No abstract

The stereogenic properties of carbocyclic spiranes with two interlocking rings are well‐established. Although some cyclophosphazene analogues of dispiranes have been reported, the molecules did not have any centres of chirality because they were symmetrically substituted. Linear tetraspiranes, in which the two inner rings are carbocyclic and symmetrical and the two outer rings are unsymmetrically‐substituted cyclotriphosphazenes, are expected to give rise to chiral molecules. We now report on the synthesis and stereogenic properties of the three structural types of pentaerythritol‐bridged disubstituted cyclophosphazenes, which all have centres of chirality. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

31P NMR spectroscopy and added chiral shift reagent (CSR) or chiral solvating agent (CSA) have been used to show that unsymmetrically substituted singly bridged macrocyclic phosphazene compounds exist as 1:1 diastereoisomers of two racemic mixtures, in contrast to previous work (ref 2) on symmetrically substituted diastereoisomeric analogues, which exist as meso and racemic forms. The cis-ansa cyclotriphosphazatriene-macrocycle, 1, is meso and monosubstitution of the >P(O-macrocycle)Cl group with 2-naphthol gives a racemic product (7), in which the macrocyclic ring exists in a trans-ansa configuration. Reaction of 7 with the di-secondary amine, piperazine, gives an unsymmetrically disubstituted racemic compound (8) having a cis-ansa configuration of the macrocyclic ring. Reaction of 8 with a further quantity of 1 forms a singly bridged derivative (9) with the macrocyclic rings in cis-trans configurations, and further reaction of 9 with pyrrolidine gives compound 10 with the macrocyclic rings in cis-cis configurations. Both 9 and 10 have four stereogenic centers giving rise to diastereoisomeric compounds existing as mixtures of two racemates. The results are consistent with inversion of configuration at phosphorus at each step of the reaction of >P(OR)Cl groups with nucleophile Z (i.e., Z = naphthoxy, piperazino, pyrrolidino) to form >P(OR)Z derivatives.

500 MHz NMR spectroscopy has been used to investigate the complexation of the anthracycline antibiotic daunomycin (DAU) with self-complementary deoxytetranucleotides, 5'-d(CGCG), 5'-d(GCGC), 5'-d(TGCA), 5'-d(ACGT) and 5'-d(AGCT), of different base sequence in aqueous salt solution. 2D homonuclear 1H NMR spectroscopy (TOCSY and NOESY) and heteronuclear 1H - 31P NMR spectroscopy (HMBC) have been used for complete assignment of the non-exchangeable protons and the phosphorus resonance signals, respectively, and for a qualitative determination of the preferred binding sites of the drug. Analysis shows that DAU intercalates preferentially into the terminal sites of each of the tetranucleotides and that the aminosugar of the antibiotic is situated in the minor groove of the tetramer duplex, partly eclipsing the third base pair. A quantitative determination of the complexation of DAU with the deoxytetranucleotides has been made using the experimental concentration and temperature dependences of the drug proton chemical shifts; these have been analysed in terms of the equilibrium reaction constants, limiting proton chemical shifts and thermodynamical parameters (enthalpies deltaH, entropies deltaS) of different drug-DNA complexes (1:1, 1:2, 2:1, 2:2) in aqueous solution. It is found that DAU interacts with sites containing three adjacent base pairs but does not show any significant sequence specificity of binding with either single or double-stranded tetranucleotides, in contrast with other intercalating drugs such as proflavine, ethidium bromide and actinomycin D. The most favourable structures of the 1:2 complexes have been derived from the induced limiting proton chemical shifts of the drug in the intercalated complexes with the tetranucleotide duplex, in conjunction with 2D NOE data. It has been found that the conformational parameters of the double helix and the orientation of the DAU chromophore in the intercalated complexes depend on base sequence at the binding site of the tetramer duplexes in aqueous solution.

