4-Phenyl-1,2,4-triazoline-3,5-dione (PTAD) | CAS 4233-33-4 | ≥95%

Hetero-Diels–Alder reactions — PTAD (4-phenyl-1,2,4-triazoline-3,5-dione), also known as the Cookson reagent, is one of the most powerful dienophiles known in organic chemistry. It reacts with conjugated dienes in [4+2] cycloaddition reactions at room temperature within seconds, often quantitatively, to give stable urazole-containing Diels–Alder adducts. This exceptional reactivity arises from the low-lying LUMO of the azo N=N bond flanked by two electron-withdrawing carbonyl groups. PTAD is the reagent of choice when rapid, clean trapping of diene intermediates is required under mild conditions (Cookson, R. C.; Gilani, S. S. H.; Stevens, I. D. R. J. Chem. Soc. C 1967, 1905).

Vitamin D₃ chemistry — diene protecting group — PTAD is widely used as a protecting group for the s-cis-diene system of vitamin D₃ (cholecalciferol) and its metabolites. The PTAD–vitamin D adduct is stable to chromatographic purification, chemical modification at other sites, and prolonged storage. Removal of the PTAD group regenerates the diene under mild conditions. This strategy is essential in the synthesis and analytical characterisation of vitamin D metabolites including 1α,25-dihydroxyvitamin D₃ and 25-hydroxyvitamin D₃ (Windaus, A.; Linsert, O. Justus Liebigs Ann. Chem. 1928, 465, 148; Holick, M. F. et al. Biochemistry 1971, 10, 2799).

Analytical derivatisation of vitamin D — PTAD is the standard derivatisation reagent for the quantification of vitamin D₂ and D₃ metabolites by LC-MS/MS in clinical and food analysis. The PTAD adduct enhances ionisation efficiency and chromatographic separation, enabling detection of vitamin D metabolites at sub-nanomolar concentrations in serum and biological matrices. This application makes PTAD an essential reagent in clinical biochemistry laboratories worldwide.

Prismane synthesis — the first successful synthesis of prismane (tetracyclo[2.2.0.0²,⁶.0³,⁵]hexane) was achieved in 1973 using PTAD as a key reagent to trap the benzvalene–cyclopentadiene Diels–Alder adduct, demonstrating the reagent’s ability to capture highly strained and transient diene intermediates (Katz, T. J.; Acton, N. J. Am. Chem. Soc. 1973, 95, 2738).

Ene reactions — PTAD undergoes ene reactions with isolated alkenes bearing allylic hydrogens, providing a mild method for C–N bond formation. This reactivity complements its Diels–Alder chemistry and has been exploited for the functionalisation of terpenes, steroids, and other natural products containing trisubstituted alkenes.

Thiol oxidation to disulfides — PTAD selectively oxidises thiols to symmetric disulfides under mild conditions without over-oxidation to sulfoxides or sulfones. This makes it a useful reagent for disulfide bond formation in peptide chemistry and protein biochemistry, where controlled oxidation is critical for maintaining native folding.

Retro-Diels–Alder studies — PTAD adducts serve as substrates for studying thermal and photochemical retro-[4+2] reactions. The well-defined stereochemistry and stability of PTAD cycloadducts make them ideal model systems for investigating the orbital symmetry rules governing electrocyclic and cycloaddition processes.