The First Enantioselective Approach to 13a‐Methyl‐14‐hydroxyphenanthroindolizidine Alkaloids – Synthetic Studies towards Hypoestestatin 2
The first enantioselective approach to 13a-methyl-14-hydroxyphenanthroindolizidine alkaloids was achieved in six linear steps from phenanthryl alcohol and features a highly substrate-dependent Parham cycloacylation and Seebach's enantioselective alkylation as the key steps. The route is concise, protecting-group free, provides access to all stereoisomers, and works on a gram scale. In addition to the putative structure of hypoestestatin 2, the other three stereoisomers and two structurally related analogues were synthesized, none of which shows identical NMR spectra to those reported for natural hypoestestatin 2, which indicates that further structure revision is required.