Recent developments in the enantioselective synthesis of polyfunctionalized pyran and chromene derivatives

Recent developments in the synthesis of 4H- and 2H-pyrans as well as structurally related chromene derivatives that have enabled the enantioselective synthesis of these scaffolds have been surveyed. The role of chiral catalysts in orienting initial reactions of active methylenes, methines and methyl ketones, to unsaturated ketones and nitriles in multi-component reactions or Friedel–Craft alkylations of phenols is discussed to show their involvement in transition states leading to end products. Chromene synthesis via [4+2] cycloadditions, [3+3] and [4+2] annulations as well as ring opening and recyclization leading to high enantio- and diasteroselectivity is also demonstrated. The enantioselectivity in such catalytic asymmetric reactions despite starting with non-chiral starting materials is discussed. On the other hand, in surveying ring opening and recyclization, the starting materials are chiral and the chiral center was not part of the reaction leading to the final product.