Alan S. F. Boyd

Heterocyclic nucleoside analogues by cycloaddition reactions of 1-vinylthymine with 1,3-dipoles.

1,3-Dipolar cycloaddition of 1-vinylthymine to azides, nitrile oxides, nitrones and nitronates has been investigated as a route to heterocyclic nucleoside analogues in which the nucleoside ribose moiety has been replaced by an alternative heterocycle. Reaction of 1-vinylthymine with highly reactive nitrile oxides affords 1-(isoxazolin-5-yl)thymine products in excellent yield at room temperature. The less reactive nitrone dipoles undergo cycloaddition to 1-vinylthymine at elevated temperature to afford 1-(isoxazolidin-5-yl)thymine cycloadducts in good-to-moderate yields, but show a tendency to eliminate thymine from the cycloaddition products over long reaction times. Azide cycloadditions to 1-vinylthymine proceed only under forcing conditions to which the fragile triazoline products are unstable.

JM47, a cyclic tetrapeptide HC-toxin analogue from a marine Fusarium species

The known metabolite, enniatin B, and a cyclic tetrapeptide, JM47, which is a new natural product, were extracted from brown rice cultures of a marine fungus, identified as a Fusarium species, isolated from the marine alga Codium fragile. NMR studies, including 15N HMQC and 15N HMBC, established the structure of JM47 as cyclo(Ala-Ala-Aoh-Pro), where Aoh is the amino acid, (2S,9S)-2-amino-8-oxo-9-hydroxydecanoic acid. The absolute stereochemistry of the Aoh side chain carbinol centre was determined using Mosher ester methodology. Analysis of NOESY data assisted by molecular modelling revealed an alternating L-, D-, L-, D-configuration for the tetrapeptide core. The absolute stereochemistry of the core was determined by acidic hydrolysis and chiral TLC analysis of the proline residue. JM47 belongs to the HC-toxin family of cyclic tetrapeptides which possess a 2-amino-8-oxo-9,10-epoxydecanoic acid residue in place of the Aoh unit. This is the first report of an analogue of HC-toxin from a marine Fusarium species.

Supraicosahedral indenyl cobaltacarboranes

13-vertex indenyl cobaltacarboranes with 4,1,6-, 4,1,10- and 4,1,2-CoC(2)B(10) architectures have been synthesised by reduction of the corresponding closo carborane and metallation with an {(eta-C(9)H(7))Co} fragment. Variants of the 4,1,6-isomer were prepared with no, one and two methyl groups on cage C atoms, whilst 4,1,2-species were obtained both with two methyl groups and a trimethylene tether on the cage C atoms. Thermolysis of the 4,1,6-isomers yielded the corresponding 4,1,8-isomers, which in turn were converted to 4,1,12-isomers by thermolysis at higher temperatures. Alternatively relatively mild heating of the 4,1,10-isomer led to the 4,1,12-isomer directly. Products were characterised by mass spectrometry, (1)H and (11)B NMR spectroscopies and, in most cases, elemental analysis, and nine compounds were studied crystallographically. The 4,1,6-, 4,1,8-, 4,1,10- and 4,1,12- species have docosahedral cages whilst the 4,1,2-species are henicosahedral. In the structural studies attention focused on the orientation of the indenyl ligand with respect to the carborane ligand since this affords experimental information on the metal-cage bonding through the structural indenyl effect. There is a general tendency for the indenyl ligand to adopt orientations in which the ring junction C atoms lie trans to cage B atoms. In cases where the orientation is not compromised by the presence of a non-H substituent on the face of the carborane there is generally good agreement between the experimental orientation and that computed by DFT calculations for the related naphthalene ferracarboranes (eta-C(10)H(8))FeC(2)B(10)H(12). The presence of C-methyl substituents in the indenyl cobaltacarboranes tends to override this preference except in the case of 1,6-Me(2)-4-(eta-C(9)H(7))-4,1,6-closo-CoC(2)B(10)H(10) where the indenyl ligand instead is forced to incline away from the cage methyl groups. In DCM solution the 4,1,6-, 4,1,8-, 4,1,10- and 4,1,12- isomers of (eta-C(9)H(7))CoC(2)B(10)H(12) exhibit two, stepwise, 1-electron reductions assigned to Co(III)/Co(II)/Co(I) couples at less negative potentials than those of the corresponding Cp compounds. Moreover these reductions are easier for those isomers (4,1,6- and 4,1,10-) in which there are two cage C atoms in the carborane face to which the metal atom is bound. By spectroelectrochemical and EPR measurements it is concluded that the reductions of these indenyl cobaltacarboranes are largely metal-based.

