- Enantioselective cyclopropanation of enals by oxidative N-heterocyclic carbene catalysis
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Carbene catalysed redox activation of α,β-unsaturated aldehydes is applied for generation of α,β-unsaturated acyl azoliums which undergo cyclopropanation upon reaction with a sulfur ylide and an alcohol to give the corresponding cyclopropanecarboxylic acid esters. With chiral carbenes good to excellent diastereo and enantioselectivities are obtained.
- Biswas, Anup,De Sarkar, Suman,Tebben, Ludger,Studer, Armido
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supporting information; experimental part
p. 5190 - 5192
(2012/06/01)
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- Photochemical synthesis of highly functionalized cyclopropyl ketones
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A series of di- and trisubstituted cyclopropyl ketones 11 were prepared by irradiation of ketones 3 and 5, which bear a leaving group adjacent to the carbonyl C-atom. The required ketones 3 could be easily synthesized either by functionalization of ketones 1 with a hypervalent iodine reagent, 2, or by O-sulfonylation of α-hydroxy ketones 7. The nitrates 5 were obtained by treatment of the corresponding α-bromo ketones with AgNO3. The irradiation of 3 and 5 must be performed in the presence of an acid scavenger (1-methyl-1H-imidazole) to obtain the cyclopropanes 11 in good yields. The synthetic efficiency of the method was, among other things, demonstrated by the preparation of a highly strained bicyclo[2.1.0]pentane 11i in good yield. The mechanism of the photochemical cyclization was investigated by means of photokinetic measurements, as well as by quantum-chemical calculations. It was shown that the presence of the leaving group substantially influences all steps of the photochemical reaction cascade. The X-ray crystal structures of 11j and exo-11k were also determined.
- Wessig, Pablo,Muehling, Olaf
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p. 865 - 893
(2007/10/03)
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- A new photochemical route to cyclopropanes
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Especially highly substituted cyclopropanes can be produced with a new photochemical approach. Starting with aromatic ketones that bear a leaving group adjacent to the carbonyl carbon atom, photolysis leads to the formation of 1,3-diradicals, which efficiently cyclize to cyclopropanes (see scheme; Ms=MeSO2).
- Wessig, Pablo,Muehling, Olaf
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p. 1064 - 1065
(2007/10/03)
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- Reactions of stabilized sulfur ylides with α,β-unsaturated alkoxychromiumcarbene complexes
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Reaction of stabilized sulfur ylides with α,β-unsaturated alkoxy-chromiumcarbene complexes in MeCN as solvent forms vinylenol ethers 3, cyclopropanes 4, and vinylcyclopropanes 5, depending on the reagent ratio, the temperature and the nature of the ylide. The results obtained in these reactions suggest that factors apart from steric hindrance may control the chemoselectivity of the addition. Less stable ylides would add preferentially to the carbene carbon, 1,4-addition increasing as the stability of the ylide does. For more stable ylides 1,4-addition is preferred and substitution at the β-carbon has little effect in the chemoselectivity. Methoxycarbonylmethylentriphenylphosphorane exclusively add to the carbene carbon while ethyl diazoacetate is unreactive towards α,β-unsaturated alkoxychromium-carbene complexes in the conditions studied.
- Alcaide, Benito,Casarrubios, Luis,Dominguez, Gema,Retamosa, Angel,Sierra, Miguel A.
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p. 13215 - 13226
(2007/10/03)
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