General Procedure C. To a solution of aldehyde (1.0 eq.) in t-BuOH (18 mL/mmol) and water (5.9 mL/mmol) was added NaH2PO4 (1.0 eq.), NaClO2 (3.95 eq.) followed by 2 M 2-methyl-2-butene in THF (5.0 eq.), and the reaction left to stir at r.t. for 16 h. The reaction was diluted with 2 M NaOH (aq) solution (1 mL) and then carefully neutralised 2 M HCl (1 mL). The mixture was extracted with DCM (4 × 10 mL), dried with MgSO4 and concentrated in vacuo to yield carboxylic acids as colourless oils without the need for further purification.