Ueno-Stork cyclization

What is Ueno-Stork cyclization?

The Ueno-Stork cyclization, which was first described by Ueno et al. in 1982 and subsequently by Stork et al. in 1983, involves a radical cyclization, with azobisisobutyronitrile (AIBN) and tributyltin hydride n-Bu3SnH, of the α-haloacetal of an allylic alcohol. Ueno-Stork cyclization, also referred as Ueno-Stork reaction, Ueno-Stork radical cyclization, leads to the formation of polysubstituted tetrahydrofuran and γ-lactone derivatives, with stereochemical control.

Ueno-Stork cyclization - general reaction scheme - Ueno-Stork reaction - Ueno-Stork radical cyclization
Ueno-Stork cyclization

R = Me, Et, Bu, etc. (see list of acronyms)

It was initially thought that the stereochemical outcome of the reaction was controlled by the chiral center of the allylic moiety and that the acetal stereogenic center did not affect the diastereoselectivity. However, experimental results suggest that the acetal center may indeed influence the stereochemistry of the reaction. In some cases, it may even control the stereochemistry of the cyclization. Additionally, the conformational anomeric effect may also affect the stereoselectivity.