Stereoselective synthesis of the pyrrolizidine alkaloids (-)-integerrimine and (+)-usaramine
                                
                                    
                                        作者:James D. White、John C. Amedio、Samuel Gut、Susumu Ohira、Lalith R. Jayasinghe                                    
                                    
                                        DOI:10.1021/jo00034a017
                                    
                                    
                                        日期:1992.4
                                    
                                    Two routes to the pyrrolizidine alkaloid (-)-integerrimine (1) are described.  The first, starting from methyl (R)-(-)-3-hydroxy-2-methylpropionate, proceeded in 19 steps to integerrinecic acid lactone (5) which was transformed to the necic acid derivative 30.  The latter was coupled to protected retronecine 31, and the synthesis of 1 was completed by lactonization employing Vedejs' protocol.  A second, shorter synthesis of (-)-1 was accomplished via 5, starting from (R)-(+)-beta-citronellol (36).  This pathway invoked Katsuki-Sharpless epoxidation of 42 for stereoselective construction of the tertiary alcohol of integerrinecic acid.  A parallel sequence proceeding via the stereoisomeric epoxide 44 led to the necic acid segment 75 of the alkaloid (+)-usaramine (2).  This acid was coupled to the retronecine borane 82 and then lactonized to 2.