Catalysis, continued rearrangements

Subsequent to the discovery of skeletal rearrangement reactions on plati-num/charcoal catalysts, the reality of platinum-only catalysis for reactions of this sort was reinforced with the observation of the isomerization of C4 and C5 aliphatic hydrocarbons over thick continuous evaporated platinum films (68,108, 24). As we have seen from the discussion of film structure in previous sections, films of this sort offer negligible access of gas to the substrate beneath. Furthermore, these reactions were often carried out under conditions where no glass, other than that covered by platinum film, was heated to reaction temperature that is, there was essentially no surface other than platinum available at reaction temperature. Studies have also been carried out (109, 110) using platinum/silica catalysts in which the silica is catalytically inert, and the reaction is undoubted confined to the platinum surface. 

Heterogeneous solid-acid catalysis has the potential to make a major contribution towards improving the environmental acceptability of terpene rearrangement and isomerization processes. There are potential heterogeneously catalyzed replacement processes for the production of campholenic aldehyde, an important intermediate for many fragrance compounds. With continuing advances in the field of solid-acid catalysis it is likely that other industrially useful heterogeneous catalysts will be discovered. 

Several reagents have been shown to convert 2-hydroxyaryloximes into benzoxazoles through a Beckmann rearrangement. Zeolite catalysis in a continuous flow reactor has been shown to be successful in benzoxazole formation,as has diethyl chlorophosphate and POCI3. Rindhe and coworkers utilized POCI3 in the formation of pyrazolyl benzoxazoles for use as antimicrobial agents. 

New synthetic methods for benzodiazepine synthesis involving Ugi-type multicomponent/post-Ugi cyclization reactions continue to be of interest. Ugi reactions of indole-2-carboxaldehydes, isocyanides, amines, and 2-iodobenzoic acid derivatives led to intermediates which, with copper(I) catalysis, underwent intramolecular indole N-arylation to produce indolo-fused benzodiazepinones, such as 134 (13CC2894). 2-Azido-benzaldehyde, isocyanides, propargylamines, and nitrophenols underwent Ugi-type reaction, Smiles-type rearrangement, and intramolecular azide-alkyne cyclization to afford triazolo-fused benzodiazepinones such as 135... 

The first general method of transforming a penicillin into a cephalosporin, the acid-catalysed rearrangement of penicillin sulphoxides, continues to receive attention. That this rearrangement proceeds via a sulphenic acid derivative has been further confirmed by isolation of the crystalline sulphenic acid (74 f, y) from the thermal rearrangement of the penicillin sulphoxide (27 f, y). This intermediate slowly reverts to the penicillin at 38 0, and, on treatment with methanesulphonic acid in dimethylacetamide, cyclizes to cephalosporin (75 f, y). Trapping of (74) by oxidation to the sulphinyl chloride (76 f, y) has been reported cyclization to a cephalosporin sulphoxide was accomplished under base catalysis/ Treatment of penicillin sulphoxides with azo-compounds also effects rearrangement to... 

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