Structural characterisation and absolute stereochemistry of some degraded carotenoids from New Zealand honeys
The structures and absolute stereochemistries of some degraded carotenoids from New Zealand native honeys were elucidated. Determination of the absolute stereochemistry of 3,5,6-trihydroxy-β-ionone was attempted by an enantioselective synthesis. A racemic synthesis was achieved starting from β-ionone. The key steps involved regioselective hydroboration of 3,4-dehydro-β-ionone, followed by molybdenum-mediated stereoselective epoxidation of the resulting homoallylic alcohol. Ring opening of the epoxide afforded the required trihydroxy-ionone. Synthesis of both enantiomers of 3,5,6-trihydroxy-β-ionone via enantio- and regioselective functionalisation of 3,4-dehydro-β-ionone was not achieved. Methods of enantioselective enzyme-mediated kinetic resolution of cyclic homoallylic alcohols were investigated. A satisfactory procedure was not developed. The absolute stereochemistry of 3,5,6-trihydroxy-β-ionone was determined by analysis of the diastereoisomeric proton chemical shifts of (S)- and (R)-α-methoxyphenylacetate (MPA) derivatives using the extended Mosher-Trost configurational model. This analysis indicated a 3S,5R,6R absolute configuration, the same configuration as 3,5,6-trihydroxy carotenoids. The natural product was extracted from thyme (Thymus vulgaris) honey using established procedures. Diethyl ether extracts of kamahi (Weinmannia racemosa) honey were found to contain three diastereoisomers (kamahines) of an unusual degraded carotenoid with a 14 carbon skeleton. After acetylation, one of the isomers was fully characterised by multidimensional ¹H and ¹³C NMR spectroscopy, and single-crystal X-ray crystallography, indicating the parent alcohol was 4,5-dihydro-1',5-dihydroxy-2',4,8',8'-tetramethylspiro[furan-2(3)H,7'[6']oxabicylco[3.2.1]oct[2']ene]-4'-one. Comparison of NMR-derived NOE and coupling constant data of kamahine A and B acetates with kamahine C acetate indicated the parent kamahines were C-4, C-5 epimers. The absolute configurations of the kamahines were determined by NMR and a molecular modeling study (MacroModel) of the diastereoisomeric hemiacetal MPA esters. The results of the determination, coupled with a consideration of crystallographically-determined hemiacetal ester conformations obtained from a search of the Cambridge Structural Database, supported the use of the Mosher-Trost model for determination of the absolute configurations of hemiacetals. Furthermore, this analysis suggested that the Mosher-Trost model should be amended to take into account a torsion angle of +25-50° between the methine proton and the carbonyl of hemiacetal MPA esters. Kamahines A-C were shown to have the absolute configurations 1'R,2R,4R,5S,5'S, 1'R,2R,4S,5S,5'S and 1'R,2R,4R,5R,5'S, respectively. The absolute configurations of kamahines A-C were consistent with an abscisic acid precursor.
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