[Challenges in chemistry: color reactions, cycloadditions and cycloreversions]

New colour reactions for the identification of drug compounds have been devised. For example, butazolidine is oxidized with permanganate in basic medium to azobenzene and butyric acid; after reduction and coupling, the former is transformed to a red azodye. On nitration, gluthetimide gives the p-nitrophenyl derivative and a second nitro group is substituted at position 3. With hydroxylamine in basic medium, this o-dinitro compound reacts to furnish a violet colour when the dinitro derivative is reduced to the ortho-quinoidal dialkali metal salt. Dipolar cycloadditions of BNO, ANO and DPNI to norbornane- and cyclohexane-condensed 1,3-oxazine dipolarophiles have been utilized to synthesize adducts containing 1,2,4-oxadiazole and -triazole rings. The norbornene double dipolarophiles contain both C=N and C=C bonds; for steric reasons, ketene formed in situ adds primarily to the C=C. The addition of dipoles with high steric demands can lead to a change in conformation. By means of retro Diels-Alder reactions, hetero- mono-, bi- and tricyclic compounds may be synthesized. With bifunctional reagents, norbornene amino acids and hydrazides are transformed to condensed cyclic derivatives; when these are heated, cyclopentadiene is cleaved off to give mostly new target compounds.