The stereochemistry of the Colombo root bitter principles

Abstract

While dihydroisocolumbin (2,3-dihydro-8-iso-l) on alkaline hydrolysis affords the diacid (IX), octahydroisocolumbic acid (XIII; stereochemistry related to dihydroisocolumbin) leads to the “isolactone”(XIV = XXV). The stereochemical implications of this transformation are discussed. When coupled with application of the Hudson–Klyne and Bose rules to position 12, and the rotatory dispersions of the columbin-derived cis-decalones (XXIX and XXXII), locked respectively in the “steroid” and “non-steroid” conformations, they lead to the absolute configuration for columbin depicted in (I). This is fully supported by an X-ray analysis of the isocolumbin derivative (XXXV). Several independent lines of evidence suggest that the epoxide in the “O₇” bitter principles is β-oriented. Previously established inter-relationships lead to the configurations of jateorin (II), chasmanthin(III), and palmarin (IV).

Attempts to rationalise certain apparent anomalies in the infrared spectra of isocolumbin derivatives lead to a postulate relating carbonyl (infrared) frequency and conformation in δ-lactones. It appears that the half-chair conformation (XL) may be associated with values in the normal range 1730–1750 cm.^–1, while the half-boat conformation (XLIV) leads to values in the region of 1760 cm.^–1. Isocolumbin features the unusual situation of a half-boat δ-lactone (XLV) adopted in response to non-bonded interactions.