[reaction: see text] Enantiomerically enriched beta-(diphenylphosphatoxy)nitroalkanes undergo radical ionic fragmentation, induced by tributyltin hydride and AIBN in benzene at reflux, to give alkene radical cations in contact radical ion pairs. These contact ion pairs are trapped intramolecularly by amines to give pyrrolidines and piperidines with significant enantioselectivity ( approximately 60% ee), indicative of cyclization competing effectively with equilibration within the ion pairs. Use of an intramolecular N-propargylamine as a nucleophile provides an enantiomerically enriched pyrrolizidine skeleton via a tandem polar/radical crossover sequence.
It is demonstrated that alpha,alpha-disubstituted-alpha-nitroketones are reduced to the corresponding trisubstituted nitro alcohols in good to excellent yield and enantiomeric excess by borane-dimethyl sulfide in the presence of a chiral oxazaborolidine catalyst. Reduction of the nitro alcohols to the corresponding amino alcohols and their subsequent conversion to enantiomerically enriched 4,4,5-trisubstituted oxazoldinones is also reported.
Enantioselective syntheses
Enantioselective syntheses O 0031Enantioselective Cyclization of Alkene Radical Cations. -On treatment with Bu 3 SnH and AIBN, enantiomerically enriched β-(diphenylphosphatoxy)nitroalkanes give pyrrolidines and piperidines with significant enantioselectivities. These products are formed from the precursors by radical ionic fragmentation to give alkene radical cations in contact radical ion pairs, which are trapped intramolecularly by the amino groups. The degree of the enantioselectivity of these reactions indicates the effective competition of cyclization with equilibration within the ion pairs. In the case of propargylamine compound (V), a tandem polar/radical crossover sequence leads to the pyrrolizidine skeleton (VI). The extension of the method to alcohol (VII) gives only racemic product (VIII) and by-product (IX), arising from cyclization of the hydroxy group onto phosphorus. -(CRICH*, D.; SHIRAI, M.; RUMTHAO, S.; Org. Lett. 5 (2003) 20, 3767-3769; Dep. Chem., Univ. Ill., Chicago, IL 60607, USA; Eng.) -Klein 05-029
Radical reactions O 0195Enantioselective Alkene Radical Cations Reactions. -The treatment of enantiomerically enriched phosphates (I), (III), and (IV) under radical reaction conditions affords pyrrolidines and piperidines with memory of the stereochemistry. The reaction proceeds via cleavage of the β-(phosphatoxy)alkyl radicals in refluxing benzene, followed by intramolecular trapping of the generated alkene radical cation/phosphate anion pairs by appended amino groups. The scope and limitations of this reaction with respect to nucleophile, leaving group, and substituents within the substrate backbone are studied for a wide range of substrates. A comparison of the cyclizations of phosphates (I), (III) and (IV) reveals that neither the ring size (five or six), the leaving group (diphenyl or diethylphosphate), nor the presence of a quaternary center has a major effect on the enantioselectivity. Hydroxy groups are much poorer nucleophiles than amino groups, and predominant or exclusive formation of phosphepane rings like compound (VIII) is observed. Alternative approaches performed with sulfamates, phosphoramides, and phosphorimidates as leaving groups fail to give the expected reaction. -(CRICH*, D.; SHIRAI, M.; BREBION, F.; RUMTHAO, S.; Tetrahedron 62 (2006) 27, 6501-6518; Dep. Chem., Univ. Ill., Chicago, IL 60607, USA; Eng.) -Klein 41-039
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