SYNTHESIS OF (+/-)-GAMMA-LYCORANE BY THE INTRAMOLECULAR CYCLOADDITION OF AN AZIDE WITH AN OMEGA-CHLOROALKENE

Cyclization of 1-[2-(chloromethyl)-4,5-(methylenedioxy)phenyl]-3-(2-azidoethyl)cyclohex-1-ene (16) in benzene at 140-degrees-C caused the following sequence of events: (1) intramolecular 1,3-dipolar cycloaddition of the azide onto the alkene: (2) formation of an imine by loss of nitrogen from the triazoline intermediate with concomitant hydrogen migration:and (3) intramolecular N-alkylation of the imine nitrogen with the benzylic chloride. Without isolation, the resultant iminium ion was reduced with sodium borohydride to give (+/-)-gamma-lycorane (3). The cyclization precursor 16 was prepared a novel allylic substitution reaction, where the allylic alcohol 10 or the allylic acetate 11 was treated with 1-ethoxy-1-[(tert-butyldimethylsilyl)oxy]ethene and lithium perchlorate in ether to produce the gamma,delta-unsaturated ester 12. An alternative synthesis of (+/-)-gamma-lycorane (3) was accomplished using a similar 1,3-dipolar cycloaddition approach, except that the benzylic chloride functionality was absent (i.e., 26 --> 27). Reduction of the resultant imine followed by a Bischler-Napieralski cyclization gave the known lactam 31, which had previously been converted to (+/-)-gamma-lycorane.