Three-carbon synthons, annulations

Three-Carbon Synthon for [3 + 2] Annulations. Danheiser and co-workers have exploited allenylsilanes as the three-carbon components in a [3 + 2] annulation strategy for the synthesis of a variety of five-membered carbocycles and heterocycles. The pathway by which a typical annulation proceeds is shown in eq 6. Reaction of the 2-carbon component (the allenophile ) at C-3 of the allenylsilane is followed by rapid rearrangement of the silicon-stabilized vinyl cation. Ring closure then affords the five-membered product. 

Allenylsilanes lacking a C-1 alkyl substituent do not function efficiently as three-carbon synthons in the [3 + 2] annulation. This phenomenon is attributable to the relative instability of the terminal vinyl cation intermediate required according to the proposed mechanism for the annulations (eq 6). Fully substituted five-membered rings result from annulations emplo3fing allenylsilanes substituted at both C-1 and C-S. ... 

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