Role of Ring Ortho Substituents on the Configuration of Carotenoid Polyene Chains

Abstract

The 9-(Z)-configuration was exclusively obtained in the carotenoid polyene chain irrespective of olefination and disconnection methods for terminal ortho-unsubstituted benzene rings. The 2,6-dimethyl substituents in the terminal rings secure an all-(E)-polyene structure. The single molecular conductance of the pure 9-(Z)-carotene was measured for the first time to be 1.53 × 10 ± 6.37 × 10G, whose value was 47% that of the all-(E)-carotene ((3.23 × 10) ± (1.23 × 10) G).

Fast Assembly and High-Throughput Screening of Structure and Antioxidant Relationship of Carotenoids

“…The first group (Table , entries 1–3) demonstrated the strongest antioxidant activities, which showed the structural features of ortho - and multiple electron-donating substituents in the aromatic rings. It suggested that ortho -substitution induced more to the all- E configuration of the polyene chain . The second-best group in radical scavenging activity contained an electron-donating substituent at the para -position of the aromatic ring, which favored 9′- Z configuration of the carotenoid polyene chain (Table , entries 4, 5, 7, and 8).…”

“…The configuration of the polyene chain was carefully assigned to be either all- E or 9′- Z , or a mixture of both by comparing their 1 H NMR spectra with those of all-( E )-carotene 1b and 9′-( Z )-carotene 1g (Table ). , The numbering sequence followed the carotenoid tradition, instead of the systematic IUPAC rule …”

“…It was found that the configuration of the polyene chain was controlled mostly by steric and electronic effects of the substituents in the terminal ring: ortho -methyl substitution preferentially produced all- E form; on the other hand, para -substituent favored 9′- Z form . It was another noteworthy feature that furan gave all- E form, but thiophene produced 9′- Z form exclusively.…”

“…Higher activities of the carotenoids in group 1, compared to those in group 2, can be ascribed not to the configuration of the polyene, but to the multiple electron-donating aromatic substituents. The ortho -substituents in group 1 induced the polyene chain more to all- E configuration by steric reason, whereas the para -substituents in group 2 favored 9′- Z configuration presumably by effective conjugation . All-( E )-carotene with furan rings is more potent than 9′-( Z )-carotene with thiophene rings for the same rationale (Table , entries 6 and 11).…”

“…The configuration of the polyene chain was carefully assigned to be either all-E or 9′-Z, or a mixture of both by comparing their 1 H NMR spectra with those of all-(E)-carotene 1b and 9′-(Z)-carotene 1g (Table 1). 10,11 The numbering sequence followed the carotenoid tradition, instead of the systematic IUPAC rule. 12 It was found that the configuration of the polyene chain was controlled mostly by steric and electronic effects of the substituents in the terminal ring: ortho-methyl substitution preferentially produced all-E form; on the other hand, parasubstituent favored 9′-Z form.…”