A problem has arisen in using chiral shift reagents (CSR) and chiral solvating agents (CSA) to determine meso and racemic forms of diastereoisomers in which the stereogenic centers of the molecules are separated by achiral spacers. It is found that NMR signals of both meso and racemic forms of diastereoisomers may exhibit doubling on addition of CSR/CSA, which means that unequivocal assignments cannot be made without characterizing the effects for separate meso and racemic forms; this is particularly important for additions of CSR/CSA at relatively low concentrations, which always result in the splitting of some NMR signals of diastereoisomers. The phenomenon is demonstrated in the (31)P NMR spectra of meso and racemic forms of three spermine-bridged gem-disubstituted cyclotriphosphazatrienes, 1a-c, and compared with analogous achiral molecules, the per-substituted spermine-bridged cyclotriphosphazatrienes 2a-d. As expected, only one set of (31)P NMR signals was observed for the achiral compounds 2a-d, even on addition of CSA. Two sets of (31)P NMR ABX multiplets corresponding to meso and racemic diastereoisomers were observed for compounds 1a-c; on addition of CSA, the signals of at least one of the multiplets for each compound separated into more than the expected groups of three lines with an intensity distribution of 2:1:1. To understand this phenomenon, the meso and racemic forms of 1a and 1b and the meso form of 1c have been separated and characterized by X-ray crystallography. On addition of CSA to the racemic forms of 1a and 1b, the (31)P NMR spectrum shows the expected doubling of signals, but, unexpectedly, the same is observed for each of the meso forms of 1a-c. Analogous results using both CSA and CSR have been obtained for the meso and racemic forms of the diastereoisomeric piperazine-bridged macrocyclic-phosphazene compound, 3, whereas no effect was observed for the two meso forms of the doubly bridged macrocyclic-phosphazene compound 4. The phenomenon of doubling of the (31)P NMR signals of the meso form of singly bridged cyclotriphosphazatrienes, 1a-c and 3, is explained by consideration of the equilibrium in solution of independent complexation of a chiral ligand with molecules that have two chiral cyclophosphazene moieties separated by an achiral spacer group. The results show that the stereogenicity of such diastereoisomeric molecules in solution cannot be characterized unequivocally by NMR measurements on addition of either CSR or CSA.

Reactions of pentaerythritol with hexachlorocyclotriphosphazene, N3P3Cl6, and gem-disubstituted cyclotriphosphazene derivatives, N3P3Cl4R2 [R = Ph, NHBu(t) or (OCH2CF2CF2CH2O)0.5] gave a series of pentaerythritol-bridged derivatives linked spiro-spiro, spiro-ansa and ansa-ansa. The structures and stereogenic properties of the products were characterised by X-ray crystallography and 31P NMR spectroscopy on addition of the chiral solvating agent, (S)-(+)-2,2,2-trifluoro-1-(9-anthryl)ethanol. Molecules with spiro-spiro and spiro-ansa bridged gem-disubstituted cyclophosphazenes [R = Ph, NHBu(t) or (OCH2CF2CF2CH2O)0.5] are found to be chiral and exist as racemates. Molecules with ansa-ansa bridged cyclophosphazenes [R = Cl or (OCH2CF2CF2CH2O)0.5] have been characterised for the first time and are shown to have meso configurations. Analysis of crystal structure data shows that the six-membered chair form of the spiro rings and the eight-membered boat-chair form of the ansa rings in the bridged compounds are similar to analogous spiro and ansa exocyclic ring conformations of 1,3-propanedioxy-derivatives of cyclophosphazenes.

No abstract

The gem-disubstituted cyclotriphosphazene 1 reacted with piperazine (pip) to give the piperazine-bridged derivative 2, which is expected to exist in meso and racemic forms because the two PCl (pip) groups are stereogenic. The proton-decoupled (31)P NMR spectrum of 2 gave rise to two similar sets of ABX signals in a 1:1 ratio, consistent with formation of diastereoisomers. The meso and racemic forms of compound 2 were separated by column chromatography on silica gel and characterised by elemental analysis, mass spectrometry, (31)P NMR spectroscopy, and X-ray crystallography. Using HPLC with a chiral stationary phase, the racemic form of compound 2 was further separated into enantiomers, which were characterised by circular dichroism (CD) spectroscopy. This is the first report of the separation of enantiomers in the field of cyclophosphazene chemistry and hence the first CD spectra of derivatives in which the cyclophosphazene ring is at the chiral centre.