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Organocatalytic conjugate addition of malononitrile to conformationally restricted dienones has been studied. A series of chiral primary and tertiary amine catalysts were screened. A piperidine-based thiourea-tertiary amine was found to be the efficient catalyst. Chiral pyran derivatives were obtained in excellent yields and enantioselectivities via a cascade conjugate addition-intramolecular cyclization pathway. The reaction is remarkably different for the corresponding reaction of conformationally flexible dienones.

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Asymmetric conjugate addition of bromonitromethane to a,b-unsaturated ketones was studied using a series of chiral primary amines as the catalysts. The cascade intramolecular cyclopropanation provided nitrocyclopropanes efficiently. Excellent enantioselctivities and good yields were obtained for several cyclic enones catalyzed by a bifunctional thiourea-primary amine. Both acid and base additives were found to be necessary for the transformation.Chiral cyclopropanes are important structural motifs in many drugs and natural products. 1 In addition they are useful synthetic intermediates for organic synthesis. 2 Great efforts have been made to develop efficient synthetic methods of chiral cyclopropanes. 3 Recently, asymmetric organocatalysis has been proved to be a powerful tool for the synthesis of chiral compounds. 4 Asymmetric organocatalytic conjugate addition of nucleophiles with aleaving groups and cascade intramolecular cyclopropanation provide a highly efficient strategy for the synthesis of chiral cyclopropanes. 5 Nitrocyclopropanes are a class of interesting cyclopropane compounds. They are presented in some natural products, such as the peptide lactone hormaomycin and also used for the synthesis of broad-spectrum antibiotic trovafloxacin. 6,7 Recently, we developed a highly enantioselective synthesis of nitrocyclopropanes via organocatalytic conjugate addition of bromomalonates to nitroalkenes. 8 Takemoto and co-workers reported another synthetic method via asymmetric conjugate addition of bromonitromethane to a,b-unsaturated cyanoimides. 9 In addition, we and the others also found that asymmetric conjugate addition of bromonitromethane to a,b-unsaturated aldehydes provided nitrocyclopropanes in excellent enantioselectivities and good yields. 10 The prolinol silyl ethers were efficient catalysts for this reaction, however, they were inefficient for the related reaction of bromonitromethane with a,b-unsaturated ketones. On the other hand, Ley and co-workers reported that the proline tetraazole was an efficient catalyst for the reaction of bromonitromethane with a,b-unsaturated ketones. 11Recently, Wang and co-workers found that 9-amino-9-deoxyepiquinine was an efficient catalyst for the reaction of bromonitromethane with cyclic enones and chalcones. The corresponding chiral nitrocyclopropanes were obtained with excellent enantioselectivities. 12 In a related study, we found that the highly enantioselective conjugate addition of bromonitromethane to a variety of alkyl vinyl ketones could be achieved using chiral primary amines as Table 1 Reaction of Cyclohex-2-enone (2a) and Bromonitromethane (3) Catalyzed by 1a-g a Entry Catalyst Time (h) Yield (%) b ee (%) c,d

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