Upon cleavage of the β-lactam ring, penems are converted to unstable Δ 4 -thiazolines, which epimerize at the ex-C-5 position, lose a nucleofugal group at C-2' (if originally present), tautomerize to Δ 3 -thiazolines, or suffer hydrolytic decomposition. Other chemical events, observed on some substrates under determinate conditions, include protomeric and oxidative aromatization to thiazoles (followed by side-chain reactions, such as epimerization at C-6, decarboxylation, dehydration, retro-aldol condensation), and conjugate addition of external and internal thiols. Mention is made of the chemical, biochemical and pharmacological significance of these findings