Synthesis of ninhydrin analogues

Ninhydrin is an important reagent which has applications not only in biochemistry and chemistry but also in forensic science as a latent fingerprint reagent. Ninhydrin reacts with minute amounts of amino acids present in fingerprint deposits to form the highly colored condensation product called Ruhemann's purple. Post-ninhydrin treatment with metal salts followed by illumination with an appropriate light source causes latent prints to luminesce. To further enhance the utility of this procedure, several ninhydrin analogues have been synthesized and evaluated. The goal of these structural modifications has been optimization of the chromophoric and luminescent properties of the Ruhemann's purple adduct. The "ideal" reagent must be highly sensitive and provide excellent background contrast. It should react to give a colored fingerprint which is also fluorescent without secondary treatment. Previous research has demonstrated that it is difficult to a priori predict which modifications will result in improved performance.; The goal of this project was to purposefully design novel ninhydrin analogues with enhanced properties and to develop efficient methodologies for their synthesis. We proposed and carried out syntheses of three types of ninhydrin analogues. The first type consisted of compounds formed by linking two ninhydrin moieties together. These "bisninhydrin" analogues have the potential to form polymeric Ruhemann's purple species. A different class of compounds have the presence of a sulfur atom as the common element and are patterned after similar sulfur-containing analogues which were shown to be highly effective. The third type of analogue relies on the extension of the conjugated system to enhance its properties. A variety of aryl ninhydrins were prepared by a versatile and highly convergent method employing a palladium-catalyzed cross-coupling reaction as the key step. All analogues were evaluated for efficacy by the United States Secret Service. The synthesis of the most promising reagent, 5-(2-thienyl)ninhydrin was optimized. Background information on ninhydrin, fingerprinting, and the synthesis of ninhydrin analogues is presented.