Boron azaenolates of chiral oxazolines: synthesis of optically active formyl oxiranes
摘要:
Boron azaenolates 2 and 8 from the optically active chloromethyloxazolines 1 and 7 have been found to couple with ketones in a highly diastereoselective fashion, which depends markedly upon the geometry (Z) of the azaenolates and the nature of the ligands which are present on the boron atom, in good accordance with semiempirical calculations (AM1). (C) 1999 Elsevier Science Ltd. All rights reserved.
The oxazoline-derived titanium azaenolate 6 couples highly stereoselectively with aldehydes affording highly optically pure oxazolinyloxiranes 7. The epoxide 7a has been deblocked to form the optically pure formyl oxirane 9. In contrast, the corresponding boron azaenolates 2 and 3 couple with very poor or no stereoselectivity. Lithium, aluminum and tin azaenolates have also been shortly studied.