A-Acyl serinate

Naturally occurring oxazoles are derived from enzymatic post-translational modifications of peptide-based precursors <1999NPR249>. The oxygen functionality on the side chain of A -acylated serines (R = H) and threonines (R = Me) 307 is capable of undergoing heterocyclization onto the preceding carbonyl group to create five-membered saturated heterocycles 308 (Scheme 86). Dehydration followed by two-electron oxidation results in aromatic oxazoles 309. 

The fragmentation pattern of deprotonated A-acyl PS species is essentially identical to that of deprotonated PS species. The only difference is the replacement of a fragment ion corresponding to the loss of serine with A-acyl serine [61]. 

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