Photochemistry of A1E, a retinoid with a conjugated pyridinium moiety: competition between pericyclic photooxygenation and pericyclization.
The photochemistry of the retinoid analogue A1E shows an oxygen and solvent dependence. Irradiation of A1E with visible light (lambda(irr) = 425 nm) in methanol solutions resulted in pericyclization to form pyridinium terpenoids. Although the quantum yield for this cyclization is low (approximately 10(-4)), nevertheless the photochemical transformation occurs with quantitative chemical yield with remarkable chemoselectivity and diastereoselectivity. Conversely, irradiation of A1E under the same irradiation conditions in air-saturated carbon tetrachloride or deuterated chloroform produced a cyclic 5,8-peroxide as the major product. Deuterium solvent effects, experiments utilizing endoperoxide, phosphorescence, and chemiluminescence quenching studies strongly support the involvement of singlet oxygen in the endoperoxide formation. It is proposed that, upon irradiation, in the presence of oxygen, A1E acts as a sensitizer for generation of singlet oxygen from triplet oxygen present in the solution; the singlet oxygen produced reacts with A1E to produce cyclic peroxide. Thus, the photochemistry of A1E is characterized by two competing reactions, cyclization and peroxide formation. The dominant reaction is determined by the concentration of oxygen, the concentration of A1E, and the lifetime of singlet oxygen in the solvent employed. If the lifetime of singlet oxygen in a given solvent is long enough, then oxidation (peroxide formation) is the major reaction. If the singlet oxygen produced is quenched by the protonated solvent molecules faster than singlet oxygen reacts with A1E, then cyclization dominates.
Jockusch
Steffen
S
Department of Chemistry, Columbia University, 3000 Broadway, New York, New York 10027, USA.
Ren
Rex X
RX
Jang
Young Pyo
YP
Itagaki
Yasuhiro
Y
Vollmer-Snarr
Heidi R
HR
Sparrow
Janet R
JR
Nakanishi
Koji
K
Turro
Nicholas J
NJ
D003500:Cyclization; D019906:Nuclear Magnetic Resonance, Biomolecular; D010084:Oxidation-Reduction; D010777:Photochemistry; D010782:Photolysis; D011726:Pyridinium Compounds / Q000737:chemistry*; D012176:Retinoids / Q000737:chemistry*; D013057:Spectrum Analysis
C112040:A2-E (N-retinylidene-N-retinylethanolamine); D011726:Pyridinium Compounds; D012176:Retinoids