NEW MATERIAL: A compound of formula I (R1 is aryl, substd. aryl, alkenyl, alkynyl or alkyl; R8 is H, ethoxyethyl, 2,2,2-trichloroethoxymethyl or other hydroxyl protecting group; R3 is R1).

EXAMPLE: Cis-2,4-diphenyl-5-(1-ethoxyethoxy)-4,5-dihydro-1,3-oxazin-6-one.

USE: Side chain precursors for the synthesis of taxol compounds useful as antitumor agents.

PROCESS: A β-lactam compound of formula II (R2 and R5 are each H, alkyl, alkenyl, aryl, etc.; R3 and R6 are each H, alkyl, aryl, etc.), is subjected to ring opening with potassium hydroxide and then cyclized in the presence of potassium tert.-butoxide and methanesulfonyl chloride to give the compound of formula I.