Organocatalytic oxa-Diels–Alder reaction of α,β-unsaturated ketones under non-classical conditions†
Non-classical reaction conditions are compatible with organocatalysis and can often improve reaction course. Oxa-Diels–Alder reaction of acyclic α,β-unsaturated ketones with aldehydes is a challenging transformation that affords valuable pyranone products. Application of microwave irradiation and solvent-free conditions during ball-milling had the best influence on this reaction, in terms of shortened reaction times and increased product yields. Other factors, such as aqueous conditions, ultrasound and flow conditions also bring notable improvements. Unfortunately, a range of chiral organocatalysts provided only small enantioselectivity of the reaction of up to e.r. 63 : 37.