New strategies for enantioselective syntheses of 1-alkyl- and 1,4-dialkyl-1,2,3,4-tetrahydroisoquinolines: diastereoselective additions of nucleophiles and electrophiles to isoquinoline mediated by an easily resolved and recycled chiral transition metal auxiliary
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The chiral rhenium isoquinoline complex [(η5-C5H5)Re(NO)(PPh3)(iso-NC9H7)]+ TfO− (1) and (CH3)3-SiCH2Li give the addition product [formula omitted] in 71% yield as a 94:6 SS,RR/SR, RS diastereomer mixture. Similar reactions with RMgX (R = (CH3)2CH, CH3CH2, C6H5CH2, CH3(CH2)2, CH3, CH3(CH2)3) give analogous adducts (3–8) as 89–82:11–18 diastereomer mixtures. Reactions of 2 and ROTf (R = H/D, (CH3)3SiCH2, CH3) give alkyl-1,4-dihydroisoquinoline complexes [formula omitted] in 84–72% yields as 94:6 diastereomer mixtures. Related complexes are prepared from 3–5 and HOTf. These react with NaBH4/CH3OH to give alkyl-1,2,3,4-tetrahydroiso-quinoline complexes, which are in turn treated with (CH3CH2)4N+ CN− to give (η5-C5H5)Re(NO)(PPh3)(CN) (17) and the title compounds. A reaction sequence starting with (+)-(S)-1 and (CH3)3SiCH2Li yields [formula omitted] (84% overall, 88% ee) and (+)-(S)-17(82%, >98% ee). Other optically active alkyl tetrahydroisoquinolines are similarly prepared. Complexes 17 and (+)-(S)-17 are converted to (η5-C5H5)Re(NO)(PPh3)(CH3) (CH3OTf/NaBH4; 88–53%) and thence to 1 or (+)-(5)-1 (92-74%, >98% ee). A crystal structure and other data confirm the configurations assigned to the preceding compounds. © 1993, American Chemical Society. All rights reserved.
author list (cited authors)
Richter-Addo, G. B., Knight, D. A., Dewey, M. A., Arif, A. M., & Gladysz, J. A.