Diels-Alder routes to prosopis alkaloids
This thesis describes the investigation of the Diels-Alder reaction of the imine (140) with the diene (141) to give four products (142,143,156, 157). At low temperatures the enone (156) is the major product while at ambient temperature the bicyclic compounds (142) and (143) predominate. The reaction is highly solvent dependent, with the best results being obtained in benzene solution. Lewis acids appear to have little effect on the course of the reaction. The reaction of the imine (140) with the TBDMSO diene (159) gives the silyl enol ethers (160) and (161) as well as the above four products. Possible mechanisms are discussed. COOM. OSIM., In order to investigate asymmetric induction the 8- phenylmenthyl derived imine (194) was reacted with the diene (141) under a variety of conditions but no asymmetric induction was seen in the exo adducts, and only a small degree in the endo adducts. The bromination and Baeyer-Villiger oxidation of the ketone (142) were investigated and the resultant lactone (209) was converted into the bromide (258). ISOPROSOPININE A (84) ISOPROSOPININE B (S3) Several methods to prepare a suitable side chain for isoprosopinine A were investigated. The prosopis alkaloids isoprosopinine A (84) and B (85) were prepared by alkylation of the sulphones (252) and (266) with the bromide (258) followed by reductive removal of the N-tosyl and sulphone moieties with sodium amalgam. The imine (274) has been prepared and shown to undergo a Lewis acid catalysed imino-Diels-Alder reaction with the diene (141) to give the adducts (283) and (284). hCH,M=CHCOjM» (2T4) Alternative imines for asymmetric synthesis, such as (299) and (288) have been investigated. Preparation of the sulphinamide imine (299) has proved difficult. Attempts to carry out aqueous Diels-Alder reactions of the silyloxydiene (159) with iminium ions, generated in situ from amines, aldehydes and acids, have proved fruitless.