Shimizu Y
Steroids. 1976 Aug;28(2):159-78. doi: 10.1016/0039-128x(76)90107-0.
Comparison of the mass spectra of 18-norsteroids with those of the corresponding normal steroids revealed that the absence of the 18-methyl group has little effect on the fragmentation pattern, especially on the D-ring and side-chain cleavage. The results indicate that the stability-of the cation or radical species, due to the quarternary nature of C-13, is not a major driving force in the key 13-17 cleavage. Hydrogen transfer observed in the D-ring fragmentation of 20-hydroxy-18-nor-5 alpha-pregnan-13-ones was not found to be specific to the 18-norcompounds. Deuterium labeling at the 20-OH and 20 alpha-H has suggested the involvement of a unique triple hydrogen rearrangement in this major cleavage. Migration of the 20-hydrogen to the 12-keto oxygen was also observed in the fragmentation of dammarane derivatives and was likewise confirmed by deuterium labeling.
18-去甲甾体与相应正常甾体的质谱比较表明，18-甲基的缺失对裂解模式影响很小，尤其是对D环和侧链裂解。结果表明，由于C-13的季碳性质，阳离子或自由基物种的稳定性不是关键的13-17裂解的主要驱动力。在20-羟基-18-去甲-5α-孕烷-13-酮的D环裂解中观察到的氢转移并非18-去甲化合物所特有。在20-OH和20α-H处进行氘标记表明，在这一主要裂解过程中涉及一种独特的三重氢重排。在达玛烷衍生物的裂解中也观察到20-氢向12-酮氧的迁移，同样通过氘标记得到证实。
