which, however, becomes flatter and flatter on passing from (1a) to (1e). As a result there is a progessive lessening of steric hindrance on the syn face which neatly parallels the observed increase in syn attack along the series (1a–e). Moreover the energy required to remove steric hindrance on passing from the boat-like to the half-planar conformation is definitively lower for (1c–e) than for (1b and
双环[3.2.0]庚-6-烯(1a)和2,4-二恶双环[3.2.0]庚-6-烯(1b)与
重氮甲烷和苯乙氧基
环氧乙烷反应,仅得到反加合物。相反,从相同的1,3-偶极与双环[3.2.0] hept-6-en-3-one(1c),2,4-二氧杂双环[3.2 ]的反应中分离出了顺式和反式加合物的混合物0.0]
庚-6-烯-3-酮(1E),甚至与显然最拥挤,在顺面,3,3-二甲基-2,4-二氧杂双环[3.2.0]庚-6-烯(1d)。广泛的从头算起MO计算(4-31G)表明化合物(1)更喜欢船形构象,但是从(1a)到(1e)时会变得越来越扁平。其结果上有空间位阻的progessive减轻顺面整齐地平行于所观察到的增加顺沿系列(攻击1A - ë)。此外,消除(1c – e)时从船形到半平面构象消除空间位阻所需的能量肯定比(1b和a)低。