With the aim of decreasing the reaction time at low temperatures and thus
maintaining high selectivity, we employed ultrasonic irradiation
using a standard commercial cleaning bath operating at a frequency
of 42 kHz.12 In a systematic study, compound 1 was reacted
with piperidine in the presence of pyridine, leading to a
mixture of the monosubstituted (2a) and the disubstituted (3a)
product shown in Scheme 1.