Enol silanes, stereogenic

Chiral a-methyl aldehydes (43) show exceptional diastereofacial preferences in their Lewis acid mediated reactions with enol silanes (equation i6) 2i 25c-26-64 selected data are reported in Table 8. The reason for this selectivity may be due to an approach trajectory of the nucleophile closer to the stereocenter when the carbonyl group is bound to the Lewis acid. Additions to chiral a-alkoxy aldehyde (48) were studied with both nonstereogenic (equation 17 Table 9) and stereogenic enol silanes (equation 18 Table 10). (Stereogenic and nonstereogenic are defined according to Mislow and Siegel.) ... 

Table 10 Ratio of Diastereoisomers in the Reactions of Stereogenic Enol Silanes with Aldehyde (48 equation 18)...

Cossy s approach for the synthesis of leucascandrolide A involved an asymmetric allylmetalation to incorporate the stereogenic centers at C(5), C(7), C(9), C(ll) and C(12), and olefin metathesis. Construction of the 2,6-cis- and 2,6-trani-tetr-ahydropyrans was achieved by the Mukaiyama enol silane addition and oxa-conjugate addition reaction, respectively. The synthesis commenced with the... 

In the addition of an achiral enolate to an aldehyde bearing a stereogenic center at Ca, the jr-facial selectivity can usually be predicted by the Felkin-Anh model (158 see also Chapter 2) [83]. However, simple 1,2-diastereoin-duction with enolates derived from typical metals such as lithium, boron, and titanium is often insufficient to be synthetically useful [16, 20). Higher levels of diastereoselectivity are generally obtained by use of enol silanes [34], as reported by Heathcock (Equation 14) [84]. 

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