Efficient preparation method of 4-hydroxybenzoic esters – Oxidation of substituted Hagemman’s ester

Role of Ring Ortho Substituents on the Configuration of Carotenoid Polyene Chains

“…28 Oxidative aromatization of 20 by NBS produced ethyl 2,6-dimethyl-4-(methylthio)benzoate ( 22) in 83% yield. 28 The key building block 22 can be better prepared by Indium reduction 29 of disulfide 21 to the monomeric thiol from, followed by S-methylation (NaH, MeI) in overall 80% yield.…”

“… Lawesson’s reagent allowed the formation of thione 19 , which was not easily purified. It was suspected to be consisted of a mixture of 19 and its dimeric form through a disulfide bond, because S -methylation (NaH, MeI) gave only 40% yield of methyl sulfide 20 ; oxidation of the Lawesson’s reaction product by NBS and catalytic TMS·OTf produced disulfide 21 in 73% yield . Oxidative aromatization of 20 by NBS produced ethyl 2,6-dimethyl-4-(methylthio)­benzoate ( 22 ) in 83% yield .…”

“…It was suspected to be consisted of a mixture of 19 and its dimeric form through a disulfide bond, because S -methylation (NaH, MeI) gave only 40% yield of methyl sulfide 20 ; oxidation of the Lawesson’s reaction product by NBS and catalytic TMS·OTf produced disulfide 21 in 73% yield . Oxidative aromatization of 20 by NBS produced ethyl 2,6-dimethyl-4-(methylthio)­benzoate ( 22 ) in 83% yield . The key building block 22 can be better prepared by Indium reduction of disulfide 21 to the monomeric thiol from, followed by S -methylation (NaH, MeI) in overall 80% yield.…”

Apocarotenals of Phenolic Carotenoids for Superior Antioxidant Activities

“…para -Hydroxybenzaldehydes 3b (without substitution), 3c ( meta -dimethyl), and 3d ( meta -dimethoxy) were commercially available. ortho-Substituted para -hydroxybenzoates 3a (Me, Me) and 3e (Me, Ph) were prepared according to our previously reported procedure . Benzenesulfonyl protection for the para -hydroxyl group was utilized throughout the carotenoid syntheses, which was proven to survive the chain extension and olefination conditions and to be deprotected selectively in the presence of acid-sensitive polyene chain by KOH in t -BuOH …”

“…ortho-Substituted para-hydroxybenzoates 3a (Me, Me) and 3e (Me, Ph) were prepared according to our previously reported procedure. 17 Benzenesulfonyl protection for the para-hydroxyl group was utilized throughout the carotenoid syntheses, which was proven to survive the chain extension and olefination conditions and to be deprotected selectively in the presence of acid-sensitive polyene chain by KOH in t-BuOH. 18 The para-hydroxybenzoate 3a with ortho-dimethyl substituents was protected first by a benzenesulfonyl group (step a), and the ester group was transformed into a formyl group (step b) after reduction (LAH) and then oxidation (PCC).…”