction: DENZO and COLLECT; program(s) used to solve structure: SHELXTL (Sheldrick, 2000); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL and local programs.

Comment
Triterpenoids represent an important class of natural products characterized by highly pronounced biological activities such as analgesic, anti-mutagenic, and anti-inflammatory (Matsuda et al. 1999;Janaki et al. 1999). Adiantum incisum, a fern, generally found in the Himalayas region is well known for its antibacterial, antiviral and antifungal activities (Kshirsagar et al. 1972;Wada et al. 1987). The plant for the present work was collected from the Murree hills of Pakistan and during its phytochemical investigations, title compound (I) was isolated and its crystal structure is being reported here for the first time. (I) has also been reported as one of the constituents of Adiantum monochlamys (Ageta et al. 1966).
The molecule (I) is shown in Fig.1 and contains five six membered rings, all adopting chair conformations, fused together in a trans fashion. All the rings at their juncture bear one methyl group while ring A has two extra methyl groups. Ring E is in a slightly distorted form due to the presence of one methyl and a hydroxyl group at one carbon, C1, and a keto group on the adjacent carbon, C2.
The arrangement of molecular units suggests that the three dimensional structure is stabilized by inter molecular hydrogen bonds. Carbonyl oxygen (O4) of one triterpenoid molecule is involved in hydrogen bonding with the hydroxyl hydrogen (H1) of the other while its hydroxyl hydrogen (H3) is H-bonded with the oxygen (O5) of the water molecule. The H atoms of the water molecule are involved in inter-molecular hydrogen bonding with hydroxyl oxygen O1 (O1···O5 = 3.009 (9) Å) and carbonyl oxygen O2i (O5···O2i = 2.834 (9) Å; symmetry operator i = x + 1, y, z + 1) of the adjacent pair of molecules making an infinite chain along [1 0 1] as shown in Figs 2 & 3.

Experimental
The plant (Adiantum incisum) collected from Murree hills, Pakistan, was shade dried. The dried stems and rhizomes (2.9 kg) were dipped in ethanol for fifteen days and filtered. The filtrate was concentrated under reduced pressure to obtain the extract (51.33 g) followed by column chromatography using a gradient solvent system from hexane-chloroform-methanol.
The eluted fractions were monitored by thin layer chromatography and combined to give 38 main fractions. Fraction 9 was rechromatographed on a silica-gel column with hexane-chloroform (1:1) to afford the pure title compound (I).

Refinement
In the absence of anomalous scatterers Friedel pairs were merged and the absolute configuration of the compound was arbitrarily set. H atoms bound to C were placed in geometric positions (C-H distance = 0.99 Å for methylene; 1.00 Å for methyl H; 0.84 Å for OH) using a riding model. H atoms on the water molecules were not located in difference maps or included in the model. U iso values were set to 1.2U eq (1.5U eq for methyl H and OH) of the carrier atom. The displacement parameters of the atoms C33 C36, C48, C51 and C53 were restrained to an isotropic behaviour.   -4,6a,6 b,9,9,12a,14b-  Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger. Flack gave no indication of absolute structure, so Friedels merged. Material isolated from a plant, but no clear idea of absolute structure. Most likely this is correct however.