Transformation of triterpene ketoximes into lactams by the action of ozone

Beckmann acid-catalyzed rearrangement of ketoximes is one of the principal methods for preparing cyclic lactams [1]. It is well studied and is used for a broad range of substrates including the class of terpenoids. Rearrangement of oximes of terpene ketones into lactams by the action of various acidic reagents is accompanied by the formation of seconitriles due to second-order Beckmann rearrangement [2–6]. Ozone is a strong oxidizing agent that does not exhibit the required acidic properties to catalyze effectively a rearrangement. However, in certain instances where ozonolysis of O-methyloximes of cycloalkanones is carried out in the presence of carbonyl compounds (Griesbaum ozonolysis [7, 8]), N-methoxylactams, i.e., analogs of the Beckmann rearrangement products, are formed as side products in small amounts. The corresponding amides of carboxylic acids are formed for linear O-methylketoximes. The mechanism of these transformations differs from that of the Beckmann rearrangement. Apparently the methoxylactams are formed via epoxidation of the C=N bond of the O-methylketoxime by the action of ozone. Then, the unstable intermediate epoxide rearranges into the corresponding cyclic N-methoxylactam (Scheme 1) [7, 8]: