Synthesis of Enantiomerically and Diastereomerically Pure 4-Hydroxy-1,2-alkadienyl Carbamates and Their Application in a Modified Nazarov Cyclization Towards Chiral Cyclopentenones

Abstract

The addition of racemic titanated alkynyl N,N-diisopropylcarbamates onto enantiopure α-dibenzylamino- or α-silyloxy­alkanals affords two (out of four possible) diastereomeric, enantiopure 5-hetero-substituted 4-hydroxy-1,2-alkadienyl carbamates. As demonstrated for hydroxyallenes 23 and 24, the reaction sequence for modified Nazarov cyclization according to Tius, yields stereohomogeneous, highly substituted 5-alkylidene-2,3-dialkyl-4-phenyl-2-cyclopenten-1-ones (Z,R)-25 and (E,R)-26.

Key words

diastereoselectivity - allenes - Nazarov reactions - carbamates - cyclizations - cyclopentenones

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1

X-ray structure analysis.

13

Programs used for X-ray crystal structure analysis of 18: data collection EXPRESS (Nonius B.V., 1994), data reduction MolEN (K. Fair, Enraf-Nonius B.V., 1990), structure solution SHELXS-97 (Sheldrick, G. M. Acta Crystalogr. Sect. A 1990, 46, 467), structure refinement SHELXL-97 (G. M. Sheldrick, Universität Göttingen, 1997), graphics SCHAKAL (E. Keller, Universität Freiburg, 1997). Crystallographic data (excluding structure factors) for the structure reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC-229908. Copies of the data can be obtained free of charge on application to The Director, CCDC, 12 Union Road, CambridgeCB2 1EZ, UK [fax: int. code [phone]; e-mail: [e-mail]].

17

Yields of compound (Z,R)-25 and (Z,R)-26 are based on enone 20a.