Herbicidal thiadiazine dioxides

A number of 1 /7-pyrido[2,3-c]-l,2,6-thiadiazin-4(3/7)-one 2,2-dioxides and 1 //-pyrido[4,3-c]-1,2,6-thiadiazin-4(3//)-one 2,2-dioxides, for example (403), display herbicidal properties <75USP392064i, 77USP4014888). Four 3-arylpyrido[3,4-e]-1,2,4-triazines (404a-c) and (252) have been studied as germination inhibitors <84Mi 717-03). 

In Section II, the chemistry of 1,2,6-thiadiazine 1,1-dioxides and fused systems will be described, but no attempt has been made to cover all the literature dealing with benzothiadiazinones. Numerous derivatives related to the herbicide bentazone (3-isopropyI-lH-2,l,3-benzothiadiazin-4-one 2,2-dioxide) have been prepared and are described in various patents, which will not be included. The reactivity of bentazone and related 2,1,3-benzo-thiadiazinones has also been extensively studied (81AG(E)151). 

Special mention should be made of bentazone, 3-isopropyl-l//-2,l,3-benzothiadiazin-4-one 2,2-dioxide (74) (Section II,C,2,b) (66GEP1542838). Basagran, the biologically active form of bentazone, is nowadays employed extensively as a selective postemergence herbicide in numerous crops. As already mentioned (Section II,C,2,b), a wide variety of thiadiazines fused to heterocyclic systems have been synthesized in the search for herbicides and success is frequently claimed in the patent literature. 

The chemistry of 1,2,6-thiadiazine 1,1-dioxides, and other heterocycles containing the sulfamide (—NHSO2NH—) fragment, have been reviewed by Spanish workers who have been extremely active in this field <88AHC(44)81>. 1,2,6-Thiadiazines, and their benzologues, other than the S,S-dioxides have also been reviewed <90AHC(50)255>. The herbicidal activity of 3-isopropyl-1/7-2,1,3-benzothiadiazin-4(3//)-one 2,2-dioxide (Bentazone 6, R = CHMej), an important commercial product has been appraised <89Cl(M)30>. 

The O-allyl oxime (302), prepared from 2,6-dimethyl-4-propionyl-4//-l,2,6-thiadiazine-3,5 (2//,6//)-dione 1,1-dioxide (Section 6.16.6.3.2) has pronounced herbicidal activity <89BRP22I3145>. 

The herbicide bentazon (3-isopropyl-2,l,3-benzo-thiadiazin-4-one-2,2-dioxide) is efficiently converted to give near quantitative yield of sulphur as sulphate ion and carbon as carbon dioxide dissolved and particulate organic carbon measurements also support nearly complete mineralization [35]. 

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