Synthesis of (−)‐Hobartine and Related Indole Alkaloids

An improved method for obtaining optically pure (S)‐(l‐p‐menthen‐8‐yl)amine (12) has led to expedient syntheses of two hypothetical biogenetic intermediates on the way to aistoteline (7), namely (S)‐(N)‐(l‐p‐menthen‐8‐yl)‐2‐(3‐indolyl)ethylamine (3) and (S)‐(N)‐(l‐p‐menthen‐8‐yl)‐2‐(3‐indolyl)ethylideneamine (4). The latter has been transformed into (−)‐hobartine (6) in 64% yield via a stereoselective biomimetic cyclization by treatment with HCOOH. This unambiguous synthesis establishes the hitherto unknown absolute configuration of (−)‐hobartin (6). Several model cyclization reactions of N‐substituted α‐(terpen‐8‐yl)imine derivatives yielding unsaturated 3azabicyclo [3.3.1]nonane compounds are described.