Asymmetric Baeyer-Villiger Oxidation of 2,3-and 2,3,4-Substituted Cyclobutanones Catalyzed by Chiral Phosphoric Acids with Aqueous H2O2 as the Oxidant

Catalytic asymmetric Baeyer-Villiger (B-V) oxidation of 2,3,4-trisubstituted cyclobutanone (4) has been realized by the catalysis of a 1,1'-bi-2-naphthol (BINOL)-derived chiral phosphoric acid (1j), which contains bulky 2,4,6-triisopropyl phenyl groups at the 3,3'-positions of the BINOL backbone, using 30 % aqueous H2O2 as the oxidant, affording the corresponding gamma-lactone (5) in 99% yield with 95 % ee. In a divergent kinetic resolution of racemic 2,3-disubstituted bicyclic cyclobutanones (6) through asymmetric B-V oxidation, the chiral phosphoric acid 1p demonstrated excellent catalytic performance, giving a range of regioisomeric chiral lactones in a normal lactone (nl)/abnormal lactone (al) ratio of up to 2.1:1, with up to 99% ee in the al product. It was found that fine tuning of the stereoelectronic properties of the backbone in chiral phosphoric acids is critically important for attaining high levels of enantioselectivity in the catalysis of B-V reactions of different type of cyclobutanones. The present work has provided a convenient approach to the synthesis of a variety of optically active chiral gamma-lactones.