Cyclic nitronates from the diastereoselective addition of 1-trimethylsilyloxycyclohexene to nitroolefins. Starting materials for stereoselective Henry reactions and 1,3-dipolar cycloadditions
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abstract
Induced by a stoichiometric excess of dichloro(diisopropoxy)-titanium, 1-trimethylsilyloxycyclohexene and (E)-nitroolefins (R—CH=CHNO2, R = C6H5, p-C6H4CH3, p-C6H4OCH3, p-C6H4CN, CH3) combine in CH2Cl2 solution at −90 °C preferentially with relative topicity ul, opposite to the corresponding reaction of enolates or enamines. The primary products are the epimeric bicyclic nitronates 4–6, which can be separated, and which are converted by KF in MeOH to the alkyl or aryl (nitroethyl)-substituted cyclohexanones 1 and 2 (Table 1). Instead of being solvolysed, the cyclic nitronates 4–6 can be used for nitroaldol additions to give the trisubstituted hexahydrobenzopyranols 15–19. Alternatively, 4–6 can be used in 1,3-dipolar cycloadditions with acrylates to furnish the tricyclic compounds 22–31. In either case, products with four asymmetric carbon atoms (not including acetal centres) are produced diastereoselectively. The addition described here is yet another example of an ul combination of trigonal centres induced by Lewis acids, overriding the influence of the configuration of the donor component.
