Synthesis of Spirolactones from Carbonyls via Carboborylation of <i>N‐</i>Sulfonylhydrazones with Alkoxycarbonylalkylboronic Acids

A straightforward synthesis of spirolactones from carbonyls via N ‐tosylhydrazones is described. The process consists of a reaction between N ‐tosylhydrazones and ethoxycarbonylethylboronic acid or ethoxycarbonylpropylboronic acid, leading to spiro γ‐butyrolactones or δ‐valerolactones, respectively. The transformation takes place by carboborylation of the N ‐tosylhydrazone followed by oxidation of the homologated boronic acid that delivers a hydroxyester, which undergoes further lactonization to yield a spirocyclic lactone. Depending on the structure of the starting N ‐tosylhydrazone, thermal or photochemical protocols are applied for the key carboborylation reaction. The usefulness of the process is illustrated by the application in a variety of natural products and biologically relevant carbonyl‐containing molecules.