Applications of allenylsilanes in organic synthesis

Abstract

(A) Reactions with Aldehydes: Allenylsilanes react with aldehydes and ketones in the presence of titanium tetrachloride to provide homopropargylic alcohols in a regioselective manner. The reaction of chiral allenylsilanes with chiral aldehydes leads to the formation of mainly syn homopropargylic alcohols.3 The reaction of chiral 3,3- disubstituted allenylsilane 3 with paraformaldehyde in the presence of TiCl42THF generates chiral homopropargylic alcohol 4, which is the key intermediate in the total synthesis of ()-histrionicotoxin 5. (B) Reactions with Aldehydes: The reaction of chiral 2-silyl-substituted ?-allenic alcohol 7 with aldehyde 6 in the presence of InBr3 give rise to chiral homopropargylic alcohol 11. The reaction proceeds via formation of oxocarbenium ion 8, which undergoes a [3,3]-sigmatropic rearrangement to form the alcohol 11. The alcohol 11 is the key intermediate in the total synthesis of the natural product (+)-neopeltolide 12. (C) Reactions with Aldehydes: The addition of ?-trimethylsilyl allene esters 13 to aldehydes 14 in the presence of i-PrOLi leads to the formation of regiospecific ?-carbinols 18. The addition of anionic catalyst i-PrOLi leads to the intermediate 15, which possess enolatelike reactivity. The nucleophilic addition of intermediate 15 to aldehyde 14 generates intermediate 16. Then, the silyl group undergoes a 1,3-shift and the nucleophile eliminates to form the intermediate 17. This reaction is the key step in the total synthesis of the [3.2.1] bicyclic natural product vitisinol D. (D) Reactions with Imines: The enantioenriched allenylsilane 20 reacts with the in situ generated iminium ion generated from t-butyl carbamate 22 and aldehydes 21 in the presence of BF3OEt2 to form substituted 4,5-dihydropyrroles 24. Similarly, the reaction of allenylsilane 20 with an iminium ion, generated in situ from methyl carbamate 23 and aldehydes 21 in the presence of TMSOTf, forms substituted 4,5-dihyrooxazines 25. (E) Gold-Catalyzed Cycloisomerization: In the presence of AuCl3 the ?-silyl-substituted allenyl ketones 29 undergoes cycloisomerization to 3-silyl furans 32. The cyclization of the allenyl ketone 29 give rise to the intermediate gold-carbene 31, and upon the 1,2-Si shift, the 3-silyl furan 32 is produced. (F) The PausonKhand Reaction of 1,1-Disubstituted Allenylsilanes: The PausonKhand reaction of 1,1-disubstituted allenylsilanes with terminal alkynes leads to 4-alkylidene-2-cyclopenten-1-ones in good yields. The reaction proceeds through a [2+2+1] pathway. A three-membered iron metacycle is generated by reaction of allenylsilane 33 with diiron nonacarbonyl. The iron metacycle undergoes complexation with alkyne 34, and finally, a reductive elimination takes place to provide the 4-alkylidene-2-cyclopenten-1- one 35. (G) [2+2+2] Cycloaddition with Benzynes: Benzynes possess a strained triple bond and are highly electrophilic. Allenylsilanes 37 react with two equivalents of benzynes 36 to generate (?-phenanthrenyl) vinylsilanes 38 in excellent yields. The reaction proceeds through a [2+2+2] pathway. © Georg Thieme Verlag Stuttgart New York.