A compound of formula or a stereoisomer thereof: wherein, R 1 is hydrogen or an amino protecting group; R 2 is an amino protecting group; R 3 is selected from the group consisting of hydrogen, C 1-6 alkyl and an amino protecting group; the amino protecting group is preferably selected from the group consisting of alkoxycarbonyl amino protecting group, acyl amino protecting group, sulfonyl amino protecting group and alkyl amino protecting group; the alkoxycarbonyl amino protecting group is selected from the group consisting of benzyloxycarbonyl, tert -butoxycarbonyl, fluorenylmethoxycarbonyl, allyloxycarbonyl, trimethylsilylethoxycarbonyl, methoxycarbonyl and ethoxycarbonyl; the acyl amino protecting group is selected from the group consisting of phthalyl, trifluoroacetyl, pivaloyl, benzoyl, formyl and acetyl; the sulfonyl amino protecting group is selected from the group consisting of p -toluenesulfonyl, o-nitrobenzenesulfonyl and p-nitrobenzenesulfonyl; and the alkyl amino protecting group is selected from the group consisting of trityl, 2,4-dimethoxybenzyl, p -methoxybenzyl and benzyl; preferably, wherein, R 1 is an alkoxycarbonyl amino protecting group, and the alkoxycarbonyl amino protecting group is preferably selected from the group consisting of benzyloxycarbonyl, tert -butoxycarbonyl, fluorenylmethoxycarbonyl and allyloxycarbonyl, and more preferably tert -butoxycarbonyl; R 2 is a sulfonyl amino protecting group, and the sulfonyl amino protecting group is preferably selected from the group consisting of p -toluenesulfonyl, o-nitrobenzenesulfonyl and p -nitrobenzenesulfonyl, and more preferably p -toluenesulfonyl; and R 3 is selected from the group consisting of hydrogen and methyl; more preferably, the compound of formula has a structure of formula (Ia), (Ib) or (Ic) below, R 1 is hydrogen or an amino protecting group; R 2 is an amino protecting group; R 3 is selected from the group consisting of hydrogen, C 1-6 alkyl and an amino protecting group; the amino protecting group is preferably selected from the group consisting of alkoxycarbonyl amino protecting group, acyl amino protecting group, sulfonyl amino protecting group and alkyl amino protecting group; the alkoxycarbonyl amino protecting group is selected from the group consisting of benzyloxycarbonyl, tert -butoxycarbonyl, fluorenylmethoxycarbonyl, allyloxycarbonyl, trimethylsilylethoxycarbonyl, methoxycarbonyl and ethoxycarbonyl; the acyl amino protecting group is selected from the group consisting of phthalyl, trifluoroacetyl, pivaloyl, benzoyl, formyl and acetyl; the sulfonyl amino protecting group is selected from the group consisting of p -toluenesulfonyl, o-nitrobenzenesulfonyl and p-nitrobenzenesulfonyl; and the alkyl amino protecting group is selected from the group consisting of trityl, 2,4-dimethoxybenzyl, p -methoxybenzyl and benzyl; preferably, wherein, R 1 is an alkoxycarbonyl amino protecting group, and the alkoxycarbonyl amino protecting group is preferably selected from the group consisting of benzyloxycarbonyl, tert -butoxycarbonyl, fluorenylmethoxycarbonyl and allyloxycarbonyl, and more preferably tert -butoxycarbonyl; R 2 is a sulfonyl amino protecting group, and the sulfonyl amino protecting group is preferably selected from the group consisting of p -toluenesulfonyl, o-nitrobenzenesulfonyl and p -nitrobenzenesulfonyl, and more preferably p -toluenesulfonyl; and R 3 is selected from the group consisting of hydrogen and methyl; more preferably, the compound of formula has a structure of formula (Ia), (Ib) or (Ic) below,