A new approach to the synthesis of optically active indole and isoquinoline alkaloids is described. The key reactions are : (i) the biogenetic-type, asymmetric Pictet-Spengler reaction of optically active α-amino acids (I) with aldehydes (II) (1, 3-asymmetric induction), and (ii) the elimination of the chiral center derived from α-amino acids (1, 3-transfer of asymmetry), shown in Chart 1. To achieve (ii), laboratory simulation of decarboxylative processes of α-amino acids was investigated by oxidative and reductive methods. The former did not give any fruitful results, as shown in Table I. However, the reduction of α-amino nitriles, easily prepared from α-amino acids, by means of sodium borohydride gave the satisfactory result, as shown in Table II. By this new method, two simple indole alkaloids, (S)-(-)-tetrahydroharman (XXIV) and (S)-(-)-1, 2, 3, 4, 6, 7, 12, 12b-octahydroindolo [2, 3-a] quinolizine (XXIX) could be obtained from L-tryptophan in a satisfactory manner.
