Tricarbonylchromium(0) Promoted Stereoselective Cyclisations of the N-3,4-Dimethoxyphenethyl Derivatives of the 1-Phenyl Ethanolamines Halostachine, Ephedrine and Pseudoephedrine to 1-Phenyl-N-Methyl-7,8-Dimethoxy-1,2,4,5-Tetrahydrobenzazepines

Article abstract of DOI:10.1016/S0957-4166(00)82268-5

Acid promoted cyclisation of homochiral (R)-N-(3,4-dimethoxyphenethyl)-halostachine proceeds with almost total racemisation to yield 1-phenyl-N-methyl-1,2,4,5-tetrahydrobenzazepine (e.e, 6percent).Coordination of the cyclisation precursor to the tricarbonylchromium(0) moiety renders the cyclisation completely stereospecific to afford, after decomplexation, homochiral (+)-(R)-1-phenyl-N-methyl-1,2,4,5-tetrahydrobenzazepine. (-)-(1R,2S)-N-(3,4-Dimethoxyphenethyl)ephedrine undergoes acid mediated cyclisation to furnish trans-(-)-(1R,2S)-1-phenyl-2-methyl-N-methyl-7,8-dimethoxy tetrahydrobenzazepine as a single diastereoisomer.In contrast, the epimeric cyclisation precursor (-)-(1R,2R)-N-(3,4-dimethoxyphenethyl)pseudoephedrine cyclises to give a mixture (ratio 91:9) of trans- and cis-1-phenyl-2-methyl-N-methyl-7,8-dimethoxy tetrahydrobenzazepine.However, cyclisation of the tricarbonylchromium(0) complex of (-)-(1R,2R)-N-(3,4-dimethoxyphenethyl)pseudo-ephedrine is completely stereoselective to yield trans-(+)-(1S,2R)-1-phenyl-2-methyl-N-methyl-7,8-dimethoxy tetrahydrobenzazepine after decomplexation.