Stereospecificity in the Pictet-Spengler Reaction. Enantiospecific Synthesis of (6S,10S)-(-)-5-Methyl-9-oxo-12-benzyl-6,7,8,9,10,11-hexahydro-6,10-imino-5H-cyclooct[b]indole, a Template for Preparation of Macroline/Sarpagine Alkaloids