Syringaldehyde differs from vanillin by a second methoxyl group at C5 position of aromatic ring as depicted in Fig. 12.6. The oxidation of softwood lignins produces exclusively vanillin whereas the oxidation of hardwood lignins leads to syring — aldehyde plus vanillin, in a proportion that depends of the original syringyl:gua — iacyl ratio in the wood.

Syringaldehyde is a valuable starting chemical for the pharmaceutical industry. For example, as vanillin, this compound is the precursor of 3,4,5-trimethoxy — benzaldehyde, which is a building block of the antibacterial agents ormetoprim and trimethoprim, with the advantage of containing already two methoxyl groups [103-105].

In the past, between 1930s and 1950s, the separation technologies to recover vanillin and syringaldehyde produced by lignin oxidation were not readily accomplished [106]. Syringaldehyde has been produced by different chemical routes and starting materials as gallic acid, vanillin, [106] and pyrogallol [107]. New alternatives for synthesis of syringaldehyde are being investigated in order to find environmental friendly and efficient processes to obtain higher yields [108]. However, many of these synthetic pathways involve complex procedures and/or include expensive materials becoming not economically feasible a large scale production.

By end of 1970s, the production process of syringaldehyde by oxidation of hardwood spent liquor was reported [109] including a step of fractional distil­lation for the separation of the two aldehydes. The oxidation of hardwood lignin to produce a syringaldehyde-rich mixture seems to be very attractive. In fact, the production of this phenolic compound from lignin, in alkaline medium with O2, has been emerging as research topic [20, 110-113]. However, the sustainability of this process must be assured by the yield of products and economical advantageous purification processes. Considering its potential applications, it is expected an increased demand for this chemical already cited in Top Value — Added Chemicals from Biomass [38]. As an example, syringaldehyde was recently considered as a promising building block to dendrimers design with high antioxidant potential: the antioxidant activity of syringaldehyde-based dendrimer showed to be two and ten times higher than that one of quercetin and trolox, respectively [114].