Supraicosahedral (metalla)carboranes

Although supraicosahedral (hetero) boranes have long been of interest to theoreticians, the area is under-developed from a synthetic viewpoint. The synthesis of supraicosahedral carboranes by reduction then capitation (RedCap) of C2B10 species is attractive, but unsuccessful as long as the cage carbon atoms are free to separate in the reduction step. Studies on 4,1,6-MC2B10 13-vertex metallacarboranes have shown that the partial degradation of such species can be a facile process, in spite of the fact that the binding energy of the metal atom to the carborane framework can be at least as high as that of a {BH}fragment. These findings support the general concept of the kinetic instability of 1,6-C2B11 species, explaining why a supraicosahedral carborane could not be made from 1,2-C2B10H12. However, tethering together the two cage C atoms with a C6H4(CH2)2 strap ultimately allowed the synthesis of the first supraicosahedral carborane. This species has a henicosahedral geometry, and there is evidence that a facile rearrangement from kinetic to thermodynamic isomer has occurred. The RedCap synthesis of this unprecedented cluster has the potential to be applied successively, yielding 14-,15-,16-, etc. vertex carboranes, the larger of which may be sufficiently kinetically stable to exist without a C,C tether.

An investigation of the role of the disparate redox states of the tetrathiafulvalene unit in modulating hydrogen bonding interactions in solution

Abstract We have investigated the electrochemically controlled hydrogen bonding interactions between tetrathiafulvalene host 3 and guests 4 or 5 . Stabilisation of the 3 + state is dependent upon the nature of the guest species, whereas both guests prevent precipitation of the electrochemically generated 3 2+ species at the working electrode via hydrogen bonded molecular recognition processes.

Coordination complexes of C-nitroso compounds. A 13C NMR study of some complexes of platinum with C-nitroso compounds

Abstract Three complexes of platinum with C -nitroso compounds have been studied by 13 C NMR spectroscopy. It has been shown that in all three cases there is a pronounced shift to higher frequency of C -NO and that for the two cases where this involves the t-butyl group the PtC coupling is evident. It is noted that this effect is stronger than for the coordination of some other ligands to platinum.

Charge-compensated carbarhodaboranes. Synthesis and molecular structures of 3-L-3,3- (I) 24-SMe2-3,1,2-closo-RhC2B9H10, L = CO or PPh3

Abstract Reaction of 3,3- (CO) 2-4-SMe2-3,1,2-closo-RhC2B9H10 with iodine results in the formation of 3- (CO) -3,3- (I) 2-4-SMe2-3,1,2-closo-RhC2B9H10, 1. Reaction between 1 and PPh3 results in substitution of the carbonyl group to afford 3-PPh3-3,3- (I) 2-4-SMe2-3,1,2-closo-RhC2B9H10, 2. Both new compounds have been fully characterised by multinuclear NMR spectroscopy and also by a single crystal X-ray diffraction studies.

The first supraicosahedral p-block metallacarboranes

Reaction of Na2[7,9-C2B10H12] or its C-dimethyl analogue with SnCl2 in THF affords 4,1,6-closo-SnC2B10H12 or 1,6-Me2-4,1,6-closo-SnC2B10H10 respectively, the first 13-vertex metallacarboranes involving p-block metals; spectroscopic, structural and theoretical studies, including an analysis of fluctionality in solution, are reported.

The first redox controlled hydrogen bonded three-pole switch

The first example of a redox controlled hydrogen bonded three-pole switch is described, which exploits both electrochemical oxidation and reduction of the host-guest dyad to modulate the intermolecular recognition properties.

The formation of isopropylidene acetals of erythritol and ribitol under conditions of kinetic control

Abstract The isopropylidenation of erythritol, under conditions of kinetic control, employing 2-methoxypropene, yielded the 1,2:3,4- ( 1 , 50%), the 1,4:2,3- ( 2 , 18%), and the novel 1,3:2,4-diacetal ( 3 , 5%). Under similar conditions, ribitol gave the known 1,2:4,5- ( 4 , 36%) and 1,4:2,3-diacetal ( 7 , 3%) together with the new 1,3:4,5- ( 8 , 12%) and 1,3:2,5- diacetal ( 11 , 5%), a further example of a dioxepane ring. 13 C-N.m.r. and 1 H-n.m.r. spectroscopy showed that the 1,3-dioxane rings in 3, 8, and 11 adopt chair conformations. In 9 , the p -nitrobenzoate of 8 , the dioxane chair is flattened. The product composition is compared with that from the equilibrated system.