Thioketal connection unit, synthetic method and purpose thereof

The invention discloses a thioketal connection unit, a synthetic method and a purpose thereof. The thioketal connection unit has a structure shown as a formula (I) in the specification, wherein, R=-OH or -COOH; R1 and R2 can satisfy one of the following conditions: 1) R1=R2=methyl; or, 2) R1=phenyl, p-methoxyphenyl, 2,4,6-trimethoxyphenyl, naphthyl or p-methoxynaphthyl, R2=H; or, and 3) R1, R2 co-forms cyclohexyl or cyclopentyl. The unit is connected with nucleotide and fluorescein to obtain fluorescein-labeled nucleotide for DNA sequencing. Compared with the prior art, when a reversible terminator of the connection unit is used for DNA sequencing, extension efficiency is close to 100%, when a template is continuous several same bases, one reversible terminator is extended at one time; the raw materials required for synthesis are simple and easily available, the synthesis processes are conventional chemical reactions, and the method can be used for large-scale popularization and usage.