Abstract
A three step synthesis leading to the title compound in good overall yield is described. The bicyclo(3.2.0)heptane framework was formed with good stereocontrol by (π2s + π2a) thermal cycloaddition of the readily available phthalimidoketene with cyclopentadiene. The racemic bicyclic β-amino alcohol 4 was conveniently obtained by mild cleavage of the phthalimide protecting group, in 60 % yield. Resolution with L-aspartic acid allowed access to the (-)- enantiomer of the required β-amino alcohol ligand 4 in 34 % yield and 98 % enantiomeric excess.
Highlights
Since Kagan's novel idea for using C2-symmetric chiral ligands for asymmetric catalysis,[1] a wide variety of such ligands have been prepared
The solution was heated at 40 °C for 10 more minutes and concentrated to 200 mL; anhydrous Et2O (200 mL) was added and the mixture was filtered through a pad of charcoal and Celite®
The filter cake was refluxed for 5 min. in MeOH (50 mL), filtered and the solvent removed under reduced pressure to leave the L-aspartate-ammonium alcohol salt (1:1 mixture by 1H NMR) as a white hygroscopic solid (0.625 g, 34 % ); ee = 98 % by chiral gas chromatography (GC)
Summary
Since Kagan's novel idea for using C2-symmetric chiral ligands for asymmetric catalysis,[1] a wide variety of such ligands have been prepared. In the reactions of phthalimidoketene with cyclopentadiene and indene, 1H NMR NOE experiments and X-ray crystallography confirmed the endo isomer as the major product. A 2.05 M solution of oxalyl chloride (39.04 g, 307.6 mmol, 1.4 eq.) in anhydrous DCM (150 mL) was added dropwise at room temperature over 1 hour.
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