Andrew Whiting

ROLE OF THE SITE OF PROTONATION IN THE LOW-ENERGY DECOMPOSITIONS OF GAS-PHASE PEPTIDE IONS

The dissociation of singly or multiply protonated peptide ions by using low-energy collisional activation (CA) is highly dependent on the sites of protonation. The presence of strongly basic amino acid residues in the peptide primary structure dictates the sites of protonation, which generates a precursor ion population that is largely homogeneous with respect to charge sites. Attempts to dissociate this type of precursor ion population by low-energy CA result in poor fragmentation via few pathways. The work described here represents a systematic investigation of the effects of charge heterogeneity in the precursor ion population of a series of model peptides in low-energy CA experiments. Incorporation of acidic residues in the peptide RLC*IFSC*FR (where C* indicates a cysteic acid residue), for example, balances the charge on the basic arginine residues, which enables the ionizing protons to reside on a number of less basic sites along the peptide backbone. This results in a precursor ion population that is heterogeneous with respect to charge site. Low-energy CA of these ions results in diverse and efficient fragmentation. Molecular modeling has been utilized to demonstrate that energetically preferred conformations incorporate an intraionic interaction between arginine and cysteic acid residues.

INTRA-IONIC INTERACTIONS IN ELECTROSPRAYED PEPTIDE IONS

Abstract Recent studies have demonstrated that interactions between cysteic acid and arginine residues have a marked effect on the low energy decompositions of protonated peptides in the gas phase. This work extends these findings to show that analogous interactions operate in the presence of other acidic amino acid residues, although the effects are less pronounced. Thus, the presence of aspartic or glutamic acid residues (X) in the sequence, RLXIFSXFR, attenuates the proton-sequestering properties of the arginine residues in the [M+2H]2+ ion, thereby promoting charge-proximal fragmentation of the peptide backbone. Significant differences in fragmentation behavior are observed for the doubly protonated peptide incorporating two aspartic acid residues, in comparison with the glutamic acid-containing analogue. Low energy collisional activation of [M+2H]2+ ions of RLDIFSDFR, but not RLEIFSEFR, yields significant d-type ions associated with cleavage at the acidic residues. The proposed mechanism invokes arginine/aspartic acid side-chain interaction and is blocked by the additional methylene group in the glutamic acid side-chain. b-Type fragmentation is promoted C-terminal to the aspartic acid residues; this is attributed to nucleophilic attack of the side-chain carboxyl group on the carbonyl carbon of the adjacent peptide bond and is promoted by an interaction between the acidic side chain and the guanidino group of the N-terminal arginine residue. This effect is not observed for the glutamic acid-containing analogue. Molecular mechanics calculations indicate lowest energy conformations consistent with the mass spectrometric data; specifically, the additional methylene group in the glutamic acid side-chain markedly increases the distance between the carboxyl group and the adjacent peptide bond.

Highly Stereoselective Palladium Catalysed Cross-Coupling Approaches to the Total Synthesis of Phthoxazolin A

Abstract The first total synthesis of racemic Phthoxazolin A 4 is described, involving a convergent series of palladium cross-coupling reactions to stereoselectively construct the Z,Z,E-trienyl unit. The most important steps involve using vinylboronate pinacol ester 1 as a vinyl dianion equivalent, by employing a Heck coupling of a vinyl iodide 9 with the vinyl boronate 1, followed by a deboronation–iodination sequence with inversion of alkene stereochemistry to provide a new alkenyl iodide 6. Final Stille coupling of the vinyl iodide 6 with an oxazolyl alkenyl stannane 40 provided Phthoxazolin A 4.

Synthesis of trans-arylvinylboronates via a palladium catalysed cross-coupling of a vinylboronate ester with aryl halides

Abstract Vinylboronate 1 , protected as its pinacol ester, can be cross-coupled with aryl halides in the presence of palladium(0) to give the styrylboronate 2 as the major or exclusive product under optimized reaction conditions. The styrylboronate Heck products 2 are obtained with higher yields when aryl iodides rather than aryl bromides are used. Heteroaromatic halides are only reactive when silver(I) salts are added to the reaction mixture.

An unusual Michael addition–dealkylation or elimination via the reaction of tertiary or secondary amines with a (Z)-iodoacrylate

Abstract A series of ( E )-ammonium or amino acrylates have been prepared via the Michael addition of methyl ( Z )-iodoacrylate and several secondary and tertiary alkylamines. Tertiary amines undergo concomitant addition–dealkylation, almost quantitatively producing ( E )-dialkylamino acrylates.

