Configuration at the 2-position of oxazolidines derived from l-ephedrine and p-bromobenzaldehyde. An x-ray structure redetermination

Abstract

We recently reported that racemic a-acetoxy aldehyde 1 could be resolved by reaction with Z-ephedrine,1,2 followed

Highly diastereoselective synthesis of 1,3-oxazolidines under thermodynamic control using focused microwave irradiation under solvent-free conditions

“…Presumably the reaction proceeds via an iminium ion or imine intermediate, to give the thermodynamically more favoured diastereomer. Furthermore the assigned stereochemistry is in full agreement with configurations assigned in previous work 16,17 on very similar compounds. 18 Assignment of the stereochemistry of the major diastereomers is based on nOe difference spectra of selected compounds 6a and 8a .…”

“…8,16.17 Solvent-free conditions and short irradiation times (80 s) at a power setting of 200 W led to moderate conversions to the 1,3-oxazolidines 6a,b and 8a,b with moderate diastereoselectivities (see entries 10, 12 and 15 in Table 1). In a separate experiment, to exclude any possible detrimental effect of the remaining starting materials, we irradiated a 3+1 mixture of diastereomers 6a for 80 s at 200 W. We observed an identical shift of the reaction equilibrium, as seen for the reaction mixture containing the 1,3-oxazolidine in the presence of aldehyde and amino-alcohol aminals 4a-e and 6a,b and 8a,b and 16. The course of the reaction was again monitored by 1 H NMR spectroscopy, which allowed the determination of conversion and diastereoselectivity during the course of the reaction.…”

“…16 Assignment of stereochemistry in this series of compounds was not straightforward. Their use can also offer often unexpected additional benefits especially with regard to reaction control.…”

Intermolecular N—H...O and C—H...O interactions form a two-dimensional network in (2S,4S,5R)-(–)-3,4-dimethyl-5-phenyl-2-(pyrrol-2-yl)-1,3-oxazolidine

“…Torsion angle differences are evident from O1--C2--C6--C7, which are -99.8 (5) and -105.0(4) ° in A and B, respectively (Table 1). The molecular geometry of (I) compares with the four independent molecules in (2S,4S,5R) (Fitzsimons & Gallagher, 1999) and a p-bromophenyl derivative (Just et al, 1983). …”

“…Torsion angle differences are evident from O1--C2--C6--C7, which are -99.8 (5) and -105.0(4) ° in A and B, respectively (Table 1). The molecular geometry of (I) compares with the four independent molecules in (2S,4S,5R) , (2S,4S,5R)-(-)-2-( 1,3-thiazol-2-yl)-3,4-dimethyl-5-phenyl-l,3-oxazolidine, (III) (Fitzsimons & Gallagher, 1999) and a p-bromophenyl derivative (Just et al, 1983). There are two primary interactions present in the crystal structure of (I).…”