Franco Marioni

Preparative synthesis of chiral alcohols by enantioselective reduction with Daucus carota root as biocatalyst

Abstract The enzymatic reduction of (±)-2-methylcyclohexanone with fresh carrot root as biocatalyst occurred in a complete diastereoisomeric way giving a 1:1 mixture of enantiomerically pure (1S,2R)- and (1S,2S)-2-methylcyclohexanol. The analogous reaction carried out on the racemic 2-hydroxycyclohexanone afforded a 1:2 mixture of (1S,2R)- and (1S,2S)-1,2-cyclohexanediol with an enantiomeric excess >95%. The low cost and the easy availability of the biocatalyst besides the very simple reaction conditions suggest the possible use of the present method for large scale preparations of important chiral alcohols.

Different enantioselectivity and regioselectivity of the cytosolic and microsomal epoxide hydrolase catalyzed hydrolysis of simple phenyl substituted epoxides

Abstract The cytosolic (cEH) and the microsomal epoxide hydrolase (mEH) hydrolyse styrene oxide and trans -1-phenylpropene oxide with different enantioselectivity and regioselectivity. While mEH always leads to a regiospecific and enantioselective opening at the non-benzylic oxirane carbon, cEH gives a non-regioselective and non-enantioselective attack to styrene oxide and a regiospecific and non-enantioselective attack at the benzylic carbon of 1-phenylpropene oxide.

Substrate enantioselectivity m the rabbit liver microsomal epoxide hydrolase catalyzed hydrolysis of trans and cis 1-phenylpropene oxides. A comparison with styrene oxide

Abstract A preferential consumption of the (1S,2S) enantiomer of (±)- trans -,1-phenylpropene oxide ( 3 ) and of the (1R,2S) enantiomer of cis -1-phenylpropene oxide (5) is observed during the rabbit liver mEH catalyzed hydrolysis of these epoxides. This preference is, respectively, much lower and much higher than that found for the consumption of the (R) enantiomer in the hydrolysis of (±)-styrene oxide. These results are rationalized in terms of the K M and V max of the respective reactions.

The epoxide hydrolase catalyzed hydrolysis of trans-3-bromo-1,2-epoxycyclohexane. A direct proof for a general base catalyzed mechanism of the enzymatic hydration.

Abstract The hydrolysis of (±)- trans -3-bromo-1,2-epoxycyclohexane in the presence of rabbit liver microsomes was investigated, and found to yield, beside c -3-bromocyclohexane- r -1, t -2-diol, 2,3-epoxycyclohexanol. It was demonstrated that the latter compound was the only product of the enzymatic reaction, whereas the diol resulted from a non enzymatic hydration in the reaction medium. These data provide the first direct proof for a general base catalysis in the enzymatic epoxide hydration, previously hypothesized on the basis of several lines of indirect evidence, and disprove alternative mechanisms involving protonation of the oxirane oxygen.

Bromination of 3-cyclohexene-1-carboxylic acid, epoxydation of methyl 3-cyclohexene-1-carboxylate and opening of methyl cis- and trans-3,4-epoxycyclohexane-1-carboxylate:Stereochemical results

Abstract Bromination of 3-cyclohexene-1-carboxylic acid ( 1 ) gives mixtures of the trans -dibromo-derivatives 3 and 4 and cis -3-hydroxy- trans -bromocyclohexane-1-carboxylic acid lactone ( 5 ). Lactone 5 is obtained by brominating 1 in the presence of triethylamine, showing that halogen preferentially attacks the double bond anti with respect to the carboxyl group. Epoxydation of the methyl ester of 1 also takes place prevalently anti to the methoxycarbonyl group. Ring opening of methyl cis -3,4-epoxycyclohexane-1-carboxylate ( 7 ) with hydrogen bromide gives methyl trans -3-bromo- cis -4-hydroxy- ( 13 ) and cis -3-hydroxy- trans -bromocyclohexane-1-carboxylate ( 6 ). Similar opening of methyl trans -3,4-epoxycyclohexane-1-carboxylate ( 11 ) affords methyl trans -3-hydroxy- cis -4-bromocyclohexane-1-carboxylate ( 14 ). The steric course of these reactions is ascribed to the effect of the electron-withdrawing substituent.

