Assisted cleavage

Release by Silver-Assisted Cleavag e. A soluble silver complex formed imagewise in the undeveloped areas of the silver haUde layer may be used to effect a cleavage reaction that releases a dye or a dye precursor. The process yields positive dye transfer images directiy with negative-working emulsions (46). An example is the silver-assisted cleavage of a dye-substituted thiazolidine compound, as shown in equation 7. 

Fig. 14. Schematic cross section of Spectra integral film. The 600 Plus film has a similar stmcture. In these films the yellow image is formed by silver-assisted cleavage of a yellow dye releaser. A colorless developer reduces exposed silver hahde in the blue-sensitive emulsion in unexposed areas dissolved silver diffuses to the dye releaser layer and triggers the release of the yellow image dye.

The following derivatives represent protective groups that contain an auxiliary functionality, which when chemically modified, results in intramolecular, assisted cleavage, thus increasing the rate of cleavage over simple basic hydrolysis. 

Scheme 12.9 Microwave-assisted cleavage of benzoic acid from Merrifield resin.

Iodine-assisted cleavage of the vicinal diol group of dextran has been investigated [77]. Dialdehyde starch (63) can... 

There are several exceptions to these generalizations which depend on the presence of particular substituents, in that the substituents, or intermediates derived from them, have a destabilizing effect on the ring system. T3rpical reactions of monocyclic triazoles are described first, and the exceptional systems, involving substituent-assisted cleavage, are then discussed. 

McMurray [151] has described the acid-assisted cleavage of the N]-C4 bond in trans 4-hydroxyphenyl p-lactams. The ring opening reaction may proceed with concomitant reduction or formation of carbon-carbon coupling products, as a function of the reagent employed. For instance, Scheme 60, treatment of 196 with 4 equivalents of triethylsilane in neat trifluoroacetic acid led to compound 197. On the contrary, treatment with anisole in trifluoroacetic acid led to compound 198. Unfortunately, no data are provided by authors regarding process yield or final diastereomeric ratio. 

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