The identification of structural fragments of enolic monoterpenes, enolic pheromones, and related natural products, from OH stretching vibration frequencies and relative intensities

Abstract

The OH stretching frequency has proved to be a powerful tool for structural investigations of enolic natural products containing diverse structural elements. The position of unsaturation can in many cases be determined directly from frequency measurements. Primary, secondary, and tertiary α–β and β–γ unsaturation can unambiguously be identified due to the possession of OH bands arising from an interaction of the OH proton with the π system. Frequently, unsaturation further removed than the γ position can be identified, particularly when a double bond is present, intermediate between the distal π system and the OH group. The identification of long-range OH–π system interactions relies on the observation that an intermediate double bond reduces the rotational freedom of the molecule. This reduction in entropy is assumed to be responsible for a significant population of the conformation in which the OH group is associated with the π system. Keywords: infrared, conformation, natural products, alcohols, molecular structure.