THE PHOTOSENSITIZED OXIDATION OF ALPHA-KETO ENOLS - A SINGLET OXYGEN APPROACH TO 2-OXASTEROIDS

Fluoride ion catalyzed photosensitized singlet oxygenation of 2-hydroxycyclohexa-2,5-dien-1-ones 15a and b and the related steroidal alpha-keto enols 18a-g, generated the cyclohexenone lactols 16a-b and the corresponding steroidal analogs 19a-g generally in moderate to good yields (60-75%). Since the lactols can be conveniently reduced to the desired 2-oxasteroids in high yields, this 1O2 route presents itself as a synthetically acceptable alternative to the previously reported BCA approach3a for the preparation of 2-oxasteroids, especially in the case of base sensitive compounds.