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The acid-promoted cyclizations of a range of N-benzylethanolamines (derived from pseudoephedrine or ephedrine) give the corresponding trans-N(2),3-dimethyl-4-phenyl-1,2,3,4-tetrahydroisoquinolines with high levels of diastereoselectivity and in good yields of isolated product. The cyclizations of the corresponding chromium tricarbonyl complexes are rendered completely stereoselective. Acid-promoted cyclization of N-(3',4'-dimethoxybenzyl)ephedrine and its chromium tricarbonyl complex occur with complementary diastereoselectivities to give trans- and cis-N(2),3-dimethyl-4-phenyl-6,7-dimethoxy-1,2,3,4-tetrahydro-isoquinoline, respectively, in >99:1 d.r. The latter is consistent with a "double inversion" mechanism, which involves neighboring group participation by the chromium tricarbonyl moiety followed by rearomatization to give the corresponding cis-tetrahydroisoquinoline with overall retention of configuration.

Original publication

DOI

10.1002/asia.200900470

Type

Journal article

Journal

Chem Asian J

Publication Date

01/03/2010

Volume

5

Pages

589 - 604

Keywords

Cyclization, Ephedrine, Ethanolamines, Isoquinolines, Stereoisomerism