Maxwell J. Parrott

Application of diphenylphosphinic carboxylic mixed anhydrides to peptide synthesis

Diphenylphosphinic carboxylic mixed anhydrides formed in situ from Nα-protected amino acids and diphenylphosphinic chloride have been critically evaluated in peptide synthesis. Wherever possible, 32.4 MHz 31P n.m.r. spectroscopy has been employed to follow the rates of both mixed anhydride formation and aminolysis.

A kinetic study of phosphinic carboxylic mixed anhydrides

Using 32.4 MHz 31P n.m.r. spectroscopy, disproportionation of a series of phosphinic carboxylic mixed anhydrides derived from protected α- amino acids has been studied both as a function of the substituents at phosphorus and structure of the α-amino acid being activated. It was found that the rates of disproportionation were insignificant from a preparative aspect compared with aminolysis at 0 °C.

Evaluation of phosphinic acid derivatives as reagents for amine protection in peptide synthesis

The results of a kinetic study of the acid-catalysed methanolysis of a series of N-(2-phenyl-ethyl)phosphinamides incorporating selected substituents on phosphorus have been evaluated in order to define the optimum reagent and conditions for amine protection of α-amino acids during peptide synthesis.

Reactions of cyclo-octatetraene and its derivatives. Part V. Addition of bromotrichloromethane, carbon tetrachloride, and thiophenol to tricyclo[4.2.2.02,5]deca-3,7-diene-9,10-dicarboxylic anhydride

The free radical addition of bromotrichloromethane and carbon tetrachloride to tricyclo[4.2.2.02,5]deca-3,7-diene-9,10-dicarboxylic anhydride (I) results in trans-addition to the cyclobutene double bond. The structures (VIa) and (VIb) proposed for the products are based on n.m.r. studies of the adducts and various related compounds; by analogy structure (VIc) is suggested for the thiophenol adduct.

Stereochemical non-rigidity of phosphoranyl radicals: relative ligand apicophilicities

A series of phosphoranyl radicals, produced by addition of photochemically generated alkoxyl or benzoyloxyl radicals to substituted 1,3,2-dioxaphospholans, have been studied by e.s.r. spectroscopy. The spectra of certain of these radicals exhibit line-shape effects which are interpreted in terms of intramolecular ligand exchange at phosphorus. On the basis of deuterium labelling studies, two types of apical–equatorial ligand exchange have been identified, involving interchange of exocyclic or of endocyclic substituents, respectively. The spectra derived from 4,4-dimethyl-1,3,2-dioxaphospholans show the presence of an approximately equimolar mixture of two isomeric phosphoranyl radicals, in, which the endocyclic CH2O group is sited either apically or equatorially. The e.s.r. spectra of some cyclic and acyclic phosphoranyl radicals show that, in general, ligand apicophilicity decreases in the order F–, Cl–, RC(O)O–>[graphic omitted]–, OCN–> RO–, R2N>H–>H3C–, closely paralleling group electronegativity. Rate constants for exchange of apical with equatorial H and Me2N ligands, repectively, in the acyclic radicals H2(Me)ṖOBut and (Me2N)2ClṖOBut have been estimated.

Phenylphosphoranyl radicals: are they tetrahedral or trigonal bipyramidal?

The e.s.r. spectra of phenylphosphoranyl radicals [PhnPX3 –nY]· show that, depending on the nature of X and Y, these radicals fall into two types in which the unpaired electron is centred mainly on phosphorus or on the aromatic ring.

Cyclisation of unsaturated alkoxyphosphoranyl radicals in solution

Phosphoranyl radicals containing 2- or 3- alkenoxy-substituents cycliose to give oxaphosphetans or xoaphospholans respectively, with the unpaired electron centred on an exocyclic carbon atom.

Steric and electrostatic effects in the addition of thiols to substituted norbornenes

Free radical addition of alkanethiols to 7,7-dimethylnorbornene, bornene, and 1,4,7,7-tetramethylnorbornene and of arenethiols to 1,4,7,7-tetramethylnorbornene to give sulphides is described. The proportions of the products are interpreted in terms of the relative steric and electrostatic interactions between a bridged polycyclic olefin and a thiyl radical approaching from the exo- and endo-directions respectively. The product sulphides have been oxidised to sulphones; the exo-sulphones may be epimerised to give endo-sulphones.

An in situ gas chromatographic and mass spectrometric investigation of the role of substrate temperature in the production of zinc selenide from dimethylzinc and hydrogen selenide via metal–organic vapour phase epitaxy (MOVPE)

Anomalous temperature-dependent decomposition of dimethylzinc, and its reaction with hydrogen selenide under MOVPE (metal–organic vapour phase epitaxy) growth conditions have been monitored by g.c. and mass spectrometry and the results correlated with optimum growth of zinc selenide epitaxial layers on single crystalline GaAs substrates.

Phosphinamides: a new class of amino protecting groups in peptide synthesis

N α -Diphenylphosphinyl protected α-amino acids have been prepared from the corresponding methyl or benzyl esters using diphenylphosphinic chloride–N-methylmorpholine followed by mild alkaline hydrolysis or catalytic hydrogenolysis, respectively. The suitability of these derivatives for use in amide bond forming reactions and their stability during the customary manipulations of peptide synthesis have been exhaustively examined. Acid-catalysed removal of the diphenylphosphinyl group has also been studied, with the aid of 32.4 MHz 31P n.m.r. spectroscopy, and compatability of cleavage with tryptophan and methionine residues—in the absence of scavengers—has been demonstrated by the synthesis of the partially protected C-terminal tetrapeptide of gastrin, Cl–H2+Trp-Met-Asp(OBut)-PheNH2.