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19.7.1.1.1.2 Variation 2: From Epoxides and Oxetanes

DOI: 10.1055/sos-SD-019-00308

Suginome, M.; Ito, Y.Science of Synthesis, (200419450.

The ring-opening reaction of epoxides 12 with silyl cyanides can take two courses, nitrile formation or isocyanide formation (Scheme 9). The use of aluminum trichloride,[‌21‌] diethylaluminum chloride,[‌22‌] or potassium cyanide/18-crown-6[‌23‌] as promoters leads to epoxide ring opening with exclusive formation of nitriles. Moreover, titanium(IV) isopropoxide mediates the ring opening of the epoxide ring with preferential formation of nitriles over isocyanides.[‌24‌] In sharp contrast, the use of zinc(II) iodide as a Lewis acid promoter gives the corresponding β-siloxy isocyanide selectively (Table 1); an example is the synthesis of isocyanide 13 (entry 3).[‌25‌‌28‌] The ring-opening reaction is highly stereospecific: a perfect stereochemical inversion occurs at the epoxide carbon atom that undergoes the nucleophilic attack. Highly regioselective ring-opening reactions of substituted cyclohexene oxides are directed by neighboring hydroxy or protected hydroxy groups, leading to the formation of highly functionalized cyclic compounds (entries 10 and 11).[‌27‌] 7-Oxabicyclo[4.1.0]hepta-2,4-diene (1,3-cyclohexadiene epoxide) undergoes a highly regioselective ring opening under the reaction conditions (entry 12).[‌25‌] The products are readily converted into β-amino alcohols by hydrolysis. With regard to the substituents of the silyl groups of the silyl cyanides, the trimethylsilyl group allows a much faster reaction with epoxides than does the tert-butyldimethylsilyl group.

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