Application of Tosylate Reductions and Molecular Rotations to the Stereochemistry of Lignans²

Neolignans

“…The similarities in the structures of 2.1e and galcatin (2.1d) are reflected in their IH NMR spectra. On such evidence, the relative configurations at C-7, 8 and 8' of 2.1d; 2.1e, 2.4a and 2.4b are identical (142), thereby confirming the stereochemical relations derived for galcatin (2.1d) by other means (28,133 Isogalcatin (2.1e) was obtained after methylation of a mixture of natural phenols which may have included otobaphenol (2.1b). This, isolated in later years, had its structure confirmed by synthesis of the dehydrogenation product of the derived ethyl ether 36 (Chart 11) (88).…”

“…The configuration of 1.2a was established by synthesis of its dimethyl ether through defunctionalization of the lactone moiety of ( -)-matairesinol (17) dimethyl ether (133) and by stepwise degradation to (2S,3S)-2,3-dimethyl succinic acid and (2S)-2-methylbutanol (39,40). While the stereochemistry of some compounds (1.4a,b) of the dihydroguaiaretic acid series is unknown (76), the absolute configuration of others (1.2b,c) was inferred by comparison of optical rotation with 1.2a (112).…”

New Lignans and Isobutylamides fromHeliopsis buphthalmoides

“…The stereochemistry followed from the observed couplings for H-8'. Compound 7 is closely related to ct-conidendrin (6), which only differs in the aromatic substitution.…”

Ariensin, a New Lignan fromBursera ariensis

“…Their spectral data, given in the experimental section and in Tables I and II are in full agreement with those corresponding to its structure. Of particular interest is the optical activity, which showed [€t]D = -34.2°in close analogy to the value of [aID = -34.0°o f (2R,3R)-2.3-bis(3.4-dimethoxybenzyl-)-1 .4-butanediol [4], thus allowing to ascertain the absolute configuration of the two chiral centres of ariensin (1).…”

“…The distribution of the substituents on the benzene rings could not be deduced completely from inspection of the 1H-NMR spectrum, since the very small chemical shift difference between H-2 and H-6 (4 Hz at 100 MHz) precluded observation of J2.6 and thus only J56 = 8. 4 Hz owing to two ortho protons is evident. However, cogent evidence for the distribution of the functionality on the aromatic rings, was readly adduced by inspection of the 13C-NMR spectrum determined under complete proton decoupling, combined with a coupled spectrum obtained by gated proton decoupling.…”