Racemic compounds, fragments chirality

Furthermore, Sato and Ikekawa (238) have obtained by chromic acid oxidation of (257 )-22,26-acetylepimino-3/3,16(8-diacetoxycholesta-5,22-diene (LX), prepared from solasodine (I) by acetylation (238, 239), 16 -hydroxy-3(8-acetoxy-22,23-bisnorchol-5-enic lactone (acetyldios-genin lactone, LXI). The side chain fragment of this degradation, which involves the carbon atoms 23-27 including the center of chirality C-26, has been identified as (R)-( — )-4-amino-3-methylbutyric acid (LXII) (240), the constitution of which was confirmed by comparison with the synthesized racemic compound. Its absolute configuration was confirmed by conversion into (R)-( +)-methylsuccinic acid (LXIII) (225, 226). 

The gross structural features, presence of a tetramic acid and E-decenoyl side chain, could be inferred from NMR studies. Methanolysis (HCl/MeOH) of 47 and pentane extraction of the quenched reaction mixture gave two compounds that were determined to be the methyl esters of decenoic acid and N-(2-decenoyl)leucine. The nature of the 3-acyl tetramic acid was deduced from the identification of 48 and 49 in the aqueous portion of the methanolysis reaction mixture following treatment with trifluoroacetic acid anhydride. The unusual C-C bond fragmentation under acidic conditions, and the structure of the antibiotic was confirmed by synthesis of racemic 47 [86]. The configuration at the lone chiral centre was established as R by chiral GC. The carbon NMR spectrum of 47 indicated an equilibrium between three tautomers in which the A2-pyrrolin-4-one form is preferred (60%) and the two internal tautomers (50, 51) make equal contributions (20% each). 

The most widely employed synthetic approach for the preparation of thiazoles is the condensation of an a-halocarbonyl compound with a reactant bearing the N—C—S fragment such as thioamides, thioureas, or dithiocarbamic acid derivatives <84CHEC-i(6)294>. When the Hantzsch s synthesis is employed for the preparation of chiral substituted thiazoles such as 2-aminomethyl thiazoles some foresight must be considered in order to prevent racemization (see Section 3.06.11). 

Many enantiomerically pure compounds can be made by a variety of strategies. Several conceivable strategies for the synthesis of the quinolone antibiotic ofloxacin 45 are shown below. The molecule contains only one chiral centre and this may be introduced as a fragment from the chiral pool 47 or by the resolution of a key intermediate 44 or indeed the final product.4 A synthesis developed5 for the preparation of racemic ofloxacin which incorporates racemic aminoalcohol ( )-47, could in principle... 

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