Although of course no condensation occurred (the nucleophile was removed!), deuterium was incorporated into the PEP substrate, suggesting that (reversible) protonation of the PEP double bond had occur...Although of course no condensation occurred (the nucleophile was removed!), deuterium was incorporated into the PEP substrate, suggesting that (reversible) protonation of the PEP double bond had occurred without participation by the S3P hydroxyl - in other words, protonation appears to occur first, as expected for an electrophilic addition.
