Aureol and Phaseol, Two New Coumestans from Phaseolus aureus Roxb

In addition to the isoflavonoid phytoalexins re­ ported earlier [1], the mung bean, Phaseolus aureus, produces coumestrol ( 1 ) and two novel coumestans, aureol (2) and phaseol (3). Coumestrol has been isolated from this plant previously [2 ] and the present communication describes the isolation and structural characterization of the latter two com­ pounds. The characterizations of aureol and phaseol as coumestans were initially suggested by their UV spectral properties. Coumestans generally exhibit high intensity absorption maxima in three wave­ length regions. The position and relative intensity of the maximum in the mid-wavelength region gives some indication of substitution patterns in the aromatic B ring [3-6], The UV spectrum of aureol possessed a mid-wavelength maximum at 257 nm which was of greater relative intensity than that at 337 nm, a feature so far known only in 3,7,9-trisubstituted coumestans [3-6]. However, 257 nm is a shorter wavelength than that of the corresponding peak in the latter compounds. Bathochromic shifts in the maximum of the UV spectrum of aureol upon addition of NaOAc and NaOMe indicated a free OH at C-3 together with one or more other phenolic functions. The formation of a triacetate established that aureol possessed three phenolic OH groups. The MS contained a plausible M+ at m /e 284 with important fragment ions at m /e 256, 228 and 2 0 0 , consistent with the behaviour of cou­ mestans in which the only significant fragmentation is loss of CO moieties from the molecular ion [3].