Novel non-exfoliated clay-nanocomposite materials by in situ co-polymerisation of intercalated monomers: a combinatorial discovery approach

We report the synthesis and qualitative testing of a novel class of clay nanocomposite meterials made by the in situ copolymerisation of small intercalating monomer molecules using combinatorial-style diversity methods. Initial screening was undertaken by treating montmorillonite clay films with combinations of selected additives in aqueous solution. The treated films were assessed for their stability in a qualitative manner based on their response to water. The mechanical stength of these films was also assessed qualitatively. Promising “lead” formulations showed no signs of water-induced swelling and/or exfoliation, while also being flexible and hard. In addition, the interlamellar d-spacings in the treated clay films were measured using X-ray diffraction, where possible; the value of the d-spacing in the treated clays was found to vary significanlty, from 12.7–17.7u200aÅ. The lead formulations were then tested on bulk montmorillonite clay, confirming that the thin film behaviour was representative of that of the bulk. Direct analysis of the treated clays by mass spectrometry using both FAB and MALDITOF did not provide any useful information. However, when the clays were subjected to extraction using chloroform, clear evidence of higher relative moleclar mass species was forthcoming, confirming that polymersation of the additives was occurring. Further supporting evidence was obtained by solid-state NMR anlaysis of treated iron-free (laponite) clay samples, which also revealed extensive polymerisation of the monomers used. Comparison of these data with the results of some simple molecular modelling studies indicates that polymerisation is indeed occurring within the clay galleries.

A facile, strain-induced 1,2-aryl migration in 5,6-diarylacenaphthenes

Abstract Treatment of 5,6-di(3-tert-butyl-4-methoxyphenyl)acenaphthene 3 with boron trichloride leads to rapid formation of the 1,2-aryl migration product 4. The rearrangement is catalysed directly by boron trichloride and not by a proton, and results from the strain added by the tert-butyl functions of 3.

Attempts to find a solution to the problem of atropisomer interconversion in 1,8-diarylnaphthalenes and 5,6-diarylacenaphthenes

A series of sterically restricted 5,6-diarylacenaphthenes 5, 11, 12, 13 and 14 have been prepared via Suzuki cross-couplings of the appropriate boronic acids with 5,6-dibromoacenaphthene 3 in an attempt to prevent atropisomer interconversion in these systems. Attempts to further functionalise bis(p-methoxyphenyl) system 5 in the position ortho to the methyl ethers by Friedel–Crafts acylation or metallation were unsuccessful; however, two unexpected products were obtained. p,p′-Dimethoxybiphenyl 6 results from an unexpected rearrangement of 5 under strongly basic conditions and is dependent on the base used, whilst acylated derivative 7 results from a Friedel–Crafts acylation of the acenaphthene scaffold in the 3-position, rather than the desired functionalisation of the peri-aryl rings, presumably due to the difficulty in forming a tetrahedral intermediate. The oxygen functionality in 5 has been used, following methyl ether cleavage via diphenol 8 and allylation via9, to demonstrate the viability of a double Claisen rearrangement yielding 11 after acetylation. However, the broad 1H NMR exhibited by 11 clearly showed that this system is not configurationally stable, hence steps were required to access more sterically demanding systems which would be configurationally stable. Molecular mechanics and semi-empirical simulations were carried out on related biaryl systems to determine if a single bulky substituent in the 3-position of the peri-aryl rings would be sufficient to prevent atropisomer interconversion. The modelling showed that the energies of the syn- and anti-atropisomeric forms, e.g. for 12–14, were surprisingly similar. With the objective of preparing conformationally stable molecules in this class in mind, 12–14 were prepared in remarkable yield for such a hindered system. In spite of extensive attempts to determine whether 13 was configurationally stable, enantiomeric separation could not be achieved. Unsuccessful attempts were thus made to detect the presence of stable atropisomeric forms of 13 through the synthesis of bis(benzyl ether) 19, in which the benzylic protons could act as enantiotopic reporters. In addition mandelate ester 20 was prepared and it was shown by 1H NMR that a mixture of anti- and syn-diastereoisomers had been obtained. It was therefore concluded that steric groups in the 3-position of the peri-aryl rings cannot be used to prevent atropisomer interconversion in 1,8-diarylnaphthalenes and 5,6-diarylacenaphthenes. During attempts to access diphenols 18 and 24, other by-products were isolated, i.e.21 and 25 respectively, resulting from a steric strain-induced 1,2-aryl shift.

Direct evidence for a ruthenium(IV) oxo complex-mediated oxidation of a hydroxamic acid in the presence of phosphine oxide donors

Ruthenium(II) complexes can be used to oxidise N-Boc hydroxylamine the the presence of tert-butylhydroperoxide to the corresponding nitroso dienophile, which is trapped using cyclohexa-1,3-diene as the hetero-Diels-Alder adduct; direct evidence has been obtained for the intervention of a triphenylphosphine oxide-stabilised ruthenium(IV) oxocomplex as the catalytically active species.

The synthesis of furan-derived calixarenes

Furan and 2-hydroxymethylfuran were reacted under Lewis acidic oligomeric precursors, for a subsequent Lewis acid catalysed cyclisation to afford the furan based calixarenes, i.e.cyclic tetramer2a, and small quantities of the cyclic pentamer2b, hexamer 2c and octamer2d.