As a test-bed for the reaction a series of potentially enolisable aldehydes 1 were trialled for SCALP reactivity using nonenolisable α-oxo aldehyde LYRAG 2 (Figure ). We hypothesised that phenylacetaldehydes would represent good nucleophilic SCALP-donors as the conjugated nature of the enol/enolates formed from these aldehydes should serve to make the enol/enolate more energetically accessible, thereby facilitating the synthesis of βhydroxyaldehyde bioconjugates 3 at neutral pH (Figure ). Our study in aqueous conditions supports this hypothesis, demonstrating aliphatic aldehydes, like butyraldehyde 12, have no appreciable SCALP reactivity, whilst with the exception of diphenylacetaldehyde 8, all phenylacetaldehydes afforded SCALP products in high conversion (Table ). Predictably benzaldehyde 11, which does not have an α-proton, showed no SCALP reactivity. Competition experiments were also conducted in which various SCALP-donors 5-14 and phenylacetaldehyde 4 were pre-mixed and delivered simultaneously to α-oxo aldehyde LYRAG 2. The relative conversions to SCALP-LYRAG products 3 were determined in order to establish the rates of reaction of the various SCALPdonors relative to phenylacetaldehyde 4. These experiments reinforce the hypothesis that the accessibility of the enolate at neutral pH determines the rate of the SCALP reaction, as the phenylacetaldehydes with strongly electron-withdrawing substituents, (4-nitrophenyl)acetaldehyde 10 and (4fluorophenyl)acetaldehyde 14, demonstrated the highest relative reactivities (see Table and Figure ).