Systematic Studies on the Role of Lewis Acids in Organic Synthesis.

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The reaction of allyltributyltin with methyl 2-[N-((4S)-4-(1-methylethyl)-2-oxazolidinone-3-carbamoyl)amino]-2-bromoacetate was accelerated at -50ﾟC in the presence of AIBN and stopped in the presense of galvinoxyl, indicating that the reaction proceeds through radical mechanism. The reaction was accelerated dramatically at -78ﾟC in the presence of ZnCl_2・OEt_2, and the ZnCl_2-mediated reaction was stopped in the presence of galinoxyl. In the presence of 2 equiv ZnCl_2・OEt_2, the reaction afforded methyl (2R)-2-[N-((4S)-4-(1-methylethyl)-2-oxazolidinone-3-carbamoyl)amino]-4-pentenoate 3a(R) with high diastereoselectivity (93 : 7). Taken together, ZnCl_2・OEt_2 works as a radical initiator as well as chelating agent. The ZnCl_2・OEt_2 mediated reaction of 1,1-diphenylmethyl bromide with allyltributytin gave 4,4-diphenyl-but-1-ene in high yield. On the other hand, no reaction took place between 1,1-diphenylmethyl bromide and BF_3・OEt_2. The ZnCl_2 mediated allylation proceeded in high yiel … More ds in cases of tertiary-, allyli-, alpha-alkoxy, and sec-benzyl-halides, whereas the reacion did not occur with primary and secondary halides. It is proved that the ZnCl_2 mediated allylation of bromomalonate and related compounds proceeds via a radical pathway.The N-acyliminium ion intermediates generated from alpha-alkoxycarbamates in the presence of Lewis acids were for the first time detected by ^1H and ^<13>CNMR.It was confirmed that there is an equilibrium between the starting carbamate and the intermediate, and the equilibrium is highly dependent upon a Lewis acid. By using the saturation transfer method, the rate constant for the formation of the intermediate was obtained. NOE experiments revealed that (4-methylphenylmethylene)methyl(methoxycarbonyl)ammonium methyltrifluoroborate has E-geometry.Lewis acid-base combinations TiCl_4・XPh_3 (X=As, Sb, Bi) have been developed to mediate the Lewis acidity of TiCl_4. The inverse addition of TiCl_4・AsPh_3 to 2-(benezyloxy)propanal and allystannane provided a chelation-controlled product with high diastereoselectivity. Diels-Alder reation of the acrylate of (S)-ethyl lactate with cyclopentadiene in the presence of TiCl_4-SbPh_3 afforted an endo-adduct with high diastereoselectivity and without cyclopentadiene polymerization. Less