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The Stereochemical Course of the Generation and Interception of aSix‐Membered Cyclic Allene: 3δ 2 ‐1 H ‐Naphthalene (2,3‐Didehydro‐1,2‐dihydronaphthalene)
Author(s) -
Christl Manfred,
Braun Martin,
Fischer Hartmut,
Groetsch Stefan,
Müller Germar,
Leusser Dirk,
Deuerlein Stephan,
Stalke Dietmar,
Ar Mario,
Engels Bernd
Publication year - 2006
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/ejoc.200600443
Subject(s) - chemistry , methyllithium , indene , allene , furan , enantiomer , steric effects , adduct , reagent , stereochemistry , medicinal chemistry , organic chemistry , catalysis
The bromofluorocarbene adduct rac ‐ 1 of indene, possessing the fluorine atom at the endo position, is a useful substrate for the generation of the isonaphthalene 4 by the Doering–Moore–Skattebøl reaction. By resolution of rac ‐ 1 , an enantiomerically pure precursor of a six‐membered cyclic allene was obtained for the first time. The treatment of (+)‐ or (–)‐ 1 , dissolved in 2,5‐dimethyl‐, 2‐ tert ‐butyl‐5‐methyl‐, or 2,5‐bis( tert ‐butyl)furan, with methyllithium gave rise to the [4+2] cycloadducts 6 of 4 to the furans. It was shown by means of HPLC on Chiralcel OD that the formation of 6 proceeded with about 40 % ee , and that this value was independent of the type and concentration of the furan, and the reaction temperature. The absolute configurations of the enantiomers 1 , as well as those of the enantiomers 6 , were determined by simulation of the CD spectra by quantum chemical methods and by the comparison of them with the experimental spectra. In the case of (+)‐ 1 , the reliability of this procedure was checked by X‐ray crystal structure analysis. On the basis of these results, a model is proposed for the steric course of the reaction sequence, leading from a pure enantiomer 1 to the 70:30 mixtures of the product enantiomers 6 . The use of indene as trapping reagent for 4 furnished the [2+2] cycloadduct 15 . For the preparation of rac ‐ 15 , the particularly simple one‐pot procedure was employed, in which indene, tetrabromomethane, and methyllithium were combined and in which the dibromocarbene adduct 2 of indene serves as precursor of 4 . Compound 2 could not be isolated, but was characterised by low‐temperature 1 H NMR spectra. The conversion of (+)‐ or (–)‐ 1 into 4 in the presence of indene afforded 15 only with a very low enantioselectivity. The constitutions and the relative configurations of 15 as well as the compounds 8 and 16 , which resulted from thermolysis of the cycloadducts 6b and 15 , respectively, were elucidated by X‐ray crystal diffraction. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)