Fourteen Brosimum species (Moraceae) contain either pyranocoumarins or furocoumarins, besides O-prenylbrosiparin which gives rise to brosiprenin by a Claisen type rearrangement. Two BRAZILIAN Brosimum species have so far been examined phytochemically: B. gaudichaudii Trécul whose roots were reported to contain psoralen (Ia) and bergapten (Ib), and B. rubescens Taubert whose trunk wood contains xanthyletin (IIa), luvangetin (IIb), 7-demethylsuberosin (IIIa), brosiparin (IIIb) and brosiprenin (IIIc). It was considered that the biosynthesis of IIIc might involve a Claisen type rearrangement, in spite of the fact that O-prenylbrosiparin (IIId) had not been detected in the extract of B. rubescens. The hypothesis, however, seemed so attractive that a search for the putative precursor IIId was undertaken in wood samples of 13 Amazonian Brosimum species. As far as TLC comparisons of extracts obtained from preserved voucher specimens merit confidence, the analysis of the results (Table 1) leads to the following comments. Phytochemically, the examined Brosimum species fall into two sections characterized by the predominant presence either of pyranocoumarins or of furocoumarins. Prenylcoumarins are, of course, precursors to both types of compounds, as indicated by the constant quantitative ratio of xanthyletin (IIa) and 7-demethylsuberosin (IIIa) in most extracts, as well as the co-occurrence of luvangetin (IIb) and brosiparin (IIIb). O-Prenylbrosiparin (IIId) was detected in the particular species of the pyranocoumarin section which contains exceptional amounts of brosiparin (IIIb) and brosiprenin (IIIc), and is ubiquitous in the furocoumarin section where its transformation into brosiparin (IIIb) and brosiprenin (IIIc) seems to be blocked. The biosynthetic association of IIIb and IIIc with O-prenylbrosiparin is thus a reasonable postulate. Indeed, both in nature and in the laboratory through pyrolysis of O-prenylbrosiparin (IIId), brosiparin (IIIb) and brosiprenin (IIIc) are formed in substantially equal amounts.