An X-ray structural analysis of a chiral α-alkoxy-ketone/SnCl4 chelate

Overriding Felkin control: a general method for highly diastereoselective chelation-controlled additions to alpha-silyloxy aldehydes

According to the Felkin-Anh and Cram-chelation models, nucleophilic additions to alpha-silyloxy aldehydes proceed through a nonchelation pathway due to the steric and electronic properties of the silyl group, giving rise to Felkin addition products. Herein we describe a general method to promote chelation-control in additions to alpha-silyloxy aldehydes. Dialkylzincs, functionalized dialkylzincs, and (E)-disubstituted, (E)-trisubstituted, and (Z)-disubstituted vinylzinc reagents add to silyl-protected alpha-hydroxy aldehydes with high selectivity for chelation-controlled products (dr of 10:1 to >20:1) in the presence of alkylzinc halides or triflates, RZnX. With the high functional group tolerance of organozinc reagents, the mild Lewis acidity of RZnX, and the excellent diastereoselectivities favoring the chelation-controlled products, this method will be useful in the synthesis of natural products. A mechanism involving chelation is supported by (1) NMR studies of a model substrate, (2) a dramatic increase in reaction rate in the presence of an alkylzinc halide, and (3) higher diastereoselectivity with larger alkyl substituents on the alpha-carbon of the aldehyde. This method provides access to chelation-controlled addition products with high diastereoselectivity previously unavailable using achiral organometallic reagents.

[3 + 2]-Annulation Reactions of Chiral Allylsilanes and Chiral Aldehydes. Studies on the Synthesis of Bis-tetrahydrofuran Substructures of Annonaceous Acetogenins

[Structure: See text] Double asymmetric [3 + 2]-annulation reactions of chiral beta-silyloxyallylsilanes with chiral 2-tetrahydrofuranyl carboxaldehydes have been studied, leading to the stereocontrolled synthesis of six diastereomeric bis-tetrahydrofuran structures corresponding to the core subunits of members of the Annonaceous acetogenin family of natural products. Transition-state models are proposed to account for the stereoselectivity of the double-stereodifferentiating [3 + 2]-annulation reactions.

Electrophilic Aromatic Substitution. 13.(1) Kinetics and Spectroscopy of the Chloromethylation of Benzene and Toluene with Methoxyacetyl Chloride or Chloromethyl Methyl Ether and Aluminum Chloride in Nitromethane or Tin Tetrachloride in Dichloromethane. The Methoxymethyl Cation as a Remarkably Selective Common Electrophile

Vacuum line kinetics studies have been made of the reaction in nitromethane between benzene and/or toluene, methoxyacetyl chloride (MAC), and AlCl(3) to produce benzyl or xylyl chlorides, CO, and a CH(3)OH(-)AlCl(3) complex. For both arenes, the rate law appears to be R = (k(3)/[AlCl(3)](0)) [AlCl(3)](2)[MAC]. When chloromethyl methyl ether (CMME) is substituted for MAC, a similar rate law is obtained. Both chloromethylation reactions yielded similar, large k(T)()/k(B)() ratios (500-600) and similar product isomer distributions with low meta percentages ( approximately 0.4) which suggest CH(3)OCH(2)(+) or the CH(3)OCH(2)(+)Al(2)Cl(7)(-) ion pair as a common, remarkably selective, electrophile. The kinetics of MAC decomposition to CMME and CO in the presence of AlCl(3) yielded the rate law R = k(2)[AlCl(3)](0)[MAC]. Here AlCl(3) is a catalyst (no CH(3)OH is formed), and thus the rate law is equivalent to the chloromethylation rate law. All three reactions have comparable reactivities, which is consistent with rate-determining production of the electrophile. Kinetics studies of benzene or toluene with SnCl(4) and MAC or CMME in dichloromethane were also completed. With MAC and benzene the rate law is R = k(3)[SnCl(4)](0)[MAC][benzene] and with toluene R = k(2)[SnCl(4)](0)[MAC]. MAC decomposition, again followed by CO production, was unaffected by the presence of either aromatic and obeyed the rate law R = k(2)' [SnCl(4)](0)[MAC] where k(2) approximately k(2)'. Chloromethylation with CMME followed the rate law R = k(3)[SnCl(4)](0)[CMME][arene] for benzene and toluene and produced a k(T)()/k(B)() ratio and product isomer distributions very similar to those determined with AlCl(3) in nitromethane, further supporting a common electrophile. Low-temperature (13)C and (119)Sn FT-NMR and Raman spectroscopic studies suggest the existence of a weak 1:1 adduct between MAC and SnCl(4) of the type RCXO --> SnCl(4), with electron donation to the metal through carboxy oxygen. Finally, an explanation is provided for the range of chloromethylation k(T)()/k(B)() values and product isomer percentages published in the literature.