Sultam chromatography

On the contrary, application of Oppolzer s chiral sultam 100 in Method 1 led to good diastereoselectivity (d.e. 60-70% after chromatography), and pure (4R)-diastereomers have been isolated by recrystallization. Moreover, the chiral auxiliary was cleaved successfully by reduction with LiAlH4.2-Aminopyran-5-carbinols 101 were produced without... 

A remarkable observation was made when the crude 4-disubstituted /3-sultam 53 obtained by reaction of compound 37 with tosyl azide was purified by column chromatography on silica gel. Two fractions were isolated which showed identical IR spectra, 13C NMR spectra, and elemental analyses. However, significant differences were exhibited in the H NMR spectra with a downfield shift for both signals, for H -C(3) and H-C(3) in one of the fractions (Scheme 3). It was suspected that two different conformers that were stable at room temperature had been isolated but the activation energy of the interconversion could not be estimated because of decomposition of the compound upon warming <2004HCA1574>. 

A variety of /3-sultams such as 65 with a poorly migratory substituent at C-3 have been treated with ethylaluminium dichloride to afford /ra r-l,2,3-oxathiazolidine-2-oxides 66 as 70 30 mixtures of isomers separable by preparative thin-layer chromatography (TLC) on silica gel. However, cA-aziridines 67 are obtained as the major products when the reaction is carried out in refluxing dichloromethane (Scheme 10) <1998T8941>. 

Monosilyl 3-sultams are prepared using [2+2] cycloaddition by treatment of methanesulfonyl chloride and an aryl or tert-butylimine in THF at room temperature. Better yields are obtained using a /-butylimine (83%) than an aryl imine (18-59%) (Equation 14). /3-Sultams obtained as a mixture of two isomers are separable by silica gel column chromatography <1998CPB757>. 

A diastereoselective synthesis of /3-sultams by 1,3-asymmetric induction in [2+2] cycloaddition of a sulfene intermediate and a chiral imine has been described (Scheme 48), and it was found that jV-alkylirnines give better diastereo-selectivity than iV-arylimines. The best selectivity was found in the case ofiV-(l-/-butylethyl)imine (67% yield, >95% de), and the diastereoselectivity was independent of the size and conformation of the N-substituents in the imines. Diastereoisomers bearing an /V-aryl group were separable by silica gel column chromatography <1998JOC8355>. 

Table 4. It is interesting to note that the tin enolate corresponding to (57), upon reaction with aldehydes, also provides syn aldol products (60), diastereomeric to (59), with high diastereoselection. This opposite sense of asymmetric induction is believed to be due to coordination of the sultam oxygen to the metal (Sn) in the transition state, which is absent in the boron counterpart. Notably, the products can be easily purified by flash chromatography and/or crystallized to nearly perfect (>99% de) diastereomeric purity.

This concept was extended to the, V-enoyl sultams 27, in which the chiral auxiliary is derived from inexpensive saccharine, but in this case the diastereoselectivity is lower than in the case of the dienophile 2079. The mixture is nevertheless separable by chromatography, and this technique can be applied to 1,3-cyclohexadiene (30) (exo/endo 1 99 d.r. 82 18) and to 2,3-dimethyl-1, 3-butadiene (d.r. 64 36)80. 

Due to the high crystallinity of the sultam auxiliary, however, the aldol adducts 278 were obtained as nearly pure diastereomers after flash chromatography and recrystallization. The cleavage from the auxiliary by means of lithium hydroxide and hydrogen peroxide yielded the corresponding p-hydroxycarboxylic acids. The stereochemical outcome was rationalized by postulating an open transition state model 277, wherein the front side is shielded by the sulfonyl moiety so that the... 

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