Enantioselectivity of the enzymatic hydrolysis of cyclohexene oxide and (±)-1-methylcyclohexene oxide: a comparison between microsomal and cytosolic epoxide hydrolases

The hydrolysis of cyclohexene oxide and (±)-1-methylcyclohexene oxide by rabbit liver microsomal and cytosolic epoxide hydrolyse (mEH and cEH) has been investigated. Microsomal preparations hydrolysed the two epoxides at respective Vs of 17.0 and 3.5 nmol min–1 mg–1 protein, cytosolic preparations at Vs of 0.95 and 1.0 nmol min–1 mg–1 protein. (–)-(R,R)-Cyclohexane-trans-1,2-diol was formed in the mEH and cEH catalysed hydrolysis of cyclohexene oxide with 94% and 22% e.e. respectively. (–)-(R,R)-1-Methylcyclohexene-r-1,t-2-diol, whose absolute configuration was deduced by c.d. measurements of its bis(p-methoxybenzoate), was obtained by partial hydrolysis of (±)-1-methylcylohexene oxide by mEH. The e.e. of the enzymatically formed diol was 94% at 8% conversion and decreased to 56% around 28% conversion. Racemic 1-methylcyclohexane-r-1,t-2-diol was instead obtained in the cEH catalysed hydrolysis of (±)-1-methylcyclohexene oxide. The substrate enantioselectivity of the mEH catalysed hydrolysis of this trisubstituted epoxide is rationalized on the basis of a better stabilization of the transition state for the anti opening at the carbon with (S) configuration of the (1R, 2S)-enantiomer of the epoxide with (3,4 M) helicity, in agreement with the stereochemical course of the analogous reaction of unsubstituted cyclohexene oxide.

Product enantioselectivity of the microsomal and cytosolic epoxide hydrolase catalysed hydrolysis of meso epoxides

1,2-Epoxycycloalkanes from C5 to C8 and cis-stilbene oxide are respectively hydrolysed to the corresponding (–)-(R,R)-trans-diols and to (+)-(R,R)-1,2-diphenylethane-1,2-diol by both the microsomal and the cytosolic epoxide hydrolase of rabbit liver, the former enzyme being more active and giving higher enantiomeric excesses.

The steric course of the conversion of some bromohydrins into dibromides with phosphorus tribromide and thionyl bromide

Abstract The conversion of (S)-(−)-2-bromo-1-butanol into 1,2-dibromobutane with SOBr2, SOBr2-pyridine. and PBr3 was investigated. While the reactions with SOBr2 produced dibromides with low specific rotations (positive in the absence and negative in the presence of pyridine), PBr3 gave a levorotatory product with much higher optical activity. Equilibrated mixtures of trans-dibromides were obtained in the reactions of the 4-t-butylcyclohexene trans-diaxial bromohydrins with SOBr2 and SOBr2-pyridine, whereas with PBr3 the dibromide was more than 90% diaxial. It can be deduced from the data that (−)-1,2-dibromobutane almost certainly has the (S) configuration. Asymmetric bromination of 1-butene in the presence of dihydrocinchonine afforded dextrorotatory 1,2-dibromobutane.

Applications of asymmetric bromination to the assignment of configurations in 3- and 4-substituted cyclohexenes and the corresponding dibromides

Abstract Previous work on the asymmetric bromination of cyclohexene derivatives in the presence of dihydrocinchonine has been extended to 4-t-butylcyclohexene, and the hypothesis that asymmetric selection in these reactions is determined by the preferential reaction of the cycloalkene in its 3,4M conformation with the alkaloid-bromine complex has been confirmed. The absolute configurations of 4-t-butylcyclohexene and of the trans -dibromides derived from it and from 3-methylcyclohexene have been independently determined, thus confirming the usefulness of the asymmetric bromination in configurational assignments.

The anomalous course of the microsomal transformation of the exo-2,3-epoxides of norbornene and norbornadiene. The possible involvement of a general acid activation during the enzymatic hydrolysis of these oxides

Abstract The enzymatic hydrolysis of exo-2,3-epoxy-norbornane ( 1 ) with a crude rabbit liver microsomal preparation occurred with a rearrangement and gave selectively (2 R ,7 S )-bicyclo[2.2.1]heptane-2,7-diol ( 3 ) , enantiomeric excess (ee) 30±2%. The analogous exo-2,3-epoxy-5-norbornene ( 2 ) gave, under the same conditions, exclusively endo-6-hydroxymethylbicyclo[3.1.0]hex-2-ene ( 4 ) , arising from the microsomal catalyzed reduction of the first formed endo-6-formylbicyclo[3.1.0]hex-2-ene ( 5 ) . A mechanistic explanation for the observed products is proposed.