Oxygen‐Substituted Isocyanates: Blocked (Masked) Isocyanates Enable Controlled Reactivity

Oxygen‐substituted isocyanates ( O ‐isocyanates) are rare isocyanates with a reported propensity to trimerize, a side‐reaction that severely limited their use in synthesis. Herein, the development of blocked (masked) O ‐isocyanate precursors that form this reactive intermediate in situ provide controlled reactivity, allowing the first examples of cascade reactions involving O ‐isocyanates. Complex hydroxylamine‐derived hydantoins and dihydrouracil compounds can be rapidly assembled from α‐ and β‐amino esters, illustrating the convenience of masked O ‐isocyanates as hydroxylamine derived building blocks. Evidence for the intermediacy of an O ‐isocyanate intermediate is provided.