Ring-contracted artemisinin derivatives: stereoselective reaction of anhydrodihydroartemisinin towards halogenating reagents

Reaction of anhydrodihydroartemisinin 1 with halogenating reagents and further rearrangement of adducts into ring-contracted aldehydes has been revisited. The stereochemistry of reactions with iodine is opposite to that with bromine, and allowed the preparation of the new aldehyde 4 with conservation of the configuration of starting artemisinin at C-9.