Acetyl ester from absolute configuration

The desilylacetylated qrcloadducts, produced from the reactions of trimethylsilyl-diazomethane with 3-crotonoyl-2-oxazolidinone or 3-crotonoyl-4,4-dimethyl-2-oxa-zolidinone, were transformed to methyl traws-l-acetyl-4-methyl-l-pyrazoline-5-car-boxylate through the reactions with dimethoxymagnesium at -20 °C. When the optical rotations and chiral HPLC data were compared between these two esters, it was found that these two products had opposite absolute stereochemistry (Scheme 7.39). The absolute configuration was identified on the basis of the X-ray-determined structure of the major diastereomer of cycloadduct derived from the reaction of trimethylsilyldiazomethane to (S)-3-crotonoyl-4-methyl-2-oxazolidi-none. 

Absolute configurations were ascribed on the basis of an empirical model by analogy to l-hydroxyaUcanephosphonates. They are opposite to those obtained from the NMR spectra of the Mosher s esters of 2-hydroxyalkanephosphonates (entries 4-6) and whose absolute configuration is known. / -Chlorophenyl acetate was used as the acetylating agent. 

The asymmetric intramolecular crossed benzoin reaction catalysed by a chiral triazolium salt has been used to synthesise 3-hydroxychroman-4-ones 34 in good to high yields and ee. The absolute configuration at the quaternary stereocentre C-3 has been shown to be S by X-ray analysis of the camphanyl ester <06SL2431>. Both enantiomers of 2-(2-phenylethyl)chroman-4-one, flindersiachromanone, have been obtained from racemic l-phenylhex-5-en-3-ol after resolution via lipase-catalysed acetylation <06H(68)483>. 

Kawabata et al. [28] resolved methyl ( )-Ar-(Boc-Z,-phenylalanyl)-m-2-aminocyclopentanecarboxylate by fractional crystallization from ethyl acetate. The phenylalanine derivative was prepared from the methyl ester of racemic m-2-ACPC (11) by acetylation with JV-Boc-Z.-phenylalanine using l-(3-dimethylaminopropyl)-3-ethylcarbodiimide (EDC). Edman degradation of the separated isomers, followed by deprotection, acid hydrolysis and desalting with anion-exchange resin, gave enantiomerically pure (15,2/ )- and (l/ ,2S)-2-ACPC. The absolute configurations were proved by X-ray diffraction of the /.-phenylalanine derivatives [28]. 

Another original synthesis of tetrahydrolipstatin comes from Stephen Davies. [278] The two central building blocks are on the one hand an enantio-mericaUypure )S-hydroxyaldehyde, which is accessible by acylation of Meldrum s acid with dodecanoyl chloride, followed by an enantioselective hydrogenation of the -keto-ester along Noyori s method, and on the other hand an enantio-mericallypure acyl-iron complex [279], which is obtained by alkylation ofthe acetyl complex. [280] It is noteworthy that from the beginning the aldehyde component possesses the correct absolute configuration. The side-specific course ofthe anh-aldol addition is exclusively determined by the iron complex. First, the iron complex is deprotonated with butyl-lithium, then transmetallated with diethylaluminium chloride, and finally reacted with the aldehyde. The re-... 

The absolute configuration of rhamnose, fucose, xylose, mannose, galactose and glucose, released from a complex polysaccharide and Isolated by cellulose column fractionation, have been determined by g.c.-m.s. of their acetylated (-)-2-octyl glycoside derivatives. In a new approach, enantiomeric pairs of nine aldoses have been separated after reaction with L-cystelne methyl ester to generate dlastereolsomerlc methyl 2-(polyhydroxyalkyl)thlazolidlne-(4R)-carboxylates, e.g., the D-galactose derivative (1), and pertri-... 

Racemic 1 and 2a were not resolved into their two enantiomers on a Cylodex B chiral stationary phase GC column (J W Scientific), and so the absolute configurations of insect-produced 1 and 2a were determined by derivatization with a chiral derivatizing reagent followed by GC analysis of the resulting diastereomers on an achiral column. Thus, racemic lavandulol, a sanq)le of (R)-(-)-lavandulol, and a hydrolyzed sample of the insect-produced conq)ound were derivatized with acetyl (5)-lactic acid chloride and pyridine in ether to form the acetyl lactate ester(s) (75). The diastereomeric derivatives from racemic lavandulol were resolved almost to baseline, with the derivative from (7 )-lavandulol (isolated from lavender oil) being the later eluting peak. 

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