Stereoselective construction of the pyrrolizidine bridgehead stereochemistry by the adjacent hydroxyl group in the synthesis of (+)-heliotridine and (−)-retronecine

Formal total synthesis of (+)-heliotridine (4) and total synthesis of (−)-retronecine (5) were accomplished by using (S)-3-acetoxysuccinimide (6) as the common starting material. The stereogenic center of 6 ended up as C-1 in both alkaloids. The chiral centers at C-7a of the alkaloids were stereoselectively constructed through the help of the adjacent functionality at C-1. The B-rings of the alkaloids were formed through α-sulfonyl radical cyclizations.