Abstract Phosphorodichloridites with sterically demanding O-aryl groups are known to undergo controlled partial hydrolysis to give 1,3,5,2,4,6-trioxatriphosphorinane derivatives, but the O-(1-adamantyl) and O-neopentyl groups failed to provide control of the hydrolysis and the cyclic products could not be obtained. The reaction of the O-aryl phosphorodichloridites or of diisopropylaminodichlorophosphine with silver oxide or lithium oxide was found to be useful for the synthesis of the trioxatriphosphorinanes, but again this reaction failed with the O-alkyl phosphorodichloridites. Some new observations on the reactivity of the ring system included: oxidation by pyridine N-oxide, sodium periodate or ozone, forming a water-sensitive trioxide; addition of sulfur in pyridine; addition of tetrachloro-1,2-quinone to form a phosphorane which with water gave a 1,3,2-dioxaphospholane oxide derivative; attack by ethanol which opened the ring and gave, surprisingly, ArO–PH(O)OH as the product rather than an O-ethyl c...