Asymmetric carbonyl olefinations diastereoselective

Generally, arsonium ylides [62] are more reactive but less accessible than phos-phonium ylides. Recently, the chiral arsonium reagent 30 has appeared, and has been applied in asymmetric Wittig-type carbonyl olefinations. This first chiral arsonium reagent also bears 8-phenylmenthyl as a chiral auxiliary on its carboalkoxy portion [63], and gave moderate chemical yields and diastereoselectivities in the conversion of 4-substituted cyclohexanone derivatives to axially chiral non-racemic alkylidene cyclohexanes under the same reaction conditions as used for the related reactions with phosphorus reagents (Scheme 7.15). On the other hand, the corre-... 

An asymmetric variant of this kind of allylic amination, based on their phenylcyclohexanol-derived chiral N-sulfinyl carbamates, was developed by Whitesell et al. (see also Sect. 3.2) (Scheme 34) [85]. After the asymmetric ene reaction with Z-configured olefins (not shown) had occurred, nearly di-astereomerically pure sulfinamides 127 were obtained which were found to be prone to epimerization. Their rapid conversion via O silylation and [2,3]-a rearrangement dehvered the carbamoylated allyhc amines 128 with around 7 1 diastereoselectivity as crystalline compounds that can be recrystallized to enhance their isomeric purity to 95 5. Obviously the imiform absolute configuration at Cl in the ene products 127 was difficult to transfer completely due to the already mentioned ease of epimerization. Unhke the sulfonamides of Delerit (Scheme 33) [84], the carbonyl moiety can easily be cleaved by base treatment. 

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