Flaming Pointer
Leading Question Why is epimerization of the C7 stereocenter problematic in the Wittig reaction of the ester (37 to 38)
but not in the analogous reaction of the carboxylic acid (37 to 39)?
At the C7 stereocenter, there is an strong electron withdrawing carbonyl. The hydrogen attached to this stereocenter is acidic enough to be deprotonated by the basic ylide, forming an enolate. This is problematic because when the reaction is then quenched with acetic acid, the nucleophilic carbon is protonated both from the front and the back of the molecule, creating the unwanted epimer. However, this is not problematic with the analogous reaction because the first step yields a carboxylate. The molecule will not enolize under the Wittig reaction conditions now as having two negatively charged atoms that close in proximity is extremely unfavorable. This leaves the C7 stereocenter unchanged, yielding the desired product.