名称:
                                Nucleophilic additions to and reductiosn of 5-formyl-and 5-acyl-2-isoxazolines (4,5-dihydeoisoxazoles): a stereoselective route to β,γ-dihydroxy ketones
                             
                            
                                摘要:
                                Reductions of readily available 5-acyl-2-isoxazolines with L-Selectride follow the Felkin-Anh model and produce syn-5-hydroxyalkyl-2-isoxazolines with excellent (> 95:5) selectivities.  Swern oxidation of 5-hydroxymethyl-2-isoxazolines, followed by direct addition of a Grignard reagent to the intermediate 5-formyl-2-isoxazolines, also follows the Felkin-Anh model and produces anti-5-hydroxyalkyl-2-isoxazolines with modest (80:20) to excellent (> 95:5) selectivity.  In contrast, additions of Grignard reagents to 5-acyl-2-isoxazolines follow the chelation model, and give syn or anti products (depending on choice of acyl substituent and Grignard reagent) with good (90:10) to excellent selectivity.  These selectivities are almost always far superior to those that can be obtained by direct nitrile oxide cycloaddition to a chiral allylic alcohol or ether.  The resulting products are readily reduced to syn- or anti-beta,gamma-dihydroxy ketones.  A speculative model to explain this surprising reversal in selectivity between formyl and acyl isoxazolines is proposed.