Butenolide Synthesis by a Molybdenum-Mediated Hetero-Pauson-Khand Reaction

Significance

The authors report a novel general CO gas free variant of the hetero-Pauson-Khand reaction with alkynyl aldehydes as substrates. Hereby, the readily available Mo(CO)₃(DMF)₃ complex is efficiently applied to the conversion of 1,5- and 1,6-alkynyl aldehydes yielding the respective fused α,β-butenolides in satisfactory to good yields. The formation of side-products during the conversion of 1,6-alkynyl aldehydes is suppressed by the addition of Et₃B. In contrast to previously reported methods, the Mo-promoted hetero-PKR also allows the conversion of terminal alkynes. The transformation of enantiopure α-substituted 1,5-alkynyl aldehydes provides the respective butenolides in good to excellent diaste­reomeric ratios with a preference for the exo product. The diastereoselectivity is reversed if 1,5-alkynyl ­aldehydes with a bulky isopropyl chain and a sterically pretentious N-Boc moiety are used.