Asymmetrical triazines

The formal derivation of the analogs, described in the foregoing, represents, from the point of view of systematic organic chemistry, a shift to the derivatives of other heterocyclic systems. In the case of pyrimidine aza analogs we arc dealing with derivatives of symmetrical or asymmetrical triazine in the case of purine aza analogs, the derivatives produced are those of imidazo[4,5]-i -triazine and z -triazolo [4,5-d] pyrimidine. 

These derivatives of asymmetric triazine are rather distantly related to the group of substances reviewed here. Only those of them which are prepared from the dioxo or dithioxo derivatives will be mentioned. 

Building on these early developments, research in triazine chemistry has continued to result in the discovery of new compounds, including asymmetrical triazine herbicides and other classes of chemistry containing the triazine ring structure that are important for weed control around the world. 

The triazine herbicides can be divided into four different structural classes chlorotriazines, methylthiotriazines, methoxytriazines, and atypical or asymmetrical triazines. The chlorotriazine group includes atrazine, simazine, pro-pazine, terbuthylazine, and cyanazine. The methylthiotriazine group includes ametryn, prometryn, and terbutryn. The methoxytriazine group will include prometon and secbumeton. Hexazinone and metribuzin were chosen to represent the atypical triazine group. The plant metabolism of the most researched member of each triazine group will be discussed in detail to cover all major biological and chemical transformations reported in the literature. 

The pKa of an organic base is the pH in an aqueous system, where half of the compound is present in the neutral form and half in the protonated form. Substitutions in the 2 position greatly influence the basicity of the -triazines (Weber, 1967). The chloro-v-triazincs are very weak bases with pKa values between 1.6 and 1.9. The mcthoxy-v-triazines and mcthylthio-v-triazincs have pKa values between 4.0 and 4.8, and the hydroxy-v-triazincs have pKa values greater than 5.0. In soil solutions (pH 4.5 to 8.0), the chioro-. -triazines are overwhelmingly present in their neutral forms. The methoxy-5-triazines, mcthylthio-v-triazines, and hydroxy-y-triazmes, however, are present as neutral species in neutral and alkaline soil solutions, but may be present as both neutral and protonated species in acidic soil solutions. This chapter also includes data on metribuzin, which is an asymmetrical triazine herbicide having somewhat different soil properties. 

Pape, B.E. and M.J. Zabik (1972). Photochemistry of bioactive compounds Solution-phase photochemistry of asymmetric triazin-5(4//)-ones. J. Agric. Food Chem., 20 72-75. 

Hazard Assessment for Selected Symmetrical and Asymmetrical Triazine Herbicides... 

The thiomethyl- and mcthoxy-v-triazines, as well as the asymmetrical triazine metribuzin, were not carcinogenic -some even in the SD rat - and at feeding levels exceeding the MTD the exception was terbutryn, where an increased incidence of mammary, thyroid, and liver tumors were observed in female SD rats at feeding levels that exceeded the MTD. 

A majority of the triazine herbicides with food uses are grouped into three chemical classes chloro-y-triazines, methylthio-y-triazincs, and asymmetric triazines. A fourth chemical class, mcthoxy-.v-triazincs, has been commercialized, but no food crop uses are registered within the United States. The NOAEL and corresponding reference dose for some of the chloro-.v-triazincs, mcthylthio-y-triazines, and an asymmetric triazine are presented in Table 27.1. 

Metribuzin is an asymmetrical triazine introduced by Bayer in 1971 (Fig. 4). Metribuzin is used alone or in combination with other herbicides in soybeans, sugarcane, and potatoes. 

Theoretical investigations of Domow et al. (1964) supplied the background for the discovery of this new herbicide group with asymmetric triazine skeleton. A reaction was elaborated which permitted the synthesis of l,2,4-triazin-3-ones, substituted by SR and NHj in position 3 and by NHj in position 4. 

The electrochemistry of triazines is well documented.Polak and Volke described a dpp procedure for the determination of symmetrical and asymmetrical triazines (desmetryne, methoprotryne, prometryne, aziprotryne, terbutryne, atrazine, cyanazine, simazine, terbuty-lazine, metribuzine) in water. The herbicides are extracted into chloroform from the sample at pH 8 to 9. The detection limit in 0.02 M H2SO4 or acetic acid buffered solution at pH 2.5 is given as 10 M. 

The asymmetrical-triazine herbicide metribuzin was found by Arvik et al. (1973) to prevent growth of five common soil algae in liquid culture when used at a concentration of 0.5 ppm (Chlorella vulgaris and Chlamydomonas) and one ppm (Chlorococcum, Anabaena and Schizo-thrix calcicola),... 

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