https://www.folate-analogue.com/index.php/2017/02/13/the-conformation-analysis-of-the-matrinic-series-left-and-its-isomeric-sophoridinic-series-right-to-deduce-the-feasiblity-of-the-alkylation-reaction-on-the-12-nitrogen-atom-in-sophoridinic-series/