NEW MATERIAL:A compound of formula I (R1 and R2 are H or the protecting group for the amino group; R3-R5 are H or the protecting group for OH) and a salt thereof.

EXAMPLE: 5-[(2R),(3R)-4-Benzyloxy-2,3-isopropylidenedioxy-trans-butylidene]-3-(4- methoxybenzyl)hydantoin.

USE: A synthetic intermediate for novel spirohydantoin compounds useful as herbicides and plant growth-regulating agents. The spirohydantoin compounds are readily synthesized in high yields.

PREPARATION: A compound of formula II is reacted with a compound of formula III in a solvent (preferably benzene) in the presence of an anion-generating reagent (preferably sodium ethoxide) at -50-150°C for 10min-15hr to provide the cis and trans isomers mixture of the compound of formula I.