Substituted vinylsilyl ether

When the (Z)-enriched 2-substituted vinylsilyl ether 184 was investigated in the same radical cyclization, only one cyclization product 185 was obtained with a (2)-configured vinyl silane [71]. This may be accounted for by a rapid equilibration of the vinyl radical prior to cyclization. While reaction of the (Z)-isomer is favorable, in the corresponding (E)-isomer, interactions between the terminal hexyl substituent and the cyclohexane ring render cyclization much less efficient (Scheme 10-59). 

Scheme 10-59 Substituted vinylsilyl ether 184 reacts stereoselectively in a radical cyclization.

When 2-iodoalkyl vinylsilyl ethers 231 (Z=Si) were reacted with 224 in the presence of the Co(dppb)Cl2 catalyst, silatetrahydrofurans 232a (Z=Si) were isolated [294], When substrates 231 with substituted allyl groups (R3 and/or R4=alkyl) were treated with catalytic amounts of Co(dppe)Cl2 and 224 or phenylmagnesium bromide, a 5-exo cyclization proceeded and tetrahydrofurans 232c with exocyclic alkenyl units were isolated in 58-89% yield. For substrates with disubstituted alkene acceptors (R3=alkyl, R4=H), the formation of 18% of reduced tetrahydrofuran 232b was also observed [280, 295],... 

An efficient method for the formation of 1,2-oxasilines 95 was developed starting from the homoallylic alcohols 94 via protection of alcohols as vinylsilyl ethers, followed by ring-closing metathesis. Fluoride-assisted deprotection of oxasilines provides the 8-substituted homoallylic... 

---