Synthesis of enantioenriched .alpha.-hydroxy-.alpha.-allenylacetic acids by [2,3] Wittig rearrangement of .alpha.-(propargyloxy)acetates
摘要:
Optically active (R)-(propargyloxy)acetic esters 5, available in ca. 90% ee through reduction of alkynones 2 with Chirald-LiAlH4 followed by alkylation with chloroacetic acid and esterification with CH2N2, undergo highly stereoselective [2,3] rearrangement upon treatment with LDA in THF at -78-degrees-C followed by Cp2ZrCl2 to afford alpha-(S)-hydroxy-beta-(R)-allenic esters 7 with complete transfer of chirality and > 90% diastereoselectivity. Upon treatment with TESOTf in Et3N the (R)-(propargyloxy) acetic esters 5 afford the diastereomeric alpha-(R)-hydroxy-beta-(R)-allenic esters 8 stereoselectively. Both hydroxy esters 7 and 8 cyclize stereospecifically to trans- and cis-2,5-dihydrofurans 13-15 and 17-19 upon treatment with AgNO3-CaCO3, PhSeCl, or NBS.