Stereogenic properties of spiranes combined with one or two equivalent conventional centres of chirality

Derivatives of the tri-spirane pentaerythritoxy-cyclophosphazene compound 1 have been used to investigate the stereogenic properties of spiranes combined with either one or two conventional centres of chirality. In compound 1, the two inner rings are carbocyclic and symmetrical and the two outer rings are cyclotriphosphazenes substituted in different positions to provide the conventional centres of chirality. Reaction of 1 in a 1:1 molar ratio with the unsymmetrical dinucleophilic reagent, 1,3-aminopropanol, gave the mono-spiro substituted derivative 2, which was shown to exist as a racemate by X-ray crystallography and 31P NMR spectroscopy on addition of a chiral solvating agent (CSA). Reaction of 1 with 1,3-aminopropanol in a 1:2 molar ratio gave three diastereoisomeric di-mono-spiro products 3a–3c, which were all shown to be racemates using a combination of X-ray crystallography (3a, 3c) and 31P NMR spectroscopy on addition of a CSA (3b), thus proving the case of the stereogenic properties of spirane molecules combined with two equivalent conventional centres of chirality. It is also shown by quantitative 31P NMR spectroscopy of the reaction mixture and by isolation of reaction products that the proportions of the diastereoisomers 3a:3b:3c are in approximate ratios of 1:2:1, respectively, and these results have been rationalised by analysis of the stereogenic properties of the series of reactions 1 → 2 → 3.

Graphical abstractSpiranes combined with two equivalent conventional centres of chirality are proven to exist as three racemic diastereoisomers by X-ray crystallography and/or 31P NMR spectroscopy with added chiral solvating agent. The relative ratios (1:2:1) of the diastereoisomers observed by 31P NMR spectroscopy are rationalised by analysis of the stereogenic properties of the formation reactions.Figure optionsDownload full-size imageDownload as PowerPoint slide