5α-Dihydrovespertilin acetate

In the title compound, C24H36O4 [systematic name: (20S)-3β-acetoxy-16α-hydroxy-22,23-bisnor-5α,17β-cholano(22-16)lactone], the three six-membered rings adopt classical chair conformations, while the five-membered rings are in envelope conformations. The ester group attached to ring A is in an equatorial position. Rings A/B, B/C and C/D are trans-fused, whereas rings D/E are cis-fused. The structure is devoid of any classical hydrogen bonds. However, non-classical inter- and intramolecular hydrogen-bonding interactions of the type C—H⋯O are present in the structure.

In the title compound, C 24 H 36 O 4 [systematic name: (20S)-3acetoxy- 16-hydroxy-22,23-bisnor-5,17-cholano(22-16)lactone], the three six-membered rings adopt classical chair conformations, while the five-membered rings are in envelope conformations. The ester group attached to ring A is in an equatorial position. Rings A/B, B/C and C/D are trans-fused, whereas rings D/E are cis-fused. The structure is devoid of any classical hydrogen bonds. However, non-classical inter-and intramolecular hydrogen-bonding interactions of the type C-HÁ Á ÁO are present in the structure.

Comment
During some investigations of the thermolysis of the N-chloro and N-bromo derivatives of secondary amides in aqueous dioxane (1:4 v/v) it was discovered that a radical-chain reaction ensued in which the first-formed N-centred radical abstracted a hydrogen atom from a proximate site. In the case of intramolecular reactions a 6-membered transition state was strongly favoured, leading to a C-centred radical which abstracted a halogen from starting material in a chain-propagating step. Under the reaction conditions the intermediate C-halo product underwent intramolecular heterolysis with the carbonyl oxygen of the amide displacing the halogen and generating an iminolactone which in turn hydrolysed to produce a γ-lactone. This process afforded a method for the functionalisation of chemically unactivated sites within the steroid nucleus as illustrated by its application to the synthesis of 5α-dihydrovespertilin acetate (I) from the 3-O-acetate of N-chloro-N-methyl-5α-bisnorcholanamide (Vohra, 1973).
The molecular structure of (I) is presented in Fig. 1 et al., 1987) and angles in (I) are as expected. The structure is devoid of any classical hydrogen bonds. However, non-classical inter and intra molecular hydrogen bonding interactions of the type C-H···O invoving O1 are present in the structure (Table 1). The crystal structure of a compound very closely related to (I), 3β-acetoxy-5α,6β-dihydroxy-bisnorcholanic acid 22,16-lactone, has been reported (Novoa de Armas et al., 2000).

Experimental
A solution of N-chloro-N-methyl-5α-bisnorcholanamide acetate (500 mg, 1.14 mmol), prepared from the parent amide by treatment with excess t-butyl hypochlorite in the dark, in aqueous 1,4-dioxane (1:4 v/v) (50 ml) containing calcium carbonate (2.5 g) and dibenzoyl peroxide (20 mg) was heated to 358 K (bath) under an atmosphere of nitrogen. After 2 hr a test for active chlorine (starch-KI paper) was negative. The mixture was filtered and the filter cake washed with dioxane (

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
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 > σ(F 2 ) is used only for calculating Rfactors(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.