Lignans yatein

Fig. 12.4 Biosynthetic pathway for lignans with 9,9 -oxygen including yatein and fxsdophyUo-toxin. DP dirigent protein, PLR pinoresinol/laiiciresinol reductase, PrR pinoresinol reductase, and SIRD secoisolariciresinol dehydrogenase...

A number of biomimetic semi- or total syntheses using an oxidative coupling of a yatein or matairesinol-type intermediate to form stegane or isostegane lignans such as in Fig. (7) have been reported [35,87]. The major contributions will be reviewed in section 3.2.2. 

The stegane lignans are thought to originate biogenetically from the same pathway as the podophyllotoxin lignans, a common precursor being the dibenzylbutyrolactone yatein, Fig. (6) [35]. 

All of the routes described so far can be readily adapted to provide asymmetric syntheses of lignans. Thus, Koga et al. have synthesised (+)-yatein (36) and (+)-/rfl/j5-burseran (37) by conjugate addition to a chiral butenolide (35) (scheme 8) [54,55]. The more readily available menthyloxybutenolide (38) has been utilised by other groups [56,57]. The products (39) and (40) after desulfurisation, serve as precursors for the synthesis of dibenzocyclooctadienes, furofurans and aryltetralins (scheme... 

Recent unpublished work by W.M. Kamil and P.M. Dewick, who I thank for permission to use these results, yields fascinating information about the ring closure process (Scheme 5). Whereas yatein and its cis-isomer are precursors of podophyllotoxin, most surprisingly the functionalized compounds anhydropodorhizol and podorhizol are not. This suggests a quinone methide intermediate with a central function. It is tempting to see matairesinol, not yet tested, as a possible precursor and branching point for all of the podophyllotoxin lignans. 

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