Comparative Synthesis Studies Towards Methyl and Phenyl 4‐Deoxy‐β‐L‐threo‐hex‐4‐enopyranosiduronic Acid as Model Compounds of Hexenuronic Acid Moieties in Hardwood Pulps

Abstract

Summary: The present study describes two approaches to synthesize hexenuronic acid model compounds in which the xylan backbone is replaced by a simple substituent, either methyl or phenyl. To general pathways towards the two products are offered: the first one, starting from glucuronic acid and involving formation and reopening of the 1,6-lactone, was especially suitable to afford the phenyl derivative 8. The second one, by contrast, is more appropriate for the methyl compound 12; it starts from galactose and involves TEMPO oxidation and conversions of the intermediate 1,3-lactone. Both pathways were optimized on a mg scale and subsequently upscaled to afford the targets in gram amounts. The ''detour'' via the methyl glucopyranosiduronate -instead of using the free acid -proved to be crucial in boosting the yield in the elimination step.

Chromophores from hexeneuronic acids (HexA): synthesis of model compounds and primary degradation intermediates

“…Although it is favored over the elimination of other substituents because the formed double bond is in conjugation with the carboxyl group (b-elimination), this effect is not very pronounced and-in general-elimination has to proceed from the disfavored synclinal conformation. It was known from previous work that a similar reaction starting from galacto-configured precursors went much more smoothly due to their favorable antiperiplanar arrangement (Tot et al 2008). However, D-galactose (or even D-galactopyranosiduronic acid) is not available in suitably 13 C-labeled forms, so the synthesis had to cope with the respective commercially available gluco-compounds.…”

“…The synthesis of the model compound was thoroughly optimized with regard to yield and recycling of side products in order to minimize the loss of rather costly 13 C-labeled material in the subsequent synthesis of the six isotopomers 1a-1f. The synthesis scheme, in principle, followed a sequence that had been previously developed in our group (Tot et al 2008), with some improvements to work more efficiently in the case of the labeled compounds. In particular, we tried to employ one-pot procedures whenever possible and reduce the number of (chromatographic) purifications between the steps to minimize inevitable compound losses.…”

“…The same synthesis sequence was repeated six times for the six selectively 13 C-labeled compounds: methyl 1- Cellulose (2017) 24:3703-3723 3705 enopyranosiduronic acid (1d), methyl 5-13 C-4-deoxy-b-L-threo-hex-4-enopyranosiduronic acid (1e), and methyl 6-13 C-4-deoxy-b-L-threo-hex-4-enopyranosiduronic acid (1f), starting from the corresponding 13 C-labeled D-glucuronic acids. The synthesis followed an optimized synthesis path towards non-labeled model compound 1, based on a path previously developed in our group (Tot et al 2008). This way, the syntheses were performed seven times, i.e.…”