Oxazine Ether Herbicides

Abstract

Benzylic ethers of oxazine-6-methanols are very effective grass herbicides which show safety to a number of major crops. We have found 2 different synthetic routes to these compounds. The first route is based on the reaction of nitroso-olefins with allylic alcohols. The second route relies on a novel reaction of oxime dianions with chloro-epoxides. We will describe our studies on the discovery and optimization of these syntheses and the biology of the oxazine ethers.Over the past 2 decades chemists from a number of companies have explored benzylic ethers as herbicides. This area began with the discovery that monocyclic oxygen heterocycles substituted with benzylic ethers had good herbicidal activity (1). Optimization of the chemistry at Shell provided the only commercial product in the area, the bicyclic ether Cinmethylin (2). The common key structural feature of active compounds was the 1,4-relationship of the oxygen of the heterocycle and the oxygen of the benzylic ether. More recently, scientists at BASF showed that nitrogen could also be present in the heterocycle with isoxazoline ethers (3). Cinmethylin ShellBayer BASF 0097-6156/95/0584-0197$12.00/0

Oxime chemistry in crop protection

“…This is demonstrated by the synthesis of the experimental herbicide 67, which belongs to the PPO inhibitors. The oximinoyl chloride 63 is converted under elimination of hydrochloric acid into the instable nitrile oxide 64…”

“…This highly reactive intermediate undergoes a hetero Diels-Alder reaction with methallyl alcohol to the oxazine derivative 73. Alkylation of its hydroxy group with 2,6-difluorobenzyl bromide delivers the experimental herbicide 74, which shows good control of A. myosuroides (blackgrass) and Digitaria sanguinalis (crabgrass) (Scheme 7) 64. 4.7 Cyclization of an oxime to a pyrrole Even pyrrole rings can be obtained by cyclization of oximes.…”