Reversed Stereochemical Course of the Michael Addition of Cyclohexanone to β‐Nitrostyrenes by Using 1‐(Trimethylsiloxy)cyclohexene/Dichloro(diisopropoxy)titanium. Preliminary Communication
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abstract
AbstractInduced by a stoichiometric excess of dichloro(diisopropoxy)titanium, 1‐(trimethylsiloxy)cyclohexene and p‐substituted β‐nitrostyrenes (Y = H,CH3,CH3O,CN) combine in CH2Cl2 solution at −90° preferentially with relative topicity ul – opposite to the corresponding reaction of enolates or enamines. The primary products are the bicyclic nitronates 3–5 which can be separated, and which are cleaved by KF in MeOH to give the aryl(nitroethyl)‐substituted cyclohexanones 1 and 2 (Tables 1 and 2, two typical procedures are given). The major products (2:1 to 4:1) are the hitherto not readily available diastereoisomers 2 of l‐configuration. Instead of being solvolyzed, the bicyclic nitronate 5 can be used for nitroaldol additions (→6) and for [3 + 2]‐dipolar cycloadditions (→7), diastereoselectively furnishing products with 4 asymmetric C‐atoms (not counting acetal centers). The Michael addition described here is yet another example of an ul‐combination of trigonal centers induced by Lewis acids, overriding the influence of the configuration of the donor component.
