Photochemical synthesis of diaza-steroids

N-Iminopyridinium ylides 12 and 13 readily undergo regio-specific photoinduced ring expansion to the isomeric 1,2-diazepines 14 and 15 respectively. Similarly UV irradiation of the steroidal N-iminopyridinium ylide 25, which can be obtained from 19-nortestosterone, leads quantitatively and in a regiospecific manner to the corresponding diazepine 26. In a second photochemical reaction 26 gives the cyclobutene derivative 27. Variable temperature ¹H NMR and OCD measurements of the optically active diazepine 26 indicate that one of the two possible diastereoisomeric conformations is preferred.