Borylated Cyclopentanes via Atom Transfer Radical [3+2] Annulation.

An atom transfer radical [3+2] annulation (ATRAn) reaction involving alkenyl boronic esters and homoallylic iodides provides a rapid access to polysubstituted borylated cyclopentanes. A variety of α‐substituted vinylboronic esters are suitable substrates and offer unique opportunities for further modification of the formed 5‐membered ring. For instance, the oxidation of the boronic ester to an alcohol allows the preparation of products that corresponds to an annulation involving the enol form of acetone. Substitution of the iodide by a nucleophile such as lithium methoxide without modifying the boronic ester moiety is also feasible. Finally, by using a (3‐acetoxyprop‐1‐en‐2‐yl)boronic ester, a facile 1,2‐elimination reaction provided a methylenecyclopentane. In this case, the alkenylboronate radical trap acts as an equivalent to allene, a building block so far elusive for preparative radical reactions. [ABSTRACT FROM AUTHOR]