The Mass Spectra of 13C Labelled Methylene Derivatives of Steroids-IV? F. Garcia, R. Cruz$ and M. zyxwvutsr Gonzalez Instituto de QuimicaO de la Universidad Nacional Aut6noma de Mtxico, Circuit0 Exterior, MCxico 20, zyx D.F., Mtxico The mass spectra of the 3-ethylene ketal of A5-pregnan[20-'3C]methylene-3-one and 17P-acetoxy[3- Clmethyleneandrost-1-ene have been examined. In both cases there are some fragments which show either relatively large retention or elimination of 13Clabel and these can be assigned to some fragmentationpaths which generally follow those found for diterpenic and steroidal hydrocarbons, and are in good agreement with the empirical fragmentations rules in mass spectrometry. 13 INTRODUCTION As a continuation of our interest in the mass spectra of C labelled compounds, we have studied the mass spectra of some modified steroidal structures. The mass spectra of these compounds are interesting since a great deal of work has been carried out on the mass spec- trometry of steroids. Some recent advances in this field have been reviewed by Djerassi.' Structure elucidation is one of the applications where the utility of mass spectrometry has been demonstrated. Yet correct interpretation requires an adequate understanding of the detailed fragmentation processes. In previous papers' it has been established that no extensive randomization is evident at the molecular ion level for some diterpenic and steroid molecules; this is an important point if one is to indulge in mechanistic interpretations. The very complex nature of hydrogen migrations, which has been demonstrated by deuterium labelling,' shows that it is naive to express fragmentation mechanisms in any but a dominant fragmentation route. If one considers that most fragments may have multiple origins, as have been shown both by deuterium and C labelling, the picture of fragmentation mechanisms necessarily becomes blurred. However, we feel that there is still a possibility of studying certain frag- mentation paths which are somehow related to struc- ture. 13 13 EXPERIMENTAL Spectra were obtained using an Hitachi Perkin-Elmer RMU-7H double focusing instrument. Samples were introduced through a direct inlet system at 150 zyxwvut "C. The ionizing current was held at 80 FA and the ionizing voltage at 70 eV. Lowr$e scans (100 s per decade) were run for both the 13 C enriched and the normal compounds. The C labelled compounds were synthesizedfrom the corresponding ketones by means of the Wittig reaction employing a technique described t Part 111, 'The mass spectra of [17-'3C]phyllocladene and zyxwvuts $ Author to whom correspondence should be addressed. [3-'3C]methylenecholestane'; see Ref. 1. Contribution No. zyxwvutsrqp 554 from Instituto de Quimica, UNAM. previ~usly.~ The labelled methyl iodide (from Bio-Rad) was 89.5% 13C enriched. The percentage of retention was calculated from the relative peak intensities with the aid of a computer program as described previously.' Relative peak intensities are reproducible within *~O/O. RESULTS The spectra of the unsaturated labelled and unlabelled compounds were compared using the computer pro- gram, and the results which include the extent of label retained at each zyxwv m/z ratio are listed in Tables 1 and 2. In the case of the 3-ethylene ketal of As-pregnanr20- Clmethylene-3-one cleavages across ring B show the largest selectivity. For some fragments a multiplicity of origins are evident, while others are quite specific. For example, fragments al, zyx a', bl and c1 show more than 90% retention of label, while fragments bZ, b3 and c4 show no more than 10% retention of I3C. These results are in good agreement with the postulated origins for the 13 Table 1. Retention of "C label at each zyx m/z value 3-Ethylene ketal of As-pregnan[20-'3Clmethylene-3-one % Label f 2% %Rel. int. Assignment' mlz 339 326 325 324 31 1 310 297 295 268 254 235 220 180 174 1 54 134 119 99 4 2 3 6 2 3 2 4 3 6 2 3 5 8 10 10 36 100 100 100 100 75 100 100 0 76 90 91 26 0 0 100 3 70 93 0 @ Heyden & Son Ltd, 1981 CCC-0306-042X/81/0008-0093$01 .OO BIOMEDICAL MASS SPECTROMETRY, VOL. 8, NO. 3, 1981 93