Phytophotodermatitis: bulla formation and hyperpigmentation during spring break.

Phytophotodermatitis is a phototoxic dermatologic reaction that occurs with exposure to ultraviolet light after contact with certain plant chemicals. Recognition is vital to proper management and avoidance of unnecessary distress for patients. Sun-sensitizing compounds, known as furocoumarins, are found in many plants (limes, celery, and natural grasses) and are excited by ultraviolet A (UVA) irradiation. UVA irradiation induces covalent linkages of nuclear DNA resulting in cutaneous photosensitivity and vesicular skin damage while increasing melanin production. Delayed erythema, hyperpigmentation, and vesicle or bullae formation are hallmarks. Literature review reveals most cases occur during or after vacation in a sunny destination with exposure to citrus fruit or in children playing outdoors in the summer when furocoumarins are abundant. Hyperpigmented lesions typically appear on the hands or around the mouth. Overall incidence is unknown and there appears to be no predilection for race, although typically more easily recognized in the fair skinned. Of concern are the many cases where misdiagnosis of child abuse has occurred with unnecessary legal action taken and emotional distress for innocent families. Phytophotodermatitis is relatively common and easily diagnosed with awareness and a careful history. Accurate diagnosis avoids unnecessary concern by patients and potential misdiagnosis of abuse. Consider phytophotodermatitis when hyperpigmentation in bizarre streaks on sun-exposed areas with vesicles in a nondermatomal distribution is present. Presented is an illustrative case.

Diphenylnitrenium ion: cyclization, electron transfer, and polymerization reactions.

Reactions of diphenylnitrenium ion were examined using laser flash photolysis (LFP), product analysis, and computational modeling using density functional theory (DFT). In the absence of trapping agents, diphenylnitrenium ion cyclizes to form carbazole. On the basis of laser flash photolysis experiments and DFT calculations it is argued that this process is a concerted cyclization/proton transfer that forms the H-4a tautomer of carbazole. Additional LFP experiments and product studies show that diphenylnitrenium ion reacts with electron-rich arenes (e.g., N,N-dimethylaniline, diphenylamine, and carbazole) through an initial one-electron transfer. The radical intermediates formed in this step then couple to form dimeric products. Secondary reactions between the diphenylnitrenium ion and these dimers results in the formation of oligomeric materials.

Photogenerated N-methyl-N-1-naphthylnitrenium ion: laser flash photolysis, trapping rates, and product study.

N-Methyl-N-1-naphthylnitrenium ion (2) was generated through photolysis of 1-(N-methyl-N-(1-naphthyl)amino)-2,4,6-trimethylpyridinium tetrafluoroborate (1). Laser flash photolysis (LFP) with time-resolved UV-vis (TRUV) detection as well as photoproduct analysis verified that the expected nitrenium ion was formed cleanly and rapidly following photolysis. Consistent with an earlier study, which used competitive trapping methods (Novak, M. et al. J. Org. Chem. 1999, 64, 6023-6031), it is found that 2 reacts rapidly with a variety of nucleophiles. The high reactivity of 2 relative to other arylnitrenium ions is discussed in terms of steric and electronic effects.

Photochemical generation of nitrenium ions from protonated 1,1-diarylhydrazines.

[reaction: see text] Laser flash photolysis experiments, chemical trapping studies, and time-dependent density functional theory calculations demonstrate that photolysis of protonated 1,1-diarylhydrazines generates N,N-diarylnitrenium ions.

Photochemically generated arylnitrenium ions: substituent effects on reactivity studied by laser flash photolysis.

Laser flash photolysis is used to examine reaction rates of four arylnitrenium ions: N-methyl-N-(4-chlorophenyl)-, -(4-biphenylyl)-, -(4-methylphenyl)-, and -(4-methoxyphenyl)nitrenium ion. These intermediates are generated from photolysis of appropriately substituted N-aminopyridinium ions. Kinetic analysis of trapping rates shows that the nitrenium ions react with weak nucleophiles in the order 4-Cl approximately 4-Me > 4-Ph > 4-OMe. Generally, amines react with the arylnitrenium ions at or near the diffusion-limited rate. Only in cases where the least reactive nitrenium ion (4-OMe) reacts with sterically hindered amines is any deviation from this generalization observed. The addition of a 2-acetyl substituent to these structures produces an interesting effect. In the case of the 4-Cl and 4-Ph analogs, the electron-withdrawing acetyl group decreases the rate of nucleophilic addition, yet with the 4-OMe and 4-Me compounds, the acetyl group increases the rate of addition. These results are discussed in terms of competing electronic and steric effects.