Diastereoisomeric substitution products

The Ir-catalyzed enantioselective allylic substitution reaction can be used for the synthesis of -substituted a-amino acids . For example, the enantioselective Ir-catalyzed allylic substitution of 3-arylallyl diethyl phosphates 5.24 with pronucleophile diphenylimino gly-cinate 5.25 is achieved up to 98% ee by using bidentate chiral phosphite ligand 5.26. By changing the base, both diastereoisomeric substitution products 5.27a and 5.27b could be formed selectively. 

A striking feature in the nitration of 4-substituted toluenes is the change in the preference of 1,2- versus 1,4-adduct formation as a function of the 4-substituent. Thus, nitration of 4-acetamido-,4-chloro-, and 4-methoxy-toluene in acetic anhydride gives in each case a cis-1,2-nitronium acetate adduct (103) in addition to the nitro substitution product(s) (Scheme 171). Nitration of 4-fluorotoluene gives a pair of diastereoisomeric 1,4-nitronium acetate adducts (104) and the cis-1,2-adduct. 

Both, diastereoisomeric P-substituted a-amino acid derivatives could be formed selectively, depending on the cation provided by the base. The (R,S)-diastereoisomer was the major product with a Hthium amide, whereas the... 

The formation of diastereoisomerically pure complexes of 90 with (-)-sparteine is also controlled by crystallisation. Treatment of the indene 89 with BuLi and (-)-sparteine in ether gives, on warming, a yellow precipitate which reacts with carbonyl electrophiles to provide the products 91 typically with good regioselectivity and >95% ee.52 An X-ray crystal structure proved the stereochemistry of the intermediate complex to be that shown as 90b, and hence proved the stereochemical course of the substitution (see section 6.1). The complex is readily decomposed by THF, in the presence of which it rearranges to a racemic V allyllithium. 

Benzophenones have been described as useful sensitisers for PET catalysed conjugate addition reactions of a-amino alkyl radicals to enones (Bertrand et al. 2000). We tried to modify this reaction and synthesised the pyrrolidinylethyl-substituted quinolone 35 from the known bromide (Bauer et al. 2005). Upon electron transfer from the pyrrolidine to a given acceptor, a radical cyclisation occurs (Scheme 15), which after electron and proton transfer generates a pyrrolizidine. We found 4,4/-dimethoxybenzophenone to be a suitable catalyst for this reaction. Remarkably, the reaction proceeded with excellent simple diastereos-electivity and a single diastereoisomeric product rac-36 was obtained. With 10 mol% of the catalyst, a chemical yield of 71% was achieved. 

The reaction of a homochiral derivatizing agent with a heterochiral sample to yield covalently linked diastereoisomeric products is a precolumn technique used for the analysis of the individual enantiomers. The diastereoisomeric products are separable under a variety of chromatographic conditions (GC and LC), including both normal and reversed-phase procedures. Fig. 23 shows the LC separation of two diastereoisomeric products formed by the reaction of a heterochiral amine and a homochiral derivatization reagent. The derivatization reaction involves the formation of an amide by treating an A-substituted 5-prolyl chloride with racemic amphetamine (Fig. 24). 

These products were characterized by H and l3C NMR spectroscopy. Among complexes incorporating unsymmetrically substituted bridging pyrazolates, [Ir(cod)(/i-mpz)]2 and [Ir(cod)(/x-3-CF3-5-Mepz)]2 exist as diastereoisomeric mixtures (C and D) with an approximate 1 1 ratio, while for [Ir(cod)(jt-3-Ph-... 

The reaction of nitriles with aromatic aldehydes is carried out at heating the reactants to 50-70°C with a 1 -h 10 (v/v) mixture of concentrated sulfuric acid and glacial acetic acid. The cycloaddition reaction is regiospecific. The oxazines 21 (equation 10) are formed as diastereoisomeric pairs which are free from their regioisomeric products in the limit of the NMR analysis. Precursors used were benzonitrile and acetonitrile as well as acetaldehyde, benzaldehyde and its substituted derivatives, and a number of the olefins having various structures. Until now, the reaction of aldehydes with nitriles was interpreted as an extension of the Ritter reaction. The initial O-protonation of aldehyde 22 is postulated to form in the presence of acid the hydroxycarbenium ion 23 which then reacts as a cationoid electrophile with the nitrile (equation 11) giving the nitrilium ion 24. 

Many of the earlier claims for the isolation of diastereoisomers have not been substantiated. However, studies of the chemical shifts of the benzylic proton in various substituted 1,3-dioxanes and dioxolanes, using proton magnetic resonance have made it possible to assign absolute configurations to the two 2,3 4,6-di-0-benzylidene acetals of methyl a-n-mannopyranoside and to the two 2-benzoates of methyl 3,4-0-benzylidene-iS-L-arabinopyranoside, thus establishing their diastereoisomeric relationship. In both cases, the isomerism arose from the five-membered ring. The method has also been used to demonstrate that several of the products... 

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