Absolute configuration carboxylic acid-containing

If a molecule contains several asymmetric C atoms, then the diastereomers show diastereotopic shifts. Clionasterol (28a) and sitosterol (28b) for example, are two steroids that differ only in the absolute configuration at one carbon atom, C-24 Differing shifts of C nuclei close to this asymmetric C atom in 28a and b identify the two diastereomers including the absolute configuration of C-24 in both. The absolute configurations of carboxylic acids in pyrrolizidine ester alkaloids are also reflected in diastereotopic H and C shifts which is used in solving problem 54. 

The Okinawan sponge Psammaplysilla purea that contains purealidins M-0 (2004-2006) also yields purealidins J (2078), K (2079), L (2080), P (2081), Q (2082), and R (2083) (1835). Purealidin J (2078) is the antipode of pseudocer-atinine A (2089). The Indian sponge Psammaplysilla purpurea, which is the source of purpurealidins F-H (2018-2020) and other bromotyrosines (vide supra), also contains purpurealidins A (2084), B (2085), C (2086), and D (2087) (1842). A Caribbean Pseudoceratina sponge has afforded the simple carboxylic acid 2088 (1868). The New Caledonian sponge Pseudoceratina verrucosa, which is the source of pseudoceratinine B (1990), also contains pseudo-ceratinines A (2089) and C (2090), the absolute configurations of which are shown (1829). 

The 20 amino acids commonly found as residues in proteins contain an a-carboxyl group, an a-amino group, and a distinctive R group substituted on the a-carbon atom. The a-carbon atom of all amino acids except glycine is asymmetric, and thus amino acids can exist in at least two stereoisomeric forms. Only the L stereoisomers, with a configuration related to the absolute configuration of the reference molecule L-glyceraldehyde, are found in proteins. 

Subsequent studies examined whether AMDase discriminates between two prochi-ral carboxyl groups. To this end, we prepared both enantiomers of a-methyl-a-phenylmalonate (6) with in one of the two carboxyl groups (Figure 3.7) [14], Starting from C-phenylacetate ( C-2), both enantiomers of chiral C-containing a-methyl-a-phenylmalonate (6) were prepared. The absolute configuration of the chiral substrate was clearly determined by the optical rotation of tire resolved hydroxyl acid (5). 

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