On the interactions of alkyl 2-hydroxycarboxylic acids with alkoxysilanes 2. Complexation and esterification of di- and tricarboxylic acids

Abstract

The selective esterifications of L-malic, L-tartaric and citric acids with tetramethoxysilane Si(OMe)4, in methanol (MeOH) have been demonstrated for the first time. The interactions between these acids and Si(OMe)4, and also between oxalic acid and Si(OMe)4, were investigated using 1H, 13C and 29Si solution phase NMR, and electrospray mass spectrometry (ES-MS). Si(OMe)4 acts as a catalyst/reagent in the selective esterification of simple 2-hydroxycarboxylic acids (2HOAs), but with the di- and tri-carboxylic acids more complex selectivities are observed: the esterification of oxalic acid proceeds slower than in MeOH alone, L-malic acid is selectively esterified approximately 1000 times faster at the 2-hydroxy acid, L-tartaric acid is esterified approximately 1000 times faster to the mono- and diester, while citric acid is selectively methylated at the terminal (3-hydroxycarboxyl) groups approximately 1000 times faster than in the absence of Si(OMe)4. Esterification is associated with the condensation of silane units to form oligomers. A mechanism is proposed in which 2HOAs attach to silicon via the alkoxy group and carboxyl groups to form various reactive cyclic intermediates. These intermediates may lead to accelerated esterification via nucleophilic attack of MeOH at the ligated carboxyl group, while a separate reaction pathway leads to condensation of silicon centres leading to oligosilanes. The mechanism has implications for the use of 2HOAs as templates in sol-gel silica preparation.

Additive-Driven Dissolution Enhancement of Colloidal Silica. 1. Basic Principles and Relevance to Water Treatment

“…Similar reaction pathways have been proposed by Lambert et al for an alkoxide-type attack, by Ansell et al for a carboxy-type attack, and by Quin et al for a phosphonate attack on silica surface …”

Antioxidant and Quinone Reductase-Inducing Constituents of Black Chokeberry (Aronia melanocarpa) Fruits

“…In this way, bioactivity-guided fractionation of ethyl acetate-soluble extract of the fruits of black chokeberry, using in vitro hydroxyl radical-scavenging and QR induction assays, led to the isolation of a new depside ( 1 ), along with 26 known compounds ( 2 – 27 ). On the basis of physical and spectroscopic data measurements ([α] D , CD, 1 H NMR, 13 C NMR, DEPT, 2D NMR, and MS) and comparison of the data obtained with published values, the structures (see Figure ) of the known compounds 2 – 27 were identified as jaboticabin ( 2 ), − 2- O -(3,4-dihydroxybenzoyl)-2,4,6-trihydroxyphenylacetic acid ( 3 ), 2,4,6-trihydroxybenzaldehyde (formylphloroglucinol) ( 4 ), benzoic acid ( 5 ), 2-furoic acid (pyromucic acid) ( 6 ), p -hydroxybenzoic aicd ( 7 ), , m -hydroxybenzoic acid ( 8 ), protocatechuic acid ( 9 ), l -malic acid ( 10 ), l -malic acid methyl ester ( 11 ), 1-β- O -benzoyl- d -glucopyranoside ( 12 ), prunasin ( 13 ), , 4,6-dihydroxy-2- O -(β- d -glucopyranosyl)acetophenone ( 14 ), vanillate glucoside ( 15 ), naringenin 7- O -β- d -glucopyranoside (prunin) ( 16 ), ,, (2 S )-5,7,3′,5′-tetrahydroxyflavanone 7- O -β- d -glucopyranoside ( 17 ), (2 E ,4 E )-8- O -β- d -glucopyranosyloxy-2,7-dimethyl-2,4-decadiene-1,10-dioic acid ( 18 ), 5- O -caffeoylshikimic acid ( 19 ), chlorogenic acid methyl ester ( 20 ), neochlorogenic acid ( 21 ), neochlorogenic acid methyl ester ( 22 ), quercetin ( 23 ), quercetin 3- O -β- d -galactopyranoside (hyperin) ( 24 ), quercetin 3- O -β- d -robinobioside ( 25 ), quercetin 3- O -β- d -rutinoside (rutin) ( 26 ), and isorhamnetin 3- O -β- d -rutinoside ( 27 ) . Compounds 1 – 3 , 5 – 9 , 12 , 14 – 20 , 22 , 23 , and 27 were isolated from A. melanocarpa for the first time.…”