Potassium hydroxide and a catalytic amound of 18-crown-6 are used, in alternative to the classical Wittig conditions, to prepare very rapidly and stereoselectively Z- and E-stilbenes. In particular, ...the use of benzyltriphenylphosphonium iodides always leads to a complete Z- stereospecificity, while benzyldiphenylchlorophosphonium salts give a complete E-stereospecificity.
An efficient, safe and highly stereoselective method to obtain E- or Z-stilbenes
Both enantiomers of
cis-β-ethyl, β-
n-propyl, β-
n-butyl, and β-
n-hexyl substituted styrene oxides undergo microsomal epoxide hydrolase catalysed hydration at the (S) carbon to give the ...corresponding (R,R)
threo diols in a > 90% e.e. A complete kinetic resolution of the racemic epoxide is also obtained with the β-ethyl substituted substrate, but not with its higher homologues.
Both enantiomers of epoxides
1b–e are hydrated by the microsomal epoxide hydrolase to give the (R,R)
threo diols
2 in a >90% e.e. Epoxides
1a and
1b also undergo a complete kinetic resolution.
Rates and products of electrophilic bromination of ring-substituted cis- and trans-stilbenes have been investigated in acetic acid, trifluoroethanol, ethanol, methanol, and water−methanol mixtures. ...The mY Br relationships (linear for nucleophilic solvents only, with m = 0.8), the deviations of the two nonnucleophilic solvents from the mY Br plots (ΔAcOH and ΔTFE positive, negative, or negligible), the kinetic solvent isotope effects (k MeOH/k MeOD = 1.1−1.6), the chemoselectivity (predominant dibromide, DB, or solvent-incorporated adducts, MA), and the high dependence of the stereochemistry on the solvent and the substituents (from stereoconvergency to stereospecificity) are discussed and interpreted in terms of a mechanistic scheme, analogous to the Jencks scheme for aliphatic nucleophilic substitutions, in which preassociation, free-ion, and ion-pair pathways compete. In particular, the stereochemical outcome of these reactions is consistent with a marked change in the nucleophilic partners of the product-forming ionic intermediate arising from different ionization routes. Return, i.e. change in the rate-limiting step from ionization to product formation, is shown to be related to substituent-dependent, but not solvent-dependent, bromine bridging.
Both enantiomers of (±)-9,10-epoxystearic acid (
1b),
cis-(±)-5,6-epoxyhexadecane (
1c) and
cis-(±)-11,12-epoxyhexadecan-1-ol (
1d) as well as the meso
cis-9,10-epoxyoctadecane (
1a) undergo ...microsomal epoxide hydrolase catalyzed hydration at the (S) carbon to give the corresponding (R,R)
threo diols in a >90 e.e.
Graphic
1-
O-Hexadecyl and 1,2-di-
O-hexadecyl-3-
O-(β-
d-glucopyranosyl)-
sn-glycerol ethers were obtained by regiospecific opening of the oxirane ring of the 2′,3′-epoxypropyl 2-
O-acetyl-3,4,6-tri-
...O-benzyl-β-
d-glucopyranoside synthetized by stereoselective epoxidation of the corresponding allyl 3,4,6-tri-
O-benzyl-β-
d-glucopyranoside asymmetrically induced by the glucide moiety.
Graphic
