Chiral auxiliaries stereoselective alkene cyclopropanation

D. Stereoselective Cyclopropanation of Alkenes using Chiral Auxiliaries. . . 266... 

Stereosectivity is a broad term. The stereoselectivity in cyclopropanation which has been discussed in the above subsection, in fact, can also be referred to as diastereoselectivity. In this section, for convenience, the description of diastereoselectivity will be reserved for selectivity in cyclopropanation of diazo compounds or alkenes that are bound to a chiral auxiliary. Chiral diazoesters or chiral Ar-(diazoacetyl)oxazolidinone have been applied in metal catalysed cyclopropanation. However, these chiral diazo precursors and styrene yield cyclopropane products whose diastereomeric excess are less than 15% (equation 129)183,184. The use of several a-hydroxy esters as chiral auxiliaries for asymmetric inter-molecular cyclopropanation with rhodium(II)-stabilized vinylcarbenoids have been reported by Davies and coworkers. With (R)-pantolactone as the chiral auxiliary, cyclopropanation of diazoester 144 with a range of alkenes provided c yield with diastereomeric excess at levels of 90% (equation 130)1... 

The range of alkenes that may be used as substrates in these reactions is vast Suitable catalysts may be chosen to permit use of ordinary alkenes, electron deficient alkenes such as a,(3-unsaturated carbonyl compounds, and very electron rich alkenes such as enol ethers. These reactions are generally stereospecific, and they often exhibit syn stereoselectivity, as was also mentioned for the photochemical reactions earlier. Several optically active catalysts and several types of chiral auxiliaries contained in either the al-kene substrates or the diazo compounds have been studied in asymmetric cyclopropanation reactions, but diazocarbonyl compounds, rather than simple diazoalkanes, have been used in most of these studies. When more than one possible site of cyclopropanation exists, reactions of less highly substituted alkenes are often seen, whereas the photochemical reactions often occur predominantly at more highly substituted double bonds. However, the regioselectivity of the metal-catalyzed reactions can be very dependent upon the particular catalyst chosen for the reaction. 

By stereoselective additions of carbenes or carbene equivalents to alkenes optically pure cyclopropanes are obtained. " In concerted [2+1] cycloadditions the stereochemistry of the alkene is conserved in the products. (Z)-configurated alkenes lead stereospecifically to cis-cyclopropanes. So far, compared to [2+1] cycloadditions involving alkenes bearing the chiral auxiliary, asymmetric reactions involving chiral carbene precursors proved to be less efficient. 

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