Asymmetric Electrophilic alpha-Amidoalkylation, VII1): Generation, Crystal Structure, and Trapping Reactions of a Chiral 6,7-Dimethoxy-1,2,3,4-tetrahydroisoquinoline Derived N-Acyliminium Ion

Abstract

The camphanic acid amide 4 has efficiently been oxidized with triphenylcarbenium tetrafluoroborate (3) to yield the chiral N-acyliminium ion 1. Trapping reactions of 1 with the silyl nucleophiles 7a-c and 10a-f proceeded with stereoselective bond formation, affording the diastereomers (R)-8/(S)-9a-c and (R)-11/(S)-12a-f, respectively, with diastereoselectivities of up to 93.9/6.1. The amido ketones (R)-8/(S)-9a-c were employed in the synthesis of the secondary amines (R)-16a-c, (S)-16a and for the preparation of (-)-homolaudanosine (R)-18. By X-ray crystallography the conformation of 1 in the crystal lattice was established and the preferred conformation of 1 in solution was elucidated by NOE experiments. Finally, the addition reaction of 7a to the iminium ion 21 derived from menthyl carbamate 20 was investigated, which reaction, however, proceeded only with insignificant asymmetric induction