Non-Enzymatic Formation of Chaxines and Natural Steroidal Derivatives via Ergosterol Air Oxidation

Mushrooms are a rich source of steroidal compounds and have attracted attention from scientists for long. Recently, we found that several naturally occurring steroidal compounds including chaxine B (1) could be generated non-enzymatically from ergosterol, a major component of the fungal cell membrane. Here, we conducted further analyses to determine that the reactions require radical intermediates by using the radical trap TEMPO but proceed without light or heat. Steric hindrance led to the formation of only a limited number of adducts. We then conducted computational analyses of the reaction pathways to propose and evaluate the mechanisms of non-enzymatic formation of the ergosterol derivatives having unusual and diverse ring systems. The analyses revealed the occurrence of a common intermediate Int-4, from which 1 and its diastereomer BB (2), as well as newly identified pleurocin-type compound 5, could be formed. The calculated activation energies of the reaction pathways from Int-4 to the precursors of 1 and 2 were low enough to proceed at 25 °C, and the identified precursors indeed transformed spontaneously to 1 and 2 in aqueous solutions. These results highlight the difficulty and complexity of biosynthetic studies of ergosterol derivatives, where the reactions could proceed either enzymatically or non-enzymatically.