Method for introducing deuterated lower alkyl into amine moiety of compound containing secondary amine

A method of introducing a mono-deuterated lower-alkyl into a secondary amine in a compound having a secondary amine, comprising

• replacing a hydrogen atom in the secondary amine by an aralkyl to obtain a tertiary amine,
• lower-alkylating the obtained tertiary amine with a deuterated-lower-alkylating agent under neutral or basic condition, and then
• eliminating the aralkyl group,
• wherein the aralkyl is a benzyl derivative selected from the group consisting of benzyl, 4-nitrobenzyl, 4-methoxybenzyl, 2-nitrobenzyl, 4-chlorobenzyl, 2,6-dichlorobenzyl, 4-methylbenzyl, and 2,4,6-trimethylbenzyl, and
• the compound having the secondary amine is a linear secondary amine or a compound having a piperidine, piperazine, pyrrolidine, azepane, diazepane, morpholine, azocane, or azetidine structure, which is a single ring system or a part of a bicyclic, tricyclic, or tetracyclic system.