To obtain the subject new compound useful as a stabilizer for organic materials liable to receive oxidative, thermal or light-induced degradation reactions, especially as a stabilizer for processing synthetic polymers.

This compound of formula I (R1-R4 are ech H, a 1-25C alkyl, a halogen, egc.; R5, R6 are each H, a 1-25C alyl, phenyl, etc.), e.g. a compound of formula II. The compound is obtained by a new method comprising reacting a compound of formula III with phosphorus trichloride and subsequently hydrolyzing the product with water, without isolating the product. The reaction of formula III with the phosphorus trichloride is preferably carried out in a molar ratio of 1:10 to 1:1 at a temperature of 20-200°C in the absence of a solvent. The hydrolysis reaction is preferably carried out at 10-120°C. The reaction product of the compound of formula III with the phosphorus trichloride is a new compound and includes 6,8-di-tert-butyl-10-chloro-2,3,4,10-tetrahydro-1H-9- oxa-10-phosphenanthrene.