Neolignanas hidrobenzofurânicas e biciclo [3,2,1] octânicas. Determinação estrutural sistemática

Abstract

Chromatography of a benzene extract of trunkwood of Endlicheria verticillnta Mez (Lauraceae), gave four benzofuranoid neolignans: (2S, 3R, 3aS)-5 allyl-3a-methoxy-2-(3',4',5'-trimethoxyphenyl)-3- methyl-2,3,3a,6-tetrahydro-6-oxobenzofuren (RFEV-3), (2S,3S, 5R)-5-allyl-5-methoxy-2-(3,4,5-trimethoxyphenyl)-3-methyl-2,3v 5,6-tetrahydro-6-oxobenzofuran (RFEV-4), (2S,3S,3aR)-3a-allyl-5-methoxy-2-(3',4',5' trimethoxyphenyl)-3-methyl-2,3,3a,6-tetrahydro-6-oxobenzofuran (RFEV-6), (2s, 3S,3aR)-3a-allyl-5-methoxy-2-(3',4'-dimethoxyphenyl)-3-methyl-2,3,3a,6-tetrahydro-6-oxobenzofuran (RFEV-7) and a bicyclooctanoid neolignan: (1S,4R,5S,6R,7R)-3-allyl-6-2-(3,'4',5'-trimethoxyphenyl)-4-hydroxi-1-methoxy-7-methyl-8-oxobicyclo [3,2,1] oct-2-ene (RFEV-5). The correlation of spectral data led to the establishment of a scheme for the structural determination of hydrobenzefuranoid and bicyclo [3,2,1] octanoid noolignans. The application of the schrms to the compounds of E. verticillata showed that RFEV-3, RFE\/-4 and RFEV-7 had already been previously described in the literature and that RFEV-5 and RFEV-6 are hitherto unknown neolignans. The structural proposals for RFEV-5 and RFEV-6 were based on RM1H, RM13C, mass and infrared spectral evidence as well as on optical rotatory dispersion curves. Comparative analysis of the RM1H, RM13C and infrared spectra of RFEV-5 and derivatives obtained by acetylation, in addition to double irradiation experiments, confirm the strucutural proposal for RFEV-5. Spectral correlation between the acid-catalyzed isomerization product of RFEV-3 [RFEV-3(delta)] and the oxidation product of RFEV-5 (RFEV-5 ox) revealed the non-identity of their structuros, showing, nevertheless, that they are configurational isomers at C-1 and C-5. Analysis of the ORD curves confirmed the deduction, stabishing the absolute configuration of the natural compound RFEV-5, since RFEV-3 (delta) is the isomerization product of a hydrobenzofuronoid neolignan of known absolute stereochemistry. A possible correlation of hydrobenzofuranoid and bicyclooctanoid neolignans was demonstrated through acid-catalyzed interconversion. An interconversion in the opposite sense was described in the literature. The present work shows that these interconversions are dependent upon stereochemical factors. Addition products of wattter and methanol of the piperenone type and epimers of the hydrobenzofurans were obtained in the reactions. The correlation of hydrobenzofuranoid neolignnns with bibyclooctanoid neolignans, added to other facts described in the literature, such as thermal derivation of hydrobenzofuranoid neolignans from a fundamental type substituted by allyl groups at severa.l positions, suggest that aryl-[1-(n-allyl-4-hydroxy-m-methoxy-2-oxo-1,2-dihydrophenyl)-ethyl]-methyl ( n=1, m=5 and n=5,.rn=1) cations play a central role in the biosyntheses of neolignans.