Photolabile groups

Benorylate (315) [4 -(acetamido)phenyl-2-acetoxybenzoate] is another example. It is the ester between two well-known antiinflamatory drugs, aspirin and paracetamol, and is employed in rheumatoid arthritis therapy. In view of the chemical structure with three photolabile groups (two esters and one amide), its possible phototoxicity has been investigated. From the preparative irradiations, it has been concluded that the PFR takes place with breaking of the central C—O bond to yield 5-acetamido-2 -acetoxy-2-hydroxybenzophenone (316). This product undergoes transacetylation to 5 -acetamido-2 -acetoxy-2-hydroxy-benzophenone (318) (Scheme 80) [300]. 

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. 

In general, group X in the prepolymer could be a number of possible photolabile groups. Benzoin alkylethers are usually superior to other photosensitizers studied for the photopolymerization of allylamines Therefore, prepolymers are prepared in which group X is derived from an unsaturated benzoin derivative of which several suitable structures are shown ... 

Protection of the same hydroxyl with a photolabile group as in 329 (Scheme 4.98) ensured its removal upon irradiation under neutral conditions. In this experiment the formation of the phenolic derivative 331 was ascertained by NMR data. Epoxide ring opening in the latter followed by the usual set of reactions proceeded readily upon exposure of 331 in THE solution to nucleophiles like EtOH, EtSH, or n-PrNH2 and yielded the respective final adducts 332a-c. ... 

Different vinyl monomers have also been reported to give block copolymers, in combination with other bifiinctional initiators, provided that the two photolabile groups differ sufficiently in the selective absorption of the light to jwrmit a two-step synthesis. Indeed, an azo-oligoperoxyester has been iM-epared [55] by reaction of hydrogen peroxide with 4,4 -azobis-(4-cyanopentanoic acid chloride) ... 

Our polymer requires a reaction between a carboxylic acid or amide and a molecule containing a photolabile group. Candidates considered for that molecule included glycldyl methacrylate 1, acryloyl chloride 2, and isocyanotoethyl methacrylate (IEM) 3. ... 

Most of the known photochemical procedures for the synthesis of block and graft copolymers are based on the modification of already existing polymers with photolabile groups incorporated at defined positions, i.e. at the chain end, at side chains, or in the main chain (see Chart 11.13) [84]. 

Upon absorption of light, the photolabile groups can dissociate into pairs of free radicals capable of initiating the polymerization of a monomer present in the system (see Scheme 11.5). 

Chart n.l3 General structures of polymers bearing photolabile groups at the chain end (a), at side chains (b), or incorporated into the main chain (c). 

Table 11.4 Photolabile groups, chemically incorporated into linear polymers at in-chain, lateral or terminal positions, giving rise to the formation of reactive free radicals [84].

Molecule 7 (Fig. 13.15). A trivial extension of the approach suggested above for the synthesis of 6 leads to a putative route to 7, illustrated in Fig. 13.15 using specifically iodide as the photolabile group. Tetraiodopyramidane 27 is shown forming... 

---