Photoreduction and Ketone‐sensitized Reduction of Alkaloids

Abstract

The photoprocesses of berberine, palmatine, coralyne, sanguinarine, flavopereirine and ellipticine were studied in several solvents. The quantum yields Φ(Δ) of singlet molecular oxygen formation of berberine, palmatine and sanguinarine are moderate in dichloromethane (0.2-0.6) and much smaller in acetonitrile or trifluoroethanol. For the other alkaloids examined, Φ(Δ) is rather independent of solvent polarity. The direct and ketone-sensitized photolysis, using steady-state irradiation at 313 nm or 248/308 nm laser pulses, was studied by absorption and fluorescence spectroscopy. Thereby, radicals were observed yielding eventually dihydro derivatives as major products, which are thermally back-converted on admission of oxygen. The quantum yield of conversion of alkaloids to dihydroalkaloids is enhanced in the presence of triethylamine. The reaction in the presence of ketones and electron or H-atom donors has a quantum yield of close to unity.

Photooxidation of phytochemicals in food and control: a review

“…Alkaloids, which are mostly distributed in the higher plants, are a group of nitrogen‐containing organic compounds with complex cyclic structures that have significant physiological activities. Alkaloids, such as berberine, coralyne, and sanguinarine, are sensitive to light and can lead to adverse photooxidation. The photochemical reactions of all three compounds under light involve the formation of 1 O 2 (type II photooxidation induced by O 2 ).…”

Photooxidation of Alkaloids: Considerable Quantum Yield Enhancement by Rose Bengal‐sensitized Singlet Molecular Oxygen Generation

“…For the structurally similar palmatine, photoinduced O 2 · ) formation has been suggested (6). The quantum yield of singlet molecular oxygen production is in the 0.39-0.42 range, irrespective of the solvent polarity for coralyne, but low (<0.05) for berberine in polar solvents (16). The low F ox values at pH 8-12 (Table 1) indicate a too low reactivity (i.e.…”

“…The photoirradiation of various alkaloids in the presence of triethylamine results in dihydroalkaloids, which were back‐converted on admission of oxygen (16). The rate of photoreduction is significantly accelerated by the combined utilization of a ketone with triethylamine or alcohol as photoinitiators and a quantum yield of close to unity was achieved for dihydroalkaloid formation (16).…”

“…Alkaloids, such as berberine, coralyne (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17) and sanguinarine (18)(19)(20)(21)(22)(23)(24), are sensitive to light and can induce adverse phototoxic reactions. The solvent and molecular structure variations markedly affect the quantum yield of singlet molecular oxygen, O 2 ( 1 D g ) generation (5)(6)(7)16). The quantum yields of triplet and O 2 ( 1 D g ) formation strongly diminish with increasing polarity of the environment in the cases of berberine, palmitine and sanguinarine, but are barely changed for coralyne (16).…”

“…The solvent and molecular structure variations markedly affect the quantum yield of singlet molecular oxygen, O 2 ( 1 Δ g ) generation (5–7,16). The quantum yields of triplet and O 2 ( 1 Δ g ) formation strongly diminish with increasing polarity of the environment in the cases of berberine, palmitine and sanguinarine, but are barely changed for coralyne (16). Binding to DNA significantly enhanced the efficiency of the berberine‐sensitized formation of O 2 ( 1 Δ g ) (11,12) and coralyne initiates easily the photochemical DNA damage (9).…”