PYRIDINE MONOTERPENE ALKALOID FORMATION FROM IRIDOID GLYCOSIDES - A NOVEL PMTA DIMER FROM GENIPOSIDE

New pyridine monoterpene alkaloids (PMTAs) have been synthesized from the iridoid glycosides 8-epi-loganin, cornin, and antirrinoside by tre atment with beta-glucosidase and aqueous NH4-OAc. The PMTA from antirr inoside contained an 8-alpha-OAc group from the opened 7,8-epoxide moi ety. Treatment of genipin, the aglycone of geniposide, with HCl(g) and NH3(g) yielded the PMTA racemigerine, a known plant isolate, in which the C=C-CH2OH side chain was converted to C=C-CH3. Reaction of genipo side with beta-glucosidase and aqueous NH4OAc led to oligomeric alkalo ids, but at high dilution a dimer was obtained whose structure was for mally that of a Diels-Alder adduct between racemigerine and a dihydrop yridine. These biomimetic semisyntheses were analyzed in terms of reac tion mechanisms and the relative paucity of known plant PMTAs in compa rison with the multitudinous occurrence of their presumed iridoid glyc oside precursors.