Sensitized Photooxidation of Ortho‐Prenyl Phenol: Biomimetic Dihydrobenzofuran Synthesis and Total ¹O₂ Quenching^†

Abstract

The sensitized photooxidation of ortho-prenyl phenol is described with evidence that solvent aproticity favors the formation of a dihydrobenzofuran [2-(prop-1-en-2-yl)-2,3dihydrobenzofuran], a moiety commonly found in natural products. Benzene solvent increased the total quenching rate constant (k T ) of singlet oxygen with prenyl phenol by ~10fold compared to methanol. A mechanism is proposed with preferential addition of singlet oxygen to prenyl site due to hydrogen bonding with the phenol OH group, which causes a divergence away from the singlet oxygen 'ene' reaction toward the dihydrobenzofuran as the major product. The reaction is a mixed photooxidized system since an epoxide arises by a type I sensitized photooxidation.

Role of curvature in acridone for 1O2 oxidation of a natural product homoallylic alcohol: A novel iso‐hydroperoxide intermediate

“…Our DFT results indicate that prenyl phenol 1 bears a weaker internal hydrogen bond compared to glycocitrine 6 (Figure 2). As we will see, the curvature of glycocitrine 6 provides a stronger internal hydrogen bond, thereby facilitating 1 O 2 oxidation of the prenyl group by an isohydroperoxide intermediate. Note that the iso form [R(H) O + -O − ] is zwitterionic (labeled as 11 in Figure 3), it is not the normal hydroperoxide form (ROOH).…”

“…In this way, the theoretical work allows us to predict a 1 O 2 path that deviates from the ‘ene’ reaction. It involves a path to dihydrobenzofuran that extends beyond our experimental work 1 …”

“…The biomimetic synthesis of dihydrobenzofuran has been reported in a singlet oxygen ( 1 O 2 ) oxidation of o-prenyl phenol 1 (Figure 1). 1 Products 2-5 were formed, where dihydrobenzofuran 2 was proposed to arise by H-bond interaction of phenolic hydrogen with 1 O 2 , directing it into the prenyl group. The formation of a peroxy intermediate was plausible, but evidence was not found in trapping experiments with triaryl phosphites.…”

Tuning the 1O2 Oxidation of a Phenol at the Air/Solid Interface of a Nanoparticle: Hydrophobic Surface Increases Oxophilicity

“…Consequently, selectivity favors dihydrofuran 1 over the “ene” hydroperoxides 2 and 3 . A recent report also found selectivity, but by the homogeneous solvent on product formation, where in C 6 D 6 , dihydrofuran 1 , whereas in CH 3 OH, dihydrofuran 1 decreased and amounts of allylic hydroperoxides 2 and 3 increased.…”

“…a Relative yields averaged from photosensitizer particles P PS [Br 2 B-OAc or Al(III)Cl 2 Pc], where in 1 H NMR product peaks were consistent with those reported in the literature 34. Hydroperoxides 2 and 3 are stable up to 2−3 days on the nanoparticles.…”