Asymmetric synthesis of homoallylic amines and functionalized pyrrolidinesvia direct amino-crotylation of in situ generated imines

The asymmetric synthesis of functionalized homoallylic amines and silyl fun ctionalized pyrrolidines through the Lewis acid promoted condensation of ch iral (E)-crotylsilanes with sulfonyl imines and in situ generated N-acyl im ines is described. We had anticipated that this bond construction could be used in the asymmetric synthesis of the N-terminal amino acid subunit of th e nikkomycins. Aryl sulfonyl imines condense with chiral silane reagents in the presence of BF3.OEt2 to form homoallylic arylsulfonyl amines with usef ul levels of syn selectivity. For cases involving aryl N-acyl imines we hav e learned that the temperature controls the course of the reaction. For ins tance, at temperatures of -78 degreesC or below the major product is the py rrolidine, while at higher temperatures (-30 to -20 degreesC) the homoallyl ic amine is produced. For the cases studied, the [3+2]-annulation is limite d to aryl imine derivatives, as alkyl- and branched- imines failed to produ ce the pyrrolidine derivatives: higher reaction temperatures promote the co nversion of the annulation product to the homoallylic amines. In double ste reodifferentiating reactions with in situ generated imines, good levels of selectivity were achieved in the formation of secondary amines bearing syn- anti and syn-syn stereochemical triads. (C) 2000 Elsevier Science Ltd. All rights reserved.