Cyclopropanated allylboration reagents participate in homoallylation reactions of aliphatic and aromatic aldehydes, generating allylicsubstituted alkenes that are difficult to produce via other methods. In studying the effect of cyclopropane substituents, we discovered that an aryl substituent completely changes the outcome to cyclopropylcarbinylation, as if the cyclopropylcarbinyl fragment were transferred intact. However, density functional theory computation suggested a novel mechanism involving ring opening and reclosure, which is supported by experimental evidence.

Cyclopropanated allylboration reagents participate in the homoallylation of aliphatic and aromatic aldehydes, generating substituted alkenes that are difficult to produce via other methods. In this study, we explored the scope and reactivity of homoallylation with cyclopropylcarbinylboronates bearing various aliphatic and aromatic αand γ-substituents. α-Alkyl substituted boronates afforded E-disubstituted alkenyl secondary alcohols in high enantiomeric ratios, while aryl substituents promoted rearrangement. γ-Alkyl substituents all resulted in diastereoselective homoallylation, while aryl substitution changed the outcome to cyclopropylcarbinylation.

An efficient multigram synthesis of alkynyl amino acid Fmoc-l-homopropargylglycine-OH is described. A double Boc protection is optimized for high material throughput, and the key Seyferth–Gilbert homologation is optimized to avoid racemization. Eighteen grams of the enantiopure (>98% ee) noncanonical amino acid was readily generated for use in solid phase synthesis to make peptides that can be functionalized by copper-assisted alkyne–azide cycloaddition.

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