The initial experiments employing thiophenol as the quenching thiol and MPA 5a as a representative alkyl thiol were unsuccessful, primarily due to significant thioester hydrolysis observed over time (see Supplementary Information). In contrast, employing p-hydroxythiophenol as the quenching thiol resulted in a satisfactory yield of the target MPA-derived peptide thioester 6a (HPLC yield 78%, 40% isolated). This success was achieved by introducing the alkyl thiol to the peptide thioester before extracting p-hydroxythiophenol. Notably, delaying the addition of the alkyl thiol until after the extraction process led to significant thioester hydrolysis. By adhering to this precautionary step, the reaction was complete in about 35 minutes after the addition of MPA (Fig. , red dots).