6 - Determination of structure

Summary

This chapter will in the main concentrate attention on spectroscopic methods for establishing lignan structures, but complementary chemical methods will be mentioned as appropriate.

Ultraviolet absorption spectra

All lignans show a basic UV absorption pattern typical of aromatic compounds with three bands in the regions of 210, 230, and 280 nm which correspond to the singlet excited states defined by Platt (1949; see Murrell, 1971) as ¹B_a,b; ¹L_a and ¹L_b respectively. As the greater majority of lignans are optically active these maxima may usually be correlated with those in circular dichroism (CD) plots (p. 000). Clearly this basic absorption pattern will be modified in compounds with additional conjugation including those lignans which are fully aromatic.

The most intense absorption maximum arising from a component of the ¹B excitation in the region of 210 nm has a molecular extinction (ε) of about 5 × 10⁴ but this is rarely recorded. Peaks in the range 220–240 nm (ε = 10 to 20 × 10³) and at about 280nm (ε = 2 to 10 × 10³) are of diagnostic value since the absorption is of sufficient intensity to allow monitoring at all stages of the isolation procedure (p. 000).

In lignans where there is no conjugation to or between the aryl groups the absorption at the longest wavelength is sensibly the sum of their molar extinction coefficients.