High-field FT NMR application of Mosher's method. The absolute configurations of marine terpenoids
                                
                                    
                                        作者:Ikuko Ohtani、Takenori Kusumi、Yoel Kashman、Hiroshi Kakisawa                                    
                                    
                                        DOI:10.1021/ja00011a006
                                    
                                    
                                        日期:1991.5
                                    
                                    Mosher's (H-1) method to elucidate the absolute configuration of secondary alcohols was reexamined by use of high-field FT NMR spectroscopy, which enables assignment of most of the protons of complex molecules.  There is a systematic arrangement of DELTA-delta (delta-S - delta-R) values obtained for the (R)- and (S)-MTPA esters of (-)-menthol, (-)-borneol, cholesterol, and ergosterol, the absolute configurations of which are known.  Analysis of the DELTA-delta values of these compounds led to a rule that could predict the absolute configurations of natural products.  When this rule was applied to some marine terpenoids including cembranolides and xenicanes, their absolute configurations were assigned and a part of the results were confirmed by X-ray structural analyses.  In the case of sipholenol A, which has a sterically hindered OH group, this rule is inapplicable.  But the problem is overcome by inverting the OH group to a less sterically hindered position; the resulting epimer gives systematically arranged DELTA-delta values, which enabled the elucidation of the absolute configuration.  Comparison of the present method with Mosher's F-19 method indicates that the latter one using F-19 NMR lacks in reliability.