化学选择性Chemoselectivity

NaBH₄ 还原醛、酮而不还原酯、羧酸、硝基；LiAlH₄ 还原几乎所有羰基（化学选择性差）；吕歇条件（NaBH₄/CeCl₃）对烯酮只还原 C=O；MPV 还原保留 C=C；林德拉催化剂只把炔还原到烯；锌/盐酸选择性还原硝基。

NaBH₄ reduces aldehydes and ketones but not esters, acids or nitro groups; LiAlH₄ reduces almost every carbonyl (poor chemoselectivity); Luche (NaBH₄/CeCl₃) reduces only the C=O of enones; MPV leaves C=C intact; Lindlar stops at the alkene; Zn/HCl reduces nitro groups selectively.

利用官能团固有活性差异、选择温和或专一试剂、控制温度和当量；不得已时使用保护基（如缩醛保护羰基、硅醚保护醇、Boc 保护胺），但会增加步骤。“无保护基合成”是追求原子与步骤经济的目标之一。

Exploit intrinsic reactivity differences, choose mild or specific reagents, control temperature and stoichiometry; otherwise use protecting groups (acetals, silyl ethers, Boc), at the cost of extra steps. ‘Protecting-group-free synthesis’ is a goal of step economy.

生物正交反应（点击化学、施陶丁格连接）是极端的化学选择性：在细胞内大量官能团存在下只与特定基团反应。

Bioorthogonal reactions (click chemistry, Staudinger ligation) are extreme chemoselectivity: reacting only with designated groups amid the functional-group soup of a cell.