I have found a synthesis with an apparently high yield of 3,4,5-trimethoxybenzaldehyde (with 5-bromovanillin as intermediate) from vanillin. It is in russian, but I hope Antoncho can help us with the translation. The information on vanillin dimerization is uninteresting, but all the preparation details from vanillin to 3,4,5-trimethoxybenzaldehyde is desired. If possible, I'd like to know the rationale behind the addition of a Cu2+ salt (usually a Cu(I) salt is used), and if there is any point in making iodovanillin as they describe, and how that is done.
The PDF file of the article can be found here: https://www.thevespiary.org/rhodium/Rhodium/bromovanillin.pdf (https://www.thevespiary.org/rhodium/Rhodium/bromovanillin.pdf)
https://www.rhodium.ws (https://www.rhodium.ws)
Yes, i'm afraid this article isn't much of a news, but still has a couple of new tidbits of information. It's another purification procedure of syringaldehyde(SA) from vanillin. The first suggested route is via dimerization of vanillin and the second one is through bromination of its mxtr w/SA. The article says that "we found the optimal amount of Br2 used to be 1,2 mole per 1 mole of vanillin....with yields of bromoV 92-95%... The reaction is most conveniently carried out in a 3-component mxtr of aqua/alcohol or aqua/acetic acid with up to 10% polyatomic alc. (glycerin, ethyleneglycol, diethyleneglycol, propyleneglycol) added, under which cond's the resulting bromovanillin precipitates and SA stays in the soln." The SA is then precip'd in cold water, dissolved in AcOH, the residual bromoV filtered out, SA re-precip'd in cold water (purity 99,0-99,4%, yield 95-97%)
The article has no mention of copper 2+ and doesn't have anything else on prep'n of TMBA :( Quite possible, it was a misprint... What about that ref, Rao D., Stuber S.A. A convenient synthesis of 3,4,5-trimethoxybenzaldehyde from vanillin // Synthesis (BDR). 1983.¹4. S. 308. ? I bet they took it from there as there's no other ref related to the subj in the article.
Hope the improved bromination proc. will be useful to Some People I Never Met :)
Antoncho
P.S. Karel, do i know you? ;) ;) ;) If not, take a look here if you care: http://www.voy.com/12987/ (http://www.voy.com/12987/)