Transformed steroids. 123. Synthesis of 5α(H)- and 5α(OH)-6-ketosteroids with an additional 17α,20ξ-dihydroxytetrahydropyran ring E
The paper is devoted to the synthesis of 3,17α,20-trihydroxy-16β,23-epoxy-21,24-dinor-5αH-cholan-6-one and 3,5α,17α,20-tetrahydroxy-16β,23-epoxy-21,24-dinorcholan-6-one and derivatives of them. It has been shown that the reduction of 3,17α-dihydroxy-16β,23-epoxy-21,24-dinorchol-5-en-20-one with sodium tetrahydroborate and with diborane takes place stereospecifically with different spatial directivities: in the products of diborane reduction, ring E exists in the boat form, as has been shown by1H and13C NMR methods. The trans linkage of rings A/B in the modified steroids has been confirmed by their circular dichroism spectra.
Circular dichroism of some steroidal α-ketols and their acetates
Abstract An examination of the CD spectra of certain steroidal α-ketols and their acetates revealed instances where the Cotton effect of the ketone n → π * transition does not obey the octant rule. This was discussed in terms of the conformational preferences of the α-substituents. The relevance of these results to earlier configurational assignments on A-ring α-ketols in the cucurbitacin series was assessed.
Couplings into Methylene Groups: a New Nuclear Magnetic Resonance Approach to Stereochemistry
A method of assigning dihedral angles to hydrogens adjacent to a methylene function by using computer analyzed n.m.r. coupling constants in a modified Karplus equation is presented. The "Dihedral Angle Estimation by the Ratio Method" (DAERM) is based on the assumption that, although the magnitudes of the Karplus constants, k1 and k2, vary, the ratio of k1 to k2 is a constant. Dihedral angles computed in this way seem not to be influenced by the effects of ring strain and substituent electronegativity. The conformational preferences of several four- to six-membered ring Systems, some containing oxygen and sulfur atoms, have been investigated by this method. Molecular features such as ring puckering and flattening are indicated by DAERM as are some subtle electrostatic influences.