Bisulfite ion adds across the 5, 6-double bond of pyrimidine nucleosides to give the 5, 6-dihydro-6 sulfonate derivatives. Under mild conditions, cytidine-bisulfite adduct is readily hydrolyzed to the corresponding uridine derivative. On the other hand, free radicals, produced when bisulfite ion is auto-oxidized in the presence of oxygen, react with 4-thiouridine, isopentenyladenosine and some other pyrimidine nucleosides. These reactions and their applications to polynucleotides are reviewed.
Some substiuents may influence a reaction by stabilizing a transition state or intermediate by becoming bonded or partially bonded with or without reagent to the reaction center. This behavior is called neighboring group effects. If the intermediate is stabilized in this way, the reaction rates are increased Therefore, the effects play an important role in producing regio-and stereoselectivity of a reaction. This review is described on the neighboring group effects in the field of natural product chemistry, especially steroids, terpeues, alkaloids and acetogenins.
Facile preparations of 4-substituted-2, 6-piperazinediones from N-substituted-iminodiacetic acids by the ring closure were studied. The crude piperazinediones were obtained by extraction of the reaction mixture with an appropriate solvent and purified by recrystallization. Isolation of the piperazinediones by sublimation, however, was found to be troublesome. 4-Phenyl- and 4-benzyl--2, 6-piperazinediones were prepared in satisfactory yields.
Improved methods for the preparation of 1, 3-dicyanoguanidine (the reaction of dicyandiamide with cyanogen bromide using DMF as solvent) and 1, 2, 3-tricyanoguanidine (the reaction of 1, 3-dicyano-2-methylisothiourea or dimethyl N-cyanodithioimidocarbonate with cyanamide) were described. Some 1-substituted 2, 3-dicyanoguanidines were also synthesised by the reaction of 1-substituted 3-cyano-2-methylisothiourea with cyanamide in the presence of alkali.