Steroids spatial arrangement

RU 58668. This is a 17/J-estradiol derivative compound, substituted in the 11/9-position with a long hydrophobic side chain, producing a spatial arrangement similar to the 7a-substituted compounds in relation to the plane of steroid nucleus (Van de Velde et al. 1994,1996). 

Direct repeat of the recognition sequence requires a nonsymmetrical spatial arrangement of the bound protein subunits (see chapter 4, Nuclear Receptors). The protein-DNA complex has, in this case, a polar character and the protein bound on each of the two halves of the recognition element can carry out different functions. Direct 2-fold repeats are commonly observed for the DNA-binding elements of the steroid hormone receptors (see chapter 4). 

The precise spatial arrangement of the six atomic nuclei forming the transition state in the Barton reaction has not been demonstrated. However, results obtained in the rigid steroid system do offer some grounds for speculation. For example, the most favorable cases for the Barton reaction are 6/3 nitrites. In the transition state such molecules must have their five atoms designated as 1, 2, 3, 4, and 5 (structure 93a) rigidly placed. The sixth atom, i.e., H (structure 93a), can then reside between... 

Model compounds, containing rings C and D, or A, C, and D of lysergic acid, were compared with polycyclic compounds of the steroid series, whose absolute configuration, with respect to the spatial arrangement corresponding to C-5 of lysergic acid, was known. 

The spatial arrangement of all steroids of the alio- and the riormaZ-series puts the substituents in equatorial (e) or axial (a) positions ... 

Steric hindrance (Sections 4.8B and 6.13A) An effect on relative reaction rates caused when the spatial arrangement of atoms or groups at or near the reacting site hinders or retards a reaction. Steroid (Section 23.4) Steroids are hpids that are derived from the following perhydrocyclopentanophenanthrene ring system ... 

Squalene, a hydrocarbon CaoHso, was first isolated from shark liver and is found in small amounts also in mammals. As is evident from the biosynthesis just discussed, squalene is an isoprenoid composed of two symmetrical halves. Among the formulas shown here, two ways of writing the same molecule are presented. The spatial arrangement indicated in the second formula permits cyclization to the steroid skeleton, after a preliminary hydroxylation. The first isolable product is lanosterol ... 

---