Meinwald J, Taggi A J, Luhan P A, McPhail A T
Proc Natl Acad Sci U S A. 1974 Jan;71(1):78-9. doi: 10.1073/pnas.71.1.78.
The C/D-cisD-norandrostanes 1a, 1b, 1c, 2a, 2b, and 2c were prepared and their reactivity studied. Whereas carbonium ion reactions of 1b and 1c are readily interpreted on the basis of their assigned stereochemistry, reactions of 2b and 2c suggest that these compounds react via conformations in which the 16beta-substituents are pseudoequatorial, requiring a ring C boat conformation as shown in formula 3b. X-ray crystallographic studies on 1a and 2a establish that while 1a has the expected conformation, 2a exists preferentially with ring C in the boat form. Molecular models suggest that this is the result of severe nonbonded interactions in the alternative conformation (3a) in which a chair C ring is retained.
制备了C/D-顺式D-降雄甾烷1a、1b、1c、2a、2b和2c，并研究了它们的反应活性。虽然1b和1c的碳正离子反应很容易根据它们指定的立体化学进行解释，但2b和2c的反应表明，这些化合物通过16β-取代基为假平伏键的构象进行反应，这需要如式3b所示的C环船式构象。对1a和2a的X射线晶体学研究表明，虽然1a具有预期的构象，但2a优先以C环为船式的形式存在。分子模型表明，这是由于保留椅式C环的另一种构象（3a）中存在严重的非键相互作用所致。
