Steric interaction between the peripheral substituents of 10(S)‐chlorophyll derivatives and its conformational consequences: A proton magnetic resonance study

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Abstract
The improved methods developed recently for the preparation of 10(S)‐chlorophyll derivatives have permitted a detailed comparison of their conformations with those of the corresponding 10(R) derivatives, utilizing 1H NMR spectroscopy. Starting from the highly purified 10(S) derivative and following its conversion to the corresponding 10(R) form by repeated 1H NMR spectral measurements in acetone‐d6, reliable values for the differences in the chemical shifts (Δδ) for each epimeric pair of compounds have been obtained. The relatively large Δδ values (0.1–0.2 ppm) observed for the P‐1‐CH2, P‐H‐2, P‐3a‐CH3 and 10b‐CH3 signals were interpreted as indicating the proximity of the C‐10 methoxycarbonyl group and the olefinic region of the phytyl group in the 10(S) derivatives. The proximity between these side‐chains was considered to be derived principally from the three closely spaced carbonyls in the 10(S) derivatives. To minimize repulsion among the carbonyls, the C‐7 and C‐10 side‐chains presumably assume conformations allowing maximal mutual distances for the carbonyls. As a compromise of various energetic factors, the phytyl group takes up a conformation where it bends to the right hand side of the macrocycle, or above rings V and III. This results in steric crowding between the olefinic region of the phytyl group and the C‐10 methoxycarbonyl group. In addition, the other chemical shift increments show that the steric crowding between the bulky side‐chains of the 10(S) derivatives increases the steric strain at the periphery of the macrocycle, relieved by pronounced conformational changes in rings IV and V and, to a lesser extent, also in the whole phorbin ring. The conformational differences were found to be larger for the magnesium‐free epimers than for the chlorophylls. In the former case, they were also manifested by relatively large Δδ values (0.1–0.5 ppm) for the pyrrole NH protons.

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