A large variety of C,C-bond forming reactions and functional group interconversions can be achieved by electron transfer. For the conversion of renewable feedstocks, electrolysis has been applied to coupling of radicals generated by anodic decarboxylation of fatty acids and carboxylic acids of carbohydrates. Furthermore, a derivative of L-gulonic acid is converted nearly quantitatively into L-xylonolacton. Trimethyl aconitate from trimethyl citronate is dimerized stereoselectively at the cathode in 72 % yield to a cyclic hexamethyl ester by an inter- and intramolecular Michael addition. Two acetoxy groups are added anodically to methyl conjuenate (obtained from methyl linoleate) to form methyl (E)-9,12-diacetoxy-10-octenoate and methyl (E)-10,13-diacetoxy-11-octenoate in 85 % yield. The primary hydroxy groups in mono- and disaccharides can be oxidized to carboxylic acid groups in good yield and high selectivity by anodic oxidation with 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) as mediator. The results demonstrate that electrolysis is in good accordance with many of the 12 principles of green chemistry.
Several enantioenriched methyl vic -dihydroxystearates 4 have been synthesized from methyl octadecenoates by enantioselective dihydroxylation in good yields (85-97%) and with satisfactory enantiomeric excess (ee 80-97%). The enantioenriched anti -9,10-dihydroxystearates ( anti -4c ) were prepared by HPLC separation of the corresponding dicamphanyl esters anti -6 . Their ee was determined by HPLC of the corresponding diastereoisomeric carbamates 7 , and their configuration was assigned by relation to a described compound or by applying the mnemonic device of Sharpless.Monolayers of chiral amphiphiles can spontaneously separate into chiral domains. 1 Most of the amphiphiles, studied so far bear chiral polar head groups. We wanted to explore how the monolayer behavior of methyl stearates is influenced by stereogenic centers located at different positions in the alkyl chain. For this purpose enantioenriched methyl vic -dihydroxystearates were prepared using the Sharpless 2 method for the enantioselective dihydroxylation of olefins. The corresponding methyl octadecenoates were synthesized by Kolbe electrolysis 3 or Wittig reaction. Methyl octadec-17-enoate (3a) has been obtained in 40% yield by coelectrolysis of one equivalent of undec-10-enoic acid (2) with four equivalents of methyl azelate (1) 4 (Scheme 1) at platinum electrodes in methanol in an undivided cell. 5 Ethyl trans -octadec-2-enoate (3b) was synthesized by a Wittig reaction from hexadecanal and (ethoxycarbonylmethylene)triphenylphosphorane in 74% yield.The synthesis of chiral vicinal diols by enantioselective dihydroxylation has been described for a variety of olefins; 2 however, to our knowledge this powerful conversion has not yet been applied to the synthesis of chiral vicdihydroxy fatty acid esters. The enantioselective dihydroxylation of 3a , b and of commercially available methyl trans -octadec-9-enoate (3c) led in excellent yield (85-97%) and satisfactory to high enantiomeric excess (ee 84-97%) to methyl vic -dihydroxystearates 4 (Scheme 2, Table 1). As reagents for the dihydroxylations commercially available AD-Mix-α and AD-Mix-β were used. 6In the case of methyl oleate (5) the enantiomeric excess of the dihydroxylation was very low, which has been reported before for the dihydroxylation of nearly symmetrical cisolefins. 7 The synthesis of enantioenriched methyl anti -9,10-dihydroxystearates was then achieved by HPLC separation of the corresponding diastereoisomeric dicamphanyl esters anti -6 (Scheme 3). For this purpose methyl oleate (5) was treated with OsO 4 to yield the methyl anti -9,10-dihydroxystearates ( anti -4c ). 8 The diastereoisomers anti -6 were obtained by diesterification of anti -4c with (-)-camphanic acid chloride, as described by Mohr and Rosini, 9 followed by HPLC separation and partial hydrolysis to the methyl anti -9,10-dihydroxystearates (-)-(9 S ,10 S )-4c and (+)-(9 R ,10 R )-4c (Scheme 3, Table 2).To determine their enantiomeric purity, the dihydroxy esters 4a , syn-4b and anti-4c were transformed into their diastereoiso...
stereochemistry stereochemistry (general, optical resolution) O 0030 -036Synthesis of Enantioenriched Methyl vic-Dihydroxystearates.-Octadecenoates (I), (III), and (V) are dihydroxylated to yield diol (II) and syn-diols (IV) and (VI) with good yields and satisfactory enantiomeric excess by the Sharpless method using either AD-mix-α or AD-mix-β. Enantioenriched anti-esters (VIII) are obtained by HPLC separation of their dicamphanyl esters and partial hydrolysis of separated diastereomers. -(PLATE, M.; OVERS, M.; SCHAEFER, H. J.; Synthesis (1998) 9, 1255-1258; Org.-Chem. Inst.,
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