Tetrahedron Letters,Vol.24,No.9,pp 969-972,1983 0040-4039/83/090969-04$03.00/O Printed in Great Britain 01983 Pergamon Press Ltd. EVIDENCE FOR THE BIOGENESIS OF TRAIVS-(lB-H;5a-H)-GUAIANOLIDES A.G. GonzBlez, A. Galindo, H. Mansilla and J.A. Palenzuela lnstituto Universitario de Quimica Org6nica - lnstituto de Productos Naturales Organicos, CSIC, La Laguna, Tenerife, Spain Swmna~: The biomimetic cycZization of I-epi-gaZZicin (‘8) into the e-(lB,Sa)-guaianolide (15) is carried out. The stereospecificityof the cycZization is explained in terms of a preferred reacting confonation (13). The LvZogenetic implications of this process are discussed. It has been postulated that the greater part of the trans-guaianes (3) derive from cis, trans-germacradiene precursors (1)) by the anti-Markovnikoff-type trams-antiparallel cyclizatior!! Another plausible suggestion for biogenesis of trans-guaianolides was presented by Herz 2) ; acid- induced cyclization of the 4n,5b-epoxide (4) would give the cation (5), showing a stereochemical arrangement typical of trans-guaianolides (Scheme I). 4 5 SCHEME I This last route has recently received strong support, since the structural revision of bai leyin from a germacrol ide3a) to a melampol ide skeleton, together with the X-ray finding that pleniradin3b) represents a trans-guaianol ide, suggest that these two co-occurring lactones 4) (Baileya pleniradiata) are biogenetically related . In a review of melampolides, Fischer and co-workers 5) suggested that the centre-to- centre distance between the two double bonds of (1) is considerably greater than in the four possible conformations of a trans,trans-germacradiene. They propose that the trans-fused guaia- ne cation (7) is formed from quasi-parallel conformation, via the 4a,5B-epoxy-trans-germa- crene (6). 969