The present invention relates to a process for preparing a compound of the formula (II):wherein R1b is a hydroxy-protecting group, R2 is optionally substituted lower alkyl, and R3 is optionally substituted lower alkyl, optionally substituted cycloalkyl, substituted or non-substituted aryl or substituted or non-substituted heteroaryl, characterized by reacting a compound of the formula (III):wherein R1b and R2 are the same as defined above, in the presence or absence of an acid with a compound of the formula: R3-N=C=O , wherein R3 is the same as defined above, or with a compound of the formula: R3-NH-C(=O)-X, wherein R3 is the same as defined above and X is a leaving group.
