Ruth M. E. Greene

The detection of ‘living’ propagating tungsten–carbene complexes in the ring-opening polymerization of bicycloalkenes

The tungsten–carbene complex W(CHCMe3)(OCH2CMe3)2Br2, when mixed with GaBr3, adds various bicyclo[2.2.1]hept-2-enes to form ‘living’ propagating carbene complexes which may be characterized by 1H n.m.r. spectroscopy, and used to make block copolymers.

Stereoselective enzyme-catalysed oxidation–reduction reactions of thioacetals–thioacetal sulphoxides by fungi

Enzymes present in the fungus Mortierella isabellina catalyse the transfer of an oxygen atom to the cyclic thioaceta 1,3-dithian and from 1,3-dithian 1-oxide, 1,3,5-trithian 1-oxide, and cis-1,3-dithian 1,3-dioxide. The oxidation of 1,3-dithian and the acyclic thioacetal bis-(p-tolythio)methane to form the corresponding monosulphoxides occurred in the presence of growing cultures of Aspergillus foetidus and a Helminthosporium species. The degree and preferred direction of stereoselectivity occurring during the asymmetric oxidation and reduction steps was deduced from the enantiomeric excess (e.e.) and absolute stereochemistry of the isolated 1,3-dithian 1-oxide and p-tolythio-(p-tolylsulphinyl)methane.

13C NMR spectra of hydrogenated polymers of exo-5-methyl-, endo-5-methyl-, and 5,5-dimethyl-derivatives of bicyclo[2.2.1]hept-2-ene prepared by ring-opening metathesis polymerization

13C NMR spectra of the three title polymers have been determined for polymers of different tacticities. Assignments to the various possible HH/HT/TT and m/r dyad structures were made. These were facilitated by the use of optically active monomers in the case of 1 and 3. In the precursor polymers the cis double bonds in the HH dyads were less readily hydrogenated than those in the other types of dyad. Methyl substitution parameters are summarized for the hydrogenated polymers and their unsaturated trans precursors.

An improved method for the synthesis of optically pure epoxide and alcohol derivatives of chiral bromohydrins

Abstract The resolution of arene oxides, diols and other derivatives of bromohydrins has been markedly improved by the use of bromohydrin esters of (−)-(S)-a-methoxy-a-trifluoromethyl- phenylacetic acid (MTPA).

Regioselective dihydroarene oxide formation during ortho-hydroxylation of halogenobenzenes by fungi

ortho-Hydroxylation of chloro- and bromo-benzene occurred in the presence of growing cultures of the fungi Rhizopus arrhizus, Rhizopus stolonifer, and Cunninghamella elegans. The absence of a primary kinetic isotope effect and the presence of the NIH shift are consistent with dihydroarene oxides being initial metabolites. N.m.r. analysis of the deuterium-labelled o-halogenophenol products suggests that enzyme-catalysed epoxidation occurs preferentially at the 2,3-bond.

Synthesis and thermal racemization of the 3,4-oxide metabolite of benzo[c] phenanthrene

Both the racemic form and the (–)-(3R,4S)-enantiomer of benzo[c] phenanthrene 3,4-oxide have been synthesized in 12 steps from naphthalene. The absolute stereochemistry of the arene oxide enantiomers and related chiral derivatives has been determined by n.m.r. methods. Spontaneous thermal racemization of (–)-benzo[c]phenanthrene 3,4-oxide occurred with a barrier to racemization (ΔG≠29 °C 24.6 kcal mol–1)(1 kcal = 4.184 kJ) in accord with predictions based upon PMO calculations.

Synthesis, resolution and racemization studies of 1,2-epoxy-1,2-dihydrochrysene

(+)-1,2-Epoxy-1,2-dihydrochrysene, a major initial mammalian metabolite of chrysene, has been synthesised and assigned the configuration (1R, 2S). Kinetic studies on the spontaneous thermal racemization of (+)-1,2-epoxy-1,2-dihydrochrysene gave a barrier to racemization (Ea) of 24.8 kcal mol–1. The kinetic results and activation parameters support a thermal racemization mechanism involving an oxepin intermediate.

Racemization of phenanthrene 3,4-oxide. Absolute stereochemistry of cis- and trans-phenanthrene 3,4-dihydrodiols

trans-3,4-Dihydroxy-1,2,3,4-tetrahydrophenanthrene (11) and cis-3,4-dihydroxy-1,2,3,4-tetrahydrophenanthrene (12) have been prepared in optically pure form via: (i) chromatographic separation of the trans-3-bromo-4-menthyloxyacetoxy-1,2,3,4-tetrahydrophenanthrene diastereoisomers (7a), (7b), (ii) conversion of (7a) into the optically pure (+)-tetrahydro-3,4-epoxide (8), (iii)acid-catalysed hydrolysis of (+)-(8) by attack at C-4 to yield (+)-trans-(11) and (–)-cis-(12). Several lines of evidence, including spectral methods and chemical transformation to compounds of known absolute stereochemistry, have been used to assign absolute stereochemistry to all chiral compounds described in the present study. The availability of optically pure diols (11) and (12) has allowed the absolute stereochemistry of the trans- and cis-3,4-dihydrodiol metabolites of phenanthrene (4) and (2) to be determined as (–)-(3R,4R) and (+)-(3S,4R) respectively. Phenantherene 3,4-oxide, an initial mammalian metabolite of phenanthrene prepared from optically pure precursors, was optically active but racemized spontaneously at ambient temperature.

Synthesis and thermal racemization of the predominant arene oxide metabolite of chrysene, (+)-(3S,4R)-chrysene 3,4-oxide

(–)-trans-4-Bromo-3-hydroxy-1,2,3,4-tetrahydrochrysene has been obtained from the chromatographic separation and cleavage of the corresponding menthyloxyacetyl (MOA) diastereoisomer. The (–)-enantiomer of this bromohydrin has been assigned (3R,4R)absolute stereochemistry and used in the synthesis of (+)-(3S,4R)-chrysene 3,4-oxide. Thermal racemization studies on this arene oxide gave a barrier to racemization (Ea) of 25.15 kcal mol–1 and an activation entropy (ΔS‡) of 0.7 cal mol–1 K–1, in accord with PMO predictions.

Thermal racemization of a chiral arene oxide metabolite: chrysene 3,4-oxide

Chrysene 3,4-oxide, a major initial mammalian metabolite of chrysene, now synthesised in optically active form and configurationally assigned, has been found to racemize thermally with an activation energy of 25·2 kcal mol–1, consistent with an earlier perturbational molecular orbital calculation-predicted oxepin intermediate mechanism.