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

Abstract

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 31 P 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 31 P 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 31 P 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.

Chiral configurations in cyclophosphazene chemistry

“…The stereogenic properties of compound 138 were also confirmed by 31 P NMR spectroscopy on addition of the CSA which causes the signals to split into two lines in a 1:1 ratio consistent with a racemate [38]. The R and S enantiomers of compound 138 are shown diagrammatically in Fig.…”

“…However, an unsymmetrically di-substituted reagent such as an aminoalcohol would lead to formation of spiro compounds with PXY groups and may be exploited to provide a readily available center of chirality in molecules. In particular, reactions of 1 with 1,3-aminoalcohol lead to derivatives having a spiro group with a stable six-membered ring [36] and this has been used to investigate the stereogenic properties of cyclophosphazene derivatives [37][38][39]. …”

“…The single additional center of chirality was conveniently provided by the reaction of 1,3-aminopropanol in a 1:1 ratio with the trispirane compound 137 to give the mono-substituted compound 138 whose structure was characterized by X-ray crystallography and shown to be a racemate [38]. The stereogenic properties of compound 138 were also confirmed by 31 P NMR spectroscopy on addition of the CSA which causes the signals to split into two lines in a 1:1 ratio consistent with a racemate [38].…”

“…The R and S enantiomers of compound 138 are shown diagrammatically in Fig. 24 in terms of the mono-spirane stick diagram representation [38]. Formation of tri-spiranes with one conventional center of chirality giving a racemate [38].…”

“…25), two of which (139a and 139c) were characterized by X-ray crystallography [38]. M a n u s c r i p t Fig.…”