8-Methoxypsoralen (8-MOP) which shows strong photosensitizing activity has two photochemical functional groups, furan double bond and pyrone double bond like other furocoumarin derivatives. Therefore 3,4-photocycloadducts (pyrone double bond), 4',5'-photocycloadducts (furan double bond) and 3,4-, 4',5'-biadducts can be anticipated when 8-MOP and pyrimidine bases are irradiated with long wavelength(320-380 nm) UV light.
The photocycloaddition reaction of 8-MOP to -electron rich olefin, tetramethylethylene (TME) and to -electron deficient olefin, fumaronitrile(FN), was studied as a model for photosensitizing 8-MOP-pyrimidine base photocycloaddition reaction.
The reaction was carried out in fluid methanol solution and frozen state of benzene solution and benzophenone sensitizer was used for comparison. 8-MOP-TME and 8-MOP-FN photoadducts was analyzed by TLC and isolated by preparative TLC and silica gel column chromatography respectively and purified by recrystallization in methanol. The structure of major photoadducts were characterized by their UV, IR, NMR, mass spectra and elemental analysis.
8-MOP-TME and 8-MOP-FN photoadducts were proved to be $C_4$-cycloadducts formed through the pyrone double bond. The same photoadducts were obtained in frozen state and benzophenone sensitization.
The fluoresence of 8-MOP was quenched by TME and FN giving linear Stern-Volmer plot and $K_q$'s of 7.27×$10^8$, 2.34×$10^9$1/mole/$\sec$. are obtained respectively. A singlet exciplex formation mechanism was proposed for the photocycloaddition reaction of 8-MOP to TME and FN. But the triplet mechanism can not be ruled out.
When the methanol solution of 8-MOP-TME photoadduct was irradiated with 254 nm light for 50 minutes, 8-MOP and TME was obtained as detected by UV ($\lambda_\max$=298 nm) and TLC even though very inefficient compared with photosplitting of 5,7-dimethoxycoumarin-TME $C_4$-cycloadduct. This can be an indirect evidence for the $C_4$-photocycloaddition reaction of 8-MOP to TME.
강한 광독성을 나타내는 8-MOP은 다른 푸로루마린 유도체들 처럼 푸란이중결합과 피론이중결합 2개의 광화학적 관능기를 가지고 있다. 따라서 3,4-광부가물과, 4',5' 광부가물 또는 이중부가물이, 8MOP과 피리미딘염기에 장파장의 자외선(320 ∼ 380nm)을 쪼일때, 예측될 수 있다.
본 실험에서는 photosensitizing 8-MOP - 피리미딘염기의 광고리화 부가반응의 모델로써, 8-MOP과 π-전자가 풍부한 올레핀인 TME와 π-전자가 부족한 올레핀인 FN 과의 반응을 시도하였다. 반응은 메탄올 용액과, 벤젠의 얼린상태에서 행하였고, 벤조페논 증감제를 사용하여서 반응시켜 비교하였다.
광반응생성물들은 preparative TLC와, 대롱크로마토그라피로 분리 하였으며 메탄올에서 재결정하여 정제하였다. 주생성물의 구조는 UV, IR, NMR, MS, 원소분석등으로 결정하였고, 그 구조는 8-MOP-TME, 8-MOP-FN 모두 피론이 중결합을 통한 $C_4$-고리화 광부가물임을 알았고, 얼려서 반응시킬때나, 벤조페논 증감제를 사용하여서도 같은 반응물을 얻었다.
TME, FN에 의한 8-MOP 형광？칭은 직선의 Stern-volmer plot을 주었고 그것으로 부터 속도 상수가 각각 7.27 × $10^8$, 2.34 × $10^9$ ℓ/mole·$\sec$ 임을 알았다.
8-MOP-TME, 8-MOP-FN의 광반응이 singlet exciplex 메카니즘을 통해서 일어날 수 있으나, 삼중상태 메카니즘도 배제 할 수 없다.
8-MOP-TME 광반응 생성물의 용액을 254nm의 빛을 쪼이면 8-MOP과 TME로 photosplitting됨을 ur와 TLC로 확인하였지만, DMC-TME의 경우처럼 효과적이지 못함을 알았다.
이것은 또한 8-MOP과 TME가 $C_4$-고리화 광부가반응을 일으켰다는 간접적인 증거가 된다.
