Synthesis of spirolactones from carbonyls via carboborylation of N‐sulfonylhydrazones with alkoxycarbonylalkylboronic acids

A straightforward synthesis of spirolactones from carbonyls via N‐tosylhydrazones is described. The process consist in the reaction between the N‐tosylhydrazone 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 the spirocyclic lactone. Depending on the structure of the starting N‐tosylhydrazone, thermal or photochemical protocols were applied for the key carboborylation reaction. The usefulness of the process is illustrated by the application on a variety of natural products and biologically relevant carbonyl containing molecules.