A 50 mL round-bottom flask was charged with a stir bar, aldehyde starting material, and CH3CN/H2O (50:1 mixture, 4 mL total volume). The reaction flask was cooled to 0 °C using an ice-water bath, and p-TsOH•H2O (10 mg, 0.053 mmol, 0.2 equiv.) was added. The reaction mixture was warmed to room temperature over a period of 4 hours. Following this time, the reaction was quenched with saturated aqueous NaHCO3 solution (10 mL), and the mixture was transferred to a separatory funnel. The aqueous layer was extracted with EtOAc (2 x 10 mL). The organic layers were collected, dried over Na2SO4, and concentrated under reduced pressure. The resulting residue was purified using a gradient of 50 to 100% EtOAc/hexanes on silica gel to yield 17 (colorless oil, 0.084 g, 0.150 mmol, 56% yield over two steps).