THE STEREOCHEMISTRY OF MALEOPIMARIC ACID AND THE LONG RANGE SHIELDING EFFECT OF THE OLEFINIC BOND

Abstract

Nuclear magnetic resonance studies of methyl maleopimarate and closely related derivatives clearly establish the stereochemistry of methyl maleopimarate and, consequently, levopimaric acid. An extension of this study to other related compounds, 22 in all, provides evidence, on the basis of an intercomparison of the chemical shifts for the angular methyl protons, to support the original suggestion by Jackman (Applications of NMR in organic chemistry. Pergamon, New York. 1959. Chap. 7) that the long range shielding effect of the olefinic linkage is analogous to that of the carbonyl double bond. Coupling constants and chemical shifts for some of the protons of the bicyclo[2.2.2]octene system present in these compounds are also discussed.