Stereoselectivity benzyl derivative /3-eliminations

All -eliminations from the benzyl derivative in Figure 4.5 exhibit -stereoselectivity. This is true regardless of whether the elimination is syn- or anti-selective or neither. The reason for the preferred formation of the E product is product development control. This comes about because there is a significant energy difference between the isomeric elimination prod-... 

The carbonyl group in 24 could be temporarily protected as the dioxolane,16 or somewhat more efficiently as the dimethyl acetal 25.17 Subsequent conventional hydroboration led stereoselectively to the P alcohol 26. Although our earliest considerations called for ultimate base-promoted elimination of the OR substituent in intermediates more advanced than 16, we later came to recognize that a methoxy derivative was not going to fulfill this role to our satisfaction. Alternatively, the tactic of forming the benzyl ether at this point ultimately proved well suited to the eventual return to an unmasked hydroxyl at this site.17,18... 

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