Investigations on phosphoric acid and thiophosphoric acid esters with a heterocyclic substituent, 10thand last communication. Aza‐analogy II: Derivatives of oxazolo[4,5‐b]‐pyridin‐2(3H)‐one, an aza‐analogue of benzoxazol‐2(3H)‐one.Oxazolo‐[4,5–b]pyridin‐2(3H)‐one (4), one of the possible aza‐analogues of benzoxazol‐2(3H)‐one (3), is readily prepared starting from furfural (8), using theClauson‐Kaas transformation(oxydative transformation of furans to pyridines with simultaneous incorporation of a nitrogen atom) and proceeding either (1)viafurfurylamine (9), 3‐hydroxy‐pyridine (10), 2‐nitro‐ and 2‐amino‐3‐hydroxy‐pyridine (11) followed by ring closure with phosgene or, on shorter routes, directly (2)via2‐amino‐3‐hydroxy‐pyridine (11) or (3)via∝‐methylamino‐(fur‐2‐yl)‐acetic acid amide (12) and 3‐hydroxypicolinic acid amide (13) followed by aHofmann degradation. Halogenation and nitration of4lead to the 6‐substituted derivatives15–17.A series of thiophosphoric and thiophosphonic acid esters22–24with high insecticidal, acaricidal and anthelminthic activities and with extremely low mammalian toxicities up to>5000 mg/kg (ratp.o.) has been disclosedviathe N‐hydroxymethyl and N‐chloromethyl‐derivatives21of the oxazolopyridin‐ones4,15,16and17and the thione14. These esters are aza‐analogues of the known benzoxazolone‐insecticide type1and present really divergent properties.The vinyl compounds25are easily accessible from the oxazolopyridin‐ones with vinyl acetate. Addition of halogen to form the 3‐(1,2‐dihaloethyl)‐derivatives26, however, occasionally is troublesome: 3‐(2‐halovinyl)‐derivatives27formed as inherent by‐products by dehydrohalogenation handicap further reactions. Some thiophosphoric acid esters28and29derived from the vinyl compounds25and the dihaloethyl compounds26resp. are active pesticides too, but their mammalian toxicity ranges from 20–200 mg/kg (ratp.o.), values generally regular in the phosphoric ester field.The one‐step condensation of a hetercyclic NH‐compound, formaldehyde and a thiophosphoric compound (Stäubli procedure) is not applicable to the preparation of the esters22because the acidity of the oxazolopyridin‐ones (e.g.4and15) is too high to fulfill the basic requirements of this procedure. An alternative procedure runningviatheMannichcompounds31and32fails due to salt formation betweenMannichbase and thiophosphoric acid.Ring‐homologues such as the benzoxazinone derivatives37and the pyrido‐oxazinone derivatives40are not pestic