¹H and ¹³C NMR spectra of the methanolic allomerization products of 13²(R)‐chlorophyll a

Abstract

The spontaneous autoxidation (allomerization) of 13'(R)-chlorophyll u in methanol produces seven main products containing three epimer pairs. The 'H and ' ' C NMR spectra of these products in acetone-d, were recorded on a 500 MHz spectrometer and fully assigned using two-dimensional HMQC and HMBC techniques. The absolute configurations of the oxidized carbons, originally C-13', were determined using the ROESY technique. In comparison with 13'(R)-chlorophyll u, the steric repulsion between the C-17 side-chain and the bulky substituents of the oxidized C-13' causes steric strain in the chiral part of an allomer, relieved by conformational changes in rings D and E and to a lesser extent also in the whole macrocycle. These changes were estimated for each allomer from the A6 values of the carbon and proton resonances and from protomproton coupling constants. Information about the orientation of the front part of the phytyl chain in 13z(R)-chlorophyll u and its methanolic allomers was obtained by analysing the variation in the form of the P1-CH, signal.

Identification of diastereomeric chlorophyll allomers by atmospheric pressure chemical ionisation liquid chromatography/tandem mass spectrometry

“…The ease of loss of the C‐13 2 substituent as water from the S diastereomer was attributed to greater steric strain in the C‐13 2 carbonoxygen bond, resulting from steric repulsion between the substituent and the C‐17 propionate ester substituent on the same plane of the macrocyclic ring. NMR studies have confirmed epimer I as S configuration and epimer II as R configuration 6…”

“…2 15 1 ‐MeO‐lact‐chl a , 13 2 ‐MeO‐chl a 2,3 and 13 2 ‐HO‐chl a 3–5 exist as two diastereomers differing at position C‐13 2 or C‐15 1 . Recently, full spectral assignment including the designation of the stereochemical configurations of three pairs of diastereomers has been achieved by complete assignment of the 1 H and 13 C nuclear magnetic resonance (NMR) and 2D ROESY spectra 6. Structural elucidation by NMR is rigorous but time‐consuming and often requires milligram quantities of highly purified sample.…”

Complete assignment of the 1H and 13C NMR spectra and conformational analysis of bonellin dimethyl ester

“…NH tautomerism) remained undetermined. Nowadays, the utilization of high-field NMR instruments and advanced NMR techniques offers an efficient way to obtain detailed solution structures for substituted chlorins [14][15][16][17][18].…”

Synthesis, Reactivity and Structure of Chlorophylls

“…oxidative degradation reactions involving oxidations, hydroxylation, ring-opening or decarboxylations of the isocyclic pentanone ring. The most comprehensive treatise on Chl chemistry remains the article by Hynninen (1991). Only a few developments have been made in the last decade, which will be the focus of this review.…”

“…Several analytical studies, mostly employing modern mass spectrometric methods, were aimed at studying the allomerization reaction of Chl (Grese et al, 1990;Rahmani et al, 1993;Brereton et al, 1994;Hyvarinen et al, 1995). A thorough study by Woolley et al (1998) clearly showed that the 13 2 -CO 2 Me group is required for allomerization Scheme 4 to occur and that a double rather than single bond between C7 and C8 (e.g., Chl vs. BChl) exerts a significant influence on the reactivity of ring E: ring E in Chl a is more susceptible to oxidative ring cleavage and lactone formation than in BChl a.…”