α‑Crotyl-α-difluoroboranyloxy-amides: Structure and Reactivity of Isolable Intermediates in Stereospecific α‑Ketol Rearrangements

The
stereospecific BF3-mediated α-ketol rearrangement
of β-hydroxy-α-ketoamides yields isolable 2-difluoroboranyloxy-3-keto-amides.
X-ray and NMR analysis reveal a carbonyl coordination of the boron
by the amide not the ketone. The boron complexes are air-stable solids,
can be purified by silica gel chromatography, and exhibit novel reactivity
in bromination and superior stereoselectivity in dipolar cycloaddition
reactions.

β-羟基-α-酮酰胺经立体专一性三氟化硼（BF3）介导的α-酮醇重排反应，可得到可分离的2-二氟硼烷氧基-3-酮酰胺。X射线（X-ray）与核磁共振（Nuclear Magnetic Resonance, NMR）分析结果显示，硼原子与酰胺的羰基发生配位，而非酮羰基。该硼配合物为空气稳定的固体，可通过硅胶柱层析进行纯化，在溴化反应中表现出新颖的反应活性，且在偶极环加成反应中具备优异的立体选择性。