Pigments from the Puffball Calvatia rubro-flava − Isolation, Structural Elucidation and Synthesis

The orange pigment rubroflavin (1) from the dried fruit bodies of Calvatia rubro-flava (Lycoperdaceae) owes its high optical rotation to a methanesulfinyl group directly attached to a 1,4-benzoquinone semicarbazone chromophore. Rubroflavin is present in fresh fungi in its leuco form 4, which is easily oxidized to 1. Thermal fragmentation of 1 yields (-)-3-methanesulfinyl-5-(methylthio)phenol (6), whose configuration was assigned as (S) by quantum mechanical calculations. This result is supported by CD comparison of 6 with (S)-4-(methanesulfinyl) toluene, and the synthesis of (S)-1 from esters of deoxyrubroflavin (8) by stereoselective sulfoxidation. In the same manner, optically active (S,S)-oxyrubroflavin (2) and (S)-craniformin (3) were obtained. NMR measurements in different solvents indicate that 1 and the related 1,4-benzoquinone semicarbazones are in equilibrium with their azophenol tautomers.