Stereoselective cyclopropanations of allylic amines and derivatives

Thesis

This thesis is concerned with the development and application of methods for the stereoselective cyclopropanation of allylic amines and derivatives. Firstly, a highly chemo- and stereoselective cyclopropanation of N,N-dibenzyl-protected allylic amines was developed using the highly reactive Shi’s carbenoid [CF₃CO₂ZnCH₂I]. Subsequent mechanistic studies revealed that the high diastereoselectivity of the reaction was likely to be due to coordination of the amine to the zinc carbenoid reagent. It is then shown that the reaction is general for a wide range of both cyclic and acyclic substrates giving the corresponding cyclopropanes in high yields and diastereoselectivities. Secondly, a novel stereodivergent cyclopropanation of allylic carbamates and amides was developed. It was found that reaction of cyclic allylic carbamates with the Wittig-Furukawa reagent [Zn(CH₂I)₂] typically gives the syn-diastereoisomer in high yields and diastereoselectivities, whilst treatment of the same substrates with Shi’s carbenoid [CF₃CO₂ZnCH₂I] gives the corresponding anti-diastereoisomers in high yields and diastereoselectivities. Mechanistic investigations suggested that reactions with the Wittig-Furukawa reagent proceed via a N-directed intramolecular cyclopropanation step whilst those with Shi’s carbenoid proceed via a sterically directed intermolecular cyclopropanation step. Unsuccessful investigations into an asymmetric variant of the cyclopropanation reaction utilising chiral carbamate protecting groups are then described. Finally, studies towards the total synthesis of the potential anti-obesity therapeutic trans-SCH-A and its epimer cis-SCH-A are described. A stereodivergent route towards the epimeric products was developed through the cyclopropanation of a common allylic carbamate intermediate with either the Wittig-Furukawa reagent or Shi’s carbenoid to give the corresponding trans-2-amino-5-arylbicyclo[3.10]hexane or cis-2-amino-5-arylbicyclo-[3.10]hexane intermediates respectively.

Authors: Ling, K; Kenneth Ling

• Publication date: 2009
• DOI: 10.5287/ora-n6vx0wyj4
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: allylic amines; cyclopropanation; Shi's carbenoid
• Subjects: Organic chemistry; Synthetic organic chemistry; Organic synthesis