Zirconium enolates reversed

The a -enolate of cyclopentenone reacts with aldehydes to give anti and syn aldols in ratios of 70 30 to 9S S, with the degree of stereoselectivity being related to the size of R (equation 62). Similar yields, with reversed diastereoselectivity, are observed with the corresponding zirconium enolates. 

Aldol reactions of chiral dioxolanones (113) and (114) are summarized in Scheme 6 and Table 9. ° With both (113) and (114), essentially perfect diasterofacial selectivity is observed. The simple dia-stereoselection is modest to good, and is dependent on the enolate counterion. For the lithium and magnesium enolates, the sense of simple diastereoselection is the same as is observed with the achiral dioxolanone (107) and the chiral dioxolanone (110). Use of the zirconium enolate generally reverses the sense of simple diastereoselection, although the isomer ratios are not very high in some cases. 

It was assumed that the hthium etiolate adopts a fraws-configuration that was believed to be maintained after transmetallation with dichloro(dicyclopenta-dienyl)zirconium that was performed below —60°C. When, however, the zirconium enolate was kept at —30 "C, the subsequent aldol addition led to nearly equal amounts of syn- andawti-diastereomers - a result that indicates a traws-cis isomerization of the zirconium enolates at elevated temperatures. Compared to the aldol addition with the acetate (fJ)-173, the topicity in the approach of the enolate is reversed Ik for the acetate, ul for the propionate. This seems to... 

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