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Photolabile Protecting Groups for Nucleosides: Mechanistic Studies of the 2‐(2‐Nitrophenyl)ethyl Group
Author(s) -
Walbert Stefan,
Pfleiderer Wolfgang,
Steiner Ulrich E.
Publication year - 2001
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/1522-2675(20010613)84:6<1601::aid-hlca1601>3.0.co;2-s
Subject(s) - chemistry , nitro , moiety , dissociation (chemistry) , medicinal chemistry , flash photolysis , nitrosobenzene , photodissociation , photochemistry , leaving group , stereochemistry , organic chemistry , reaction rate constant , kinetics , alkyl , physics , quantum mechanics , catalysis
Abstract The photochemistry of several 2‐(2‐nitrophenyl)ethyl‐caged compounds including caged thymidine nucleosides was studied by nanosecond laser flash photolysis and stationary illumination experiments with quantitative HPLC analysis for quantum yields and product distribution. Effects of solvent basicity and acidity were investigated by varying the H 2 O content and HCl concentration, respectively, in MeCN/H 2 O mixtures. For all compounds 1 – 7 investigated, intramolecular H abstraction by the nitro group from the exocyclic α ‐position with respect to the aryl moiety was found to be the primary process. The protolytic dissociation equilibrium of the resulting aci ‐nitro compound was kinetically characterized in the 0.1 – 10 μs time region. In general, two reaction channels compete for the aci ‐nitro compound and its anion: β ‐elimination of the caged compound occurs from the anion, while from the undissociated aci ‐nitro compound, a nitrosobenzene derivative is formed with no release of the caged compound. The yield ratio of these two reaction channels can be controlled through shifts in the protolytic dissociation equilibrium of the aci ‐nitro compound. In solutions with either low basicity (H 2 O‐free MeCN) or high acidity (higher concentration of HCl in H 2 O/MeCN), two as yet unidentified products are formed, each one specifically for one of the mentioned conditions.