Journal of the American Chemical Society, Vol.118, No.35, 8266-8277, 1996
Tandem (4+2)/(3+2) Cycloadditions of Nitroalkenes .9. Synthesis of (-)-Rosmarinecine
(-)-Rosmarinecine (2) is the necine base portion of the pyrrolizidine alkaloid (-)-rosmarinine. (-)-Rosmarinecine is a representative of the group of pyrrolizidines that show a cis relationship between adjacent stereocenters C(1), C(7), and C(7a), in addition to a highly oxygenated skeleton. (-)-Rosmarinecine (2) has been synthesized in eight steps and 14.8% overall yield, as an illustration of a general approach for the construction of pyrrolizidines having this stereochemical feature. The key step in the asymmetric synthesis is a Lewis acid-promoted, tandem inter-[4 + 2]/intra-[3 + 2] cycloaddition between a fumaroyloxy nitroalkene and a chiral vinyl ether.
Keywords:PYRROLIZIDINE ALKALOID BIOSYNTHESIS;O-ALKYL CLEAVAGE;BASES NECINES;METHYL-ESTERS;INTER (4;REAGENT;INTERMEDIATE;DEALKYLATION;CONVENIENT;HYDRIDE