220906-00-3Relevant academic research and scientific papers
Effect of the leaving group on the alkylation diastereoselectivity of the Schollkopf chiral auxiliary
Ma, Chunrong,He, Xiaohui,Liu, Xiaoxiang,Yu, Shu,Zhao, Shuo,Cook, James M.
, p. 2917 - 2918 (1999)
Tosylates, diphenylphosphates and bromides were employed to study the effect of the leaving group on the alkylation diastereoselectivity of the Schollkopf chiral auxiliary 1 at aliphatic, benzylic, propargylic and allylic electrophilic centers. The diastereoselectivity generally follows the pattern: diphenylphosphate > tosylate > bromide. In terms of both diastereoselectivity and yield, each leaving group possesses a different advantage.
Efficient asymmetric synthesis of biologically important tryptophan analogues via a palladium-mediated heteroannulation reaction
Ma,Liu,Li,Flippen-Anderson,Yu,Cook
, p. 4525 - 4542 (2007/10/03)
A novel and concise synthesis of optically active tryptophan derivatives was developed via a palladium-catalyzed heteroannulation reaction of substituted o-iodoanilines with an internal alkyne. The required internal alkyne 14a or 25 was prepared in greater than 96% de via alkylation of the Schoellkopf chiral auxiliary 19 employing diphenyl phosphate as the leaving group. The Schoellkopf chiral auxiliary was chosen here for the preparation of L-tryptophans would be available from D-valine while the D-isomers required for natural product total synthesis would originate from the inexpensive L-valine (300-g scale). Applications of the palladium-catalyzed heteroannulation reaction were extended to the first asymmetric synthesis of L-isotryptophan 38 and L-benz[f]tryptophan 39. More importantly, the optically pure 6-methoxy-D-tryptophan 62 was prepared by this protocol on a large scale (>300 g). This should permit entry into many ring-A oxygenated indole alkaloids when coupled with the asymmetric Pictet-Spengler reaction. In addition, an improved total synthesis of tryprostatin A (9a) was accomplished in 43% overall yield employing this palladium-mediated process.
Concise synthesis of optically active ring-A substituted tryptophans
Ma, Chunrong,Liu, Xiaoxiang,Yu, Shu,Zhao, Shuo,Cook, James M.
, p. 657 - 660 (2007/10/03)
A concise synthesis of optically active tryptophan derivatives was developed via diastereoselective alkylation of the Schollkopf chiral auxiliary 4 to provide alkyne 2 which underwent palladium-catalyzed heteroannulation with iodoanilines 1 to furnish protected tryptophans 3. Hydrolysis and subsequent saponification of 3 provided the desired tryptophans 12 in good yields.
