γ-Fluorophenyl-GABA derivatives from fluorobenzonitriles in high diastereomeric and enantiomeric excess

An enantioselective synthesis of α-fluoroaryl homoallylic amines in 52–71% yields and 76–93% enantioselectivities has been achieved via the allylboration of the corresponding fluorinated N-aluminobenzaldimines with B-allyldiisopinocampheylborane in the presence of methanol, followed by alkaline hydrogen peroxide workup. Crotylboration of these aluminobenzaldimines with potassium B-methoxy B-E- or -Z-crotyldiisopinocampheylborinate provided the corresponding β-anti- or -syn-methyl α-fluoroarylhomoallylamines, respectively in high de and ee. Similarly, alkoxyallylboration with lithium B-methoxy B-γ-OMEMallyldiisopinocampheylborinate provided the corresponding β-syn-alkoxyhomoallylamines in excellent de and ee. Representatives of these amino alkenes were converted to the corresponding optically active N-Boc-protected fluorinated amino alcohols via hydroboration-oxidation. Further chromium-mediated oxidation provided N-Boc-protected γ-fluorophenyl-γ-aminobutyric acids, which upon deprotection provided the corresponding γ-lactams.