Lariciresinol type lignan

Another simple synthesis of lignans of the lariciresinol type (11) involves a straightforward reduction of a functionalised dibenzylbutyrolactone such as (75) or (76) followed by acid-catalysed cyclisation (scheme 24) [77,78]. Cyclisation of the doubly functionalised precursor (77) affords access to the furofuran skeleton (78) (scheme 25). A second synthesis which has also been extended to the furofuran series involves a radical cyclisation step, which proceeds with high stereoselectivity (scheme 26) [79]. This route has been used to synthesise (79) which is a precursor for the synthesis of sesamin and eudesmin [80]. 

Fig. 6.13 Furofuran and dibenzylbutyrolactone type lignans isolated from convolvulaceous species (glu = P-D-glucose api = P-D-apiose xyl = P-D-xylose). The arrows in the structure of pinoresinol indicate that this bond is not existing in the structure of lariciresinol, i.e., there is a free hydroxyl group which is glycosylated in case of its 9-glucoside (a constituent of Solarium tuberosum, Solanaceae)...

The morama bean is an excellent source of lignans (Holse et al., 2010). The content of secoisolariciresinol in morama beans (305-406 gg/100 g) is higher than that of soybean (13-273 gg/100 g) and peanut (333 pg/100 g) (Mazur et al., 1998). Likewise, the level of lariciresinol in morama beans (614-825 [tg/100 g) is higher than that of soybeans (287 gg/lOO g), while the level of pinoresinol in morama beans (21-23 gg/lOO g) is lower than the level in soybeans (446 gg/lOO g) (Penalvo et ah, 2004). These lignans can be converted by intestinal bacteria into enterolignans, which possess biological activities such as (anti) estrogenic and antioxidant action. Therefore, they may reduce the risk of certain types of cancers as well as cardiovascular diseases (Adlercreutz, 2007). 

Lignans derived from (la), (lb), (2b), (3a), (3b), and (4b) are known to occur naturally. Assignment of chirality at the 1-position may be carried out by ORD/CD [1], from which the complete stereochemistry may be adduced by methods such as nmr. Alternatively evidence from cyclisation reactions such as lariciresinol isolariciresinol can be used, but caution is necessary in the interpretation of this type of cyclisation because epimerisation at Cj may occur [2]. 

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