1-Aryl-1-alkenyl N,N-diisopropylcarbamates (1) are obtained from alkyl aryl ketones and N,N-diisopropylcarbamoyl chloride (CbCl) by heating with excess pyridine. These undergo facile syn-carbolithiation by alkyllithium/diamine and produce configurationally stable lithiated benzyl carbamates, which have been trapped with different electrophiles. If the reaction is carried out in the presence of chiral diamines, such as (-)-sparteine or (-)-a-isosparteine, moderate enantiofacial differentiation is observed.The intermolecular nucleophilic addition of organolithium to an alkene is the first step in anionic polymerisation reactions of styrene. 1 For synthetically useful carbolithiation reactions, 2 the formation of intermediate A must proceed much more rapidly than the next addition step. Thus, special stabilization of A is required 3 as was demonstrated by Normant et al., utilizing cinnamyl derivatives C bearing a Lewis-basic substituent X (X = OR, NR 2 ) as substrates in carbolithiation reactions. 3,4 The complexinduced proximity effect (CIPE), 5 arising from the formation of the complex D, causes a rapid addition reaction to form the stabilized adduct E, which can be trapped by many different electrophilic reagents. 6 In addition to two carbon-carbon bond formations, up to two stereogenic centers are established. Usually, high diastereoselectivity is observed and, moreover, in the presence of chiral ligands such as (-)-sparteine, 7 high enantiofacial selectivity at the prostereogenic double bond is achieved (Scheme 1). 8 We expected that 1-aryl-1-alkenyl carbamates of type 1 could be ideal substrates for CIPE-driven stereoselective formation of secondary a-carbamoyloxy-benzyllithium compounds G by carbolithiation reactions. 9,10-12 A related study, concerning the carbolithiation of different 1-arylethenyl N,N-diethylcarbamates was published as a short communication by Snieckus et al. 13 when our work was in progress. We had discovered that chiral compounds G, prepared by deprotonation, are configurationally stable at low temperatures and these are substituted by proton acids with retention and by most other electrophiles with inversion of the configuration. Thus a convenient stereoselective approach for the preparation of more complex chiral benzyl derivatives of type H seemed possible (Scheme 2). 14 The vinyl carbamates 1a-f were prepared by heating the appropriate ketones 3 with N,N-diisopropylcarbamoyl chloride (4) 15 and pyridine (1.9-3.0 equiv) for 2-9 days. 16 Presumably, the reaction proceeds via the enol of 3 (Scheme 3, Table 1). The stilbene derivatives Z-1d/E-1d were obtained in a ratio of 68:32 and could be separated by chromatography on silica gel. The crystalline isomer Z-1d was subjected to a X-ray structure analysis (Figure 1). 17 As expected, the torsion around the O-C=O bond in Z-1d is more restricted than that in E-1d; whereas the 1 H NMR-absorption in CDCl 3 of the two isopropyl protons in E-1d coincide at 3.97 ppm, two separate, sharp signals (3.77 and 4.34 ppm) appear for Z-1d. The latter Schem...

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Homoenolat‐Äquivalente: Durch enantiotopos‐differenzierende Deprotonierung achiraler 1‐Alkenylcarbamate mit dem chiralen Basenpaar n‐Butyllithium/(−)‐Spartein werden konfigurativ stabile Lithium‐Homoenolat‐Äquivalente erzeugt. Im anschließenden syn‐ oder anti‐Substitutionsschritt (siehe Schema; ElX=Elektrophil) bilden sich enantio‐ und diastereoselektiv γ‐substituierte O‐(1‐Aryl‐1‐alkenyl)‐N,N‐diisopropylcarbamate.

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