Photolabile base

For experimental cleavage procedures carried out with strong acids (e.g., liq HF, CF3SO3H) and high concentrations of TFA, see Section 4.3.5. The cleavage of peptides from photolabile-based resins is carried out as discussed above for acid handles, and from XAL and its A-alkyl-based resins as discussed for highly acid-labile resins. 

The crucial requirement of excited-state proton transfer (ESPT) is suggested by the failure of 1-naphthyl methyl ether to undergo self-nitrosation under similar photolysis conditions. The ESPT is further established by quenching of the photonitrosation as well as 1-naphthol fluorescence by general bases, such as water and triethylamine, with comparable quenching rate constants and quantum yield. ESPT shows the significance in relation to the requirement of acid in photolysis of nitrosamines and acid association is a photolabile species. 

Various materials have been examined for use as deep UV resists poly(methyl methacrylate) (PMMA) (1), poly(methyl isopropenyl ketone) (PMIPK) fS.7L and the novolak-Meldrum s acid solution inhibition system (S). Each however has a problem related to sensitivity and/or resolution. While PMMA is insensitive to light of X > 230 nm because of its weak absorption, its high resolution properties make it an attractive starting point for the design of a resist that will perform well in the 230-280 region. The photochemical properties of PMMA could be modified by the incorporation of a small percentage of photolabile groups so as to have both the desired sensitivity and base polymer properties. 

An interesting application of the 3-methoxybenzoin-based photolabile linker has been described in which the dithiane, ( )-(3- 2-(l,3-dithian-2-yl)-l-[(9-fluorenylmethoxy-carbonyl)oxy]-2-phenylethyl)benzyl)oxy] acetic acid (126, Scheme 33), a transiently masked form of the caging moiety, is incorporated into a peptide chain generated by solid-phase synthesis. 248 ... 

Photolabile linkers for amides are most often based on 2-nitrobenzyl derivatives (Table 3.10 for the preparation of these and similar linkers, see also [147,153,235]). The mechanism of photolysis is the same as for the related 2-nitrobenzyl alcohol linkers (Section 3.1.3). 

The stable alkylmetal(III) porphyrins are accessible from the hydrides via their corresponding bases (i.e. Rh(I) or Ir(I) porphyrins) by action of alkyl halides (paths 1, m) or from the respective chlorometal(III) porphyrins by action of lithium alkyls (path c,n). They form a very important class of compounds in view of their close relation to the alkylcobalt corrinoids. Of course, the Rh or Ir alkyl group is much more stable than the corresponding Co alkyl group which, in the case of the porphyrins, is photolabile. A variety of axial ligands have been added to the iridium alkyl porphyrins (path o). 

The ligand field theory of transition metal photochemistry is based on the idea that the bonding changes in excited electronic states are correlated with ligand photolabilization (11-13). Populating the dz2 or dx2 y2 orbitals increases sigma antibonding in the z and xy directions respectively. 

The photophysical properties of compounds 31—34 are presented in Table 7. Compared to the Pd-based materials, the Pt-based ones exhibit longer emission lifetimes. This is rationalized by the more stable Pt-Pt and Pt-L bonds compared to the more photolabile Pd-Pd and Pd-L ones. Therefore, the energy-wasting photo-induced cleavage (here bond cleavage process) does not occur or at least in a much less efficiency for the Pt-materials, thus reducing efficient nonradiative excited state deactivation. 

Although ethers have been used less frequently than alcohols, it has recently been reported that tetrahydrofuran (THF) is photocatalytically activated by TBADT, and the alkylation of unsaturated nitriles is obtained in good yield [15]. As an alternative, the C—Br bond in various glycosyl bromides has been homolytically cleaved, and the resulting radical trapped by acrylonitrile to form the corresponding C-glycosides. The halogen abstraction step is initiated by a photolabile iron-based dimeric metal complex [16]. 

Buehler, S., Lagoja, I., Giegrich, H., Stengele, K.-P. and Pfleiderer, W. (2004) New types of very efficient photolabile protecting groups based upon the [2-(2-nitrophenyl)propoxy]carbonyl (NPPOC)... 

For the automated synthesis of CS hexasaccharides, Peter Seeberger s group designed a photolabile nitrobenzyl ether-based linker to the Merrifield resin (Scheme 10.15) [31]. Their advanced strategy also differs by the use of monosaccharide donors for the... 

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