2-Nitrobenzyl ester

The preparation of the phenacyl-, />-bromophenacyl or />-nitrobenzyl esters of sulphonic acids is usually unsatisfactory. 

Carboxylic Acids, Benzylthiouronium salts (p. 349), Phenacyl esters (p. 349), /(-Bromophenacyl esters (p. 349), N Benz lamides (p. 350), -Nitrobenzyl esters (p. 350). 

The o-nitrobenzyl ester, used in this example to protect penicillin precursors, can be cleaved by irradiation (H20/dioxane, pH 7). Reductive cleavage of benzyl or p-nitrobenzyl esters occurred in lower yields. 

With dichloroethane as solvent, approximately fourth-order kinetics were obtained, the order in aromatic being approximately one. The rates of reaction of 4-chlorobenzyl benzenesulphonate with mesitylene and pentamethylbenzene at 50 °C were 0.12 and 1.1 respectively, and for reaction of the 3-nitrobenzyl ester with pentamethylbenzene, 0.009 (at 70 °C) and 0.012 (at 80 °C) these results correspond to the previously observed orders of reactivity. 

N-[N-(benzyloxycarbonyl)-L-aspart-l-oyl-(L-alanyl-L-threonine p-nitrobenzyl ester)-4-oyl]-... 

N-[N-(benzyloxycarbonyl)-L-aspart-l-oyl-(L-alanyl-L-threonyl-L-leucyl-L-alanine p-nitrobenzyl ester)-4-oyl]-N-[N-(benzyloxycarbonyl)-L-aspart-l-oyl-(L-alanyl-L-threonyl-L-leucyl-L-alanyl-L-serine p-nitrobenzyl ester)-4-oyl]-N-[N-(benzyloxycarbonyl)-L-aspart-1 -oyl-(glycine ethyl ester)-4-oyl -N-[N-(benzyloxycarbonyl)-L-aspart-l-oyl-(glycyl-L-serine methyl ester)-4-oyl]-... 

NITROBENZYL ACETATE (Acetic acid, />-nitrobenzyl ester)... 

While "conventional positive photoresists" are sensitive, high-resolution materials, they are essentially opaque to radiation below 300 nm. This has led researchers to examine alternate chemistry for deep-UV applications. Examples of deep-UV sensitive dissolution inhibitors include aliphatic diazoketones (61-64) and nitrobenzyl esters (65). Certain onium salts have also recently been shown to be effective inhibitors for phenolic resins (66). A novel e-beam sensitive dissolution inhibition resist was designed by Bowden, et al a (67) based on the use of a novolac resin with a poly(olefin sulfone) dissolution inhibitor. The aqueous, base-soluble novolac is rendered less soluble via addition of -10 wt % poly(2-methyl pentene-1 sulfone)(PMPS). Irradiation causes main chain scission of PMPS followed by depolymerization to volatile monomers (68). The dissolution inhibitor is thus effectively "vaporized", restoring solubility in aqueous base to the irradiated portions of the resist. Alternate resist systems based on this chemistry have also been reported (69,70). 

Recently, nonionic acid precursors based on nitrobenzyl ester photochemistry have been developed for chemically amplified resist processes (78-80). These ester based materials (Figure 8) exhibit a number of advantages over the onium salt systems. Specifically, the esters are easily synthesized, are soluble in a variety organic solvents, are nonionic in character, and contain no potential device contaminants such as arsenic or antimony. In addition, their absorption characteristics are well suited for deep-UV exposure. 

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