“…By increasing H 2 SO 4 loading to 50 mol%, the reaction could reach completion rapidly within 5 h, furnishing 4a in excellent yield (93%, Table 1, entry 4). With lower amounts of aldehyde 2a (1.2 equiv, Table 1, entry 5) or sulfinic acid 3a (2.0 equiv, Table 1, entry 6), satisfactory yields (83% and 80%) were still obtained after a prolonged reaction time, but with the formation of undesired by-product A [14c, 16] in 8% and 12% yields, respectively. Next, other solvents instead of CH 3 CN were tested.…”