Addition of aldehyde dimethyl acetals (here acetaldehyde) to unisolated O-trimethylsilyl derivatives of 1-acetyl-3-arylmethylpiperazine-2, 5-diones (here 2, 5-dimethoxyphenyl), in the presence of TMSOTf as the catalyst, gave nearly quantitatively the corresponding N- methoxyalkyl derivatives which, under acidic treatment, gave in very good yield through a Pictet–Spengler-type reaction involving N-acyliminium cations (6S*, 11aR*)-2-acetyl-6- ...