Unexpected Propargylic Retro-Brook Rearrangements in Alkynes

1,2-Silyl Shift-Induced Heterocyclization of Propargyl Silanes: Synthesis of Five-Membered Heterocycles Containing a Functionalized Olefin Side Chain

Propargyl silanes with a terminal alkyne moiety undergo a 1,2-silyl shift when activated with electrophiles such as H+, Br+, I+, and PhSe+. A method was developed to trap 1,3-transposed electrophilic centers with various internal O-, N-, and S-nucleophiles in a 5-exo manner. This synthetic procedure provided five-membered heterocycles containing a trisubstituted olefin side chain. The scope of the method includes access to tetrahydrofuran, γ-butyrolactone, 2-isooxazoline, pyrrolidine, and thiolane derivatives in yields ranging from 25 to 85% (23 examples in total). Reactions with TsNBr2 ensured complete (E)-selectivity of the newly formed olefins. Further functionalization of the obtained 1-trialkylsilyl-2-bromovinyl side chain was demonstrated by double-bond geometry-preserving electrophilic substitution and cross-coupling reactions that provided heterocycles with a trisubstituted vinyl moiety.

Synthesis of 4-(tert-Butyldimethylsilyl)-4,5,6,7-tetrahydro-[1,2,3]triazolo[1,5-a]pyridine

SN2 rection between 4-(tert-butyldimethylsilyl)hex-5-yn-1-yl 4-methylbenzenesulfonate and NaN3 in DMF at 80 °C provided (6-azidohex-1-yn-3-yl)(tert-butyl)dimethylsilane intermediate, which underwent in situ intramolecular thermal Huisgen azide–alkyne cycloaddition reaction. This one-pot process gave 4-(tert-butyldimethylsilyl)-4,5,6,7-tetrahydro-[1,2,3]triazolo[1,5-a]pyridine in 78% yield.