NOVEL NOR-SESQUITERPENOIDS IN NEW-ZEALAND HONEYS - THE RELATIVE AND ABSOLUTE STEREOCHEMISTRY OF THE KAMAHINES - AN EXTENSION OF THE MOSHER METHOD TO HEMIACETALS

The relative stereochemistries of the diastereoisomeric kamahines A (1 a) and B (1b) were determined by correlation of the NMR-derived NOE an d coupling constant data of kamahine A and B acetates (2a and 2b) with those from the previously crystallographically solved kamahine C acet ate (2c). The kamahines were shown to be C4,C5 epimers of an-2(3)H,7'[ 6']oxabicyclo[3.2.1]oct[2']en]-4'-one. Absolute configurations of the kamahines were determined by an NMR and molecular modeling study (Macr oModel) of the diastereoisomeric hemiacetal methoxymandelate esters (6 a-c and 7a-c). The results of the determination, coupled with a consid eration of crystallographically determined hemiacetal ester conformati ons obtained from a search of the Cambridge Structural Database, sugge sted that the Mosher-Trost model used for prediction of absolute confi guration should be amended to take into account a torsion angle of +25 to +50 degrees between the methine hydrogen and the carbonyl of hemia cetal mandelate esters. The kamahines were shown to have the absolute configurations (1'R,2R,4R,5S,5'S)1a, (1'R,2R,4S,5S,5'S)-1b, and (1'R,2 R,4R,5R,5'S)-1c.