A chemo- and enantioselective [3 + 2] annulation of Morita-Baylis-Hillman carbonates of isatins with propargyl sulfones was catalyzed by a β-ICD O-MOM ether 1c, affording spirocyclic 2-oxindoles bearing an unusual cyclopentadiene motif in outstanding ee values (up to >99%). More electrophiles, such as N-phenylmaleimide, have been also utilized to deliver complex spirocyclic 2-oxindoles with good results.
The redox interconversion
between Co(III) thiolate and Co(II) disulfide
compounds has been investigated experimentally and computationally.
Reactions of cobalt(II) salts with disulfide ligand L1SSL1 (L1SSL1 = di-2-(bis(2-pyridylmethyl)amino)-ethyl
disulfide) result in the formation of either the high-spin cobalt(II)
disulfide compound [CoII2(L1SSL1)Cl4] or a low-spin, octahedral cobalt(III) thiolate
compound, such as [CoIII(L1S)(MeCN)2](BF4)2. Addition of thiocyanate anions to
a solution containing the latter compound yielded crystals of [CoIII(L1S)(NCS)2]. The addition of chloride
ions to a solution of [CoIII(L1S)(MeCN)2](BF4)2 in acetonitrile results in conversion
of the cobalt(III) thiolate compound to the cobalt(II) disulfide compound
[CoII2(L1SSL1)Cl4], as monitored with UV–vis spectroscopy; subsequent addition
of AgBF4 regenerates the Co(III) compound. Computational
studies show that exchange by a chloride anion of the coordinated
acetonitrile molecule or thiocyanate anion in compounds [CoIII(L1S)(MeCN)2]2+ and [CoIII(L1S)(NCS)2] induces a change in the character
of the highest occupied molecular orbitals, showing a decrease of
the contribution of the p orbital on sulfur and an increase of the
d orbital on cobalt. As a comparison, the synthesis of iron compounds
was undertaken. X-ray crystallography revealed that structure of the
dinuclear iron(II) disulfide compound [FeII2(L1SSL1)Cl4] is different from that
of cobalt(II) compound [CoII2(L1SSL1)Cl4]. In contrast to cobalt, reaction of ligand
L1SSL1 with [Fe(MeCN)6](BF4)2 did not yield the expected Fe(III) thiolate compound.
This work is an unprecedented example of redox interconversion between
a high-spin Co(II) disulfide compound and a low-spin Co(III) thiolate
compound triggered by the nature of the anion.
Acetohydroxyacid synthase (AHAS; EC 2.2.1.6) catalyzes the first common step in branched-chain amino acid biosynthesis. The enzyme is inhibited by several chemical classes of compounds and this inhibition is the basis of action of the sulfonylurea and imidazolinone herbicides. The commercial sulfonylureas contain a pyrimidine or a triazine ring that is substituted at both meta positions, thus obeying the initial rules proposed by Levitt. Here we assess the activity of 69 monosubstituted sulfonylurea analogs and related compounds as inhibitors of pure recombinant Arabidopsis thaliana AHAS and show that disubstitution is not absolutely essential as exemplified by our novel herbicide, monosulfuron (2-nitro-N-(4'-methyl-pyrimidin-2'-yl) phenyl-sulfonylurea), which has a pyrimidine ring with a single meta substituent. A subset of these compounds was tested for herbicidal activity and it was shown that their effect in vivo correlates well with their potency in vitro as AHAS inhibitors. Three-dimensional quantitative structure-activity relationships were developed using comparative molecular field analysis and comparative molecular similarity indices analysis. For the latter, the best result was obtained when steric, electrostatic, hydrophobic and H-bond acceptor factors were taken into consideration. The resulting fields were mapped on to the published crystal structure of the yeast enzyme and it was shown that the steric and hydrophobic fields are in good agreement with sulfonylurea-AHAS interaction geometry.
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