Rearranged 8.1-Neolignans

The structure of aurein (10.1a) was deduced from MS, NMR (66) and NMR 144) data. It shows an O-allyl which is easily cleaved by pyrolysis (to 67a) and acid treatment (to 67b). The former reaction is relevant, since loss of the allyl group demonstrates that the ortho- and / ra-positions relative to the O-allyl group are substituted 66). 

The NMR spectra of neolignans pertaining to structural types 7.1, 7.7, 7.8, 8.1, 8.3, 8.4, 9.1, 9.2 and 16.2 were analyzed and their carbon shifts assigned. Clearly, CMR constitutes a powerful analytical method, of use in the recognition not only of specific neolignan types, but also of configuration and conformation 144). 

In this connection, the C-methyl (C-9) shift of 8.T- and 8.3 meolignans is of diagnostic importance (Table 10). Its similarity in the licarins 

The orientation of the hydroxyl towards the pentacycle became evident upon consideration of NMR coupling and dihedral angle involving the CHOH-CH system. Since this hydroxyl is easily acetylated at room temperature and addition of hydrogen on the P-side of the molecule is favoured, the bulky aryl group should be located on the endo-face of this molecule. 

Compound 13.2a diverges from macrophyllin not only in aromatic substitution and the lack of an aliphatic methyl, but also by the presence of the carbonyl and carbinol functions at interchanged positions, facts which were deduced by the comparison of IR and NMR spectra. At the time of writing, the type of doubt mentioned in the case of 13.1a also applied here (32). 

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Naphthyl acetates, Fries rearrangement of 639 Naphthylamines—see also 8-Hydroxy-A, Ai-dimethyl-1-naphthylamine as phenol precursors 401, 402 Naringin, complexes of 365 National Institute of Health (NIH) shift 14 Negative ion chemical ionization (NlCl) mass spectrometry 310, 314-316 Negishi reaction 1406 Nematic liquid crystals 372-375 Neolignans 1278... 

The first 8.T-neolignan to be isolated was burchellin to which structure 8.1a was accorded by consideration of IR, UV and 220 MHz NMR spectral data 21). The MS was of little help in the analysis. The rearrangements which seem to account for some of the major peaks could only be formulated with confidence a posteriori (Chart 22). 

Fig. 7.3.1. Skeletons of 8.8 -lignans and neolignans and of the rearrangement products of the latter. In this and the following figures the presently known numbers of lignans (L) and neolignans (N) are registered for each structural subtype...

The fruits of Virola sebifera have been found to contain oxootobain (7), Unusual neolignans were also foimd in fiuits of this species and these new indanone neolignans were proposed to be biosynthesized by a pinacol-pinacolone type rearrangement of trihydroxyaiyltetralones (8) as depicted in Figure 4. 

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