New Facile Syntheses of Monolignols Glucosides; p- Glucocoumaryl Alcohol, Coniferin and Syringin

The monolignols glucosides pglucocoumaryl alcohol, coniferin and syringin, have been found in cambial sap of gymnospermous trees (Freudenberg and Harkin 1963) and some species of angiospermous trees (Terazawa et al. 1984). Coniferin has been shown to bean effective precursor of lignin biosynthesis in pine (Freudenberg et al. 1955). When three kinds of monolignols glucosides labeled with C or H were administered to growing stems of both gymnosperm and angiosperms, including monocotyledons, they were effectively incorporated into cell wall lignins in a biochemically regulated manner (Terashima et al. 1993). When monolignol glucosides enriched with C at specific side-chain carbon were fed to plants, lignin was shown to be C enriched at the corresponding positions (Terashima et al. 1991). A hypothetical mechanism for formation of lignified cell wall has been proposed in which monolignols glucosides play an important role as precursors of lignin biosynthesis (Terashima et al. 1995, 1996). Using this theory as a basis, a new type of synthetic guaiacyl lignin was prepared from coniferin that has a closer resemblance to natural lignin than do other synthetic lignins prepared from coniferyl alcohol by conventional procedures (Terashima et al. 1995, 1996). In previous work, labeled or unlabeled monolignols glucosides employed as lignin precursors were synthesized mainly on the basis of procedures described by Kratzl and Billek (1953a, 1953b, 1954) and Fuchs (1955) as shown in Figure 1. First, the aromatic aldehydes phydroxybenzaldehyde, vanillin and syringaldehyde (Ia, Ib and Ic) wet-e converted to their tetra-O-acetyl-b-D-glucosides (IIIa, IIIb and IIIc), which were then condensed with malonic acid to give tetra-O-acetyl-~ -D-glucosides of corresponding pcournaric, ferulic and sinapic acids (IVa, IVb and IVc) (Kratzl and Billek 1953). The acids were converted to corresponding acid chlorides (Va, Vb and Vc) by treatment with thionyl chloride, and the acid chlorides were reduced with sodium trimethoxyborohydride to give the corresponding alcohols (VIa, VIb and VIc) (Fuchs 1955). Deacetylation with sodium methoxide gave monolignols glucosides (VIIa, VIIb and VIIC) in 60–80% of theoretical yield. More recently, Teutonico et al. (1991) prepared the glucoside of ~-5-dehydrodiconiferyl alcohol from the corresponding tetra-O-acety]-f) -D-glucoside of ~-5-dehydrodiferulic acid dimethyl ester by reduction and deacetylation with isopropyl aluminum hydride. Quideau and Ralph (1992) also showed that diisobutylaluminum hydride (DIBAL-H) is useful for reduction of ethyl esters of pcoumaric, ferulic and sinapic acids to give corresponding monolignols in good yield.