Decomposition of Alkynyl Hydrazones: Synthesis of Allenoates, Dihaloallenoates, and Angularly Fused Tricyclic Azepines

An unprecedented decomposition of unprotected alkynyl
hydrazones
is attempted that has provided allenoates, tetrasubstituted α,γ-dihaloallenoates,
and functionalized tricyclic azepines. A reaction of alkynyl hydrazones
with N-halosuccinimides captures the electrophile
in 2-fold that delivers fully substituted dibromo- and diiodoallenoates
in good yields. In addition, a DABCO-promoted Wolff–Kishner
reduction of hydrazones, followed by isomerization, provides versatile
allenoates under mild conditions. In contrast, a similar decomposition
with ambiphilic DBU furnishes a completely different tricyclic azepine
scaffold in excellent yield and diastereoselectivity.