- Construction of Quaternary Carbon Centers Using Organomolybdenum Chemistry
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Two approaches are described for the construction of quaternary carbon centers using organomolybdenum complexes.The first involves carbon nucleophile addition to the methylated diene terminus of dicarbonyl(η5-indenyl)(1-4-η-1,4-dimethylcyclohexa-1,3-diene)molybdenum tetrafluoroborate (12).A wide range of nucleophiles add successfully to 12 giving gem-disubstituted cyclohexenylmolybdenum complex.This methodology, coupled with a lactonization/decomplexation reaction, has led to a short synthesis of 2-(2-hydroxy-1,4-dimethylcyclohex-3-enyl)propionic acid lactone(14), which has previously been employed as a key intermediate for trichothecene synthesis.The second method involves alkylation of cyano-stabilized carbanions generated from dicarbonyl(η5-cyclopentadienyl)(1-3-η-4-cyanocyclohexenyl)molybdenum and dicarbonyl(η5-cyclopentadienyl)(1-3-η-4-cyanocycloheptenyl)molybdenum.While a rather limited range of electrophiles could be used in this coupling reaction, it was found that stereochemical control is excellent and allows the construction of quaternary carbon centers with defined stereochemistry relative to other substituents on the cyclohexenyl or cycloheptenyl ligand.During the course of this study it was found that treatment of the exo-cyano-substituted ?-allylmolybdenum complexes with butyllithium led to novel decyanation reaction.This allowed the generation of nonstabilized carbanion species that could be alkylated regio- and stereospecifically.
- Pearson, Anthony J.,Khetani, Vikram D.
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- Isomer differentiation by tri-osmium cluster complexation of substituted 1,3-cyclohexadienes
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A series of methyl- and dimethyl-substituted 1,3-cyclohexadienes have been prepared from their aromatic analogues via Birch reduction and subsequent isomerisation with Fe(CO)3 fragments. These ligands were reacted with [Os3(CO)10(CH3CN)2] to form tri-osmium decacarbonyl cluster compounds containing the η4-coordinated substituted 1,3-cyclohexadienes. The various isomers of the substituted dienes show a dramatic difference in their reactivity towards the tri-osmium cluster and it is likely that this is due to the steric interactions between the methyl substituents and the cluster framework, with this effect being more marked for the di-substituted ligands.
- Ingham,Johnson,Sadler,Nairn
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p. 237 - 242
(2007/10/03)
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- IMIDAZOLE DERIVATIVES AND PHARMACEUTICAL COMPOSITIONS CONTAINING THE SAME
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The present invention provides a novel imidazole derivative having the chemical formula STR1 These compounds are useful for preventing or treating hypertension or congestive heart failure and have high activity, rapid action upon intravenous injection, go
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- 66. β-Cleavage of Potassium bicyclooct-5-en-2-olates. Stereoselective Synthesis of (+-)-Trichodiene
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The transformation of 12 bicyclooct-5-en-2-ols (V or VI) to 3-(cyclohex-3-enyl)-2-alkanones (III or IV), via β-cleavage of their potassium alkoxides in HMPA, has been investigated (cf. table I).As an illustration of this synthetic methodology, a stereoselective synthesis of (+)-trichodiene ((+)-1) is described wich involves the β-cleavage of the tricyclic potassium alkoxides 46a and 47a to cyclopentanone 4 (cf.Scheme 7).
- Snowden, Roger L.,Brauchli, Robert,Sonnay, Philippe
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p. 570 - 593
(2007/10/02)
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- WAVELENGTH EFFECT ON THE PHOTOCHEMICAL REACTIONS OF (Z)-2,5-DIMETHYL-1,3,5-HEXATRIENE: SELECTIVE EXCITATION OF ROTAMERS
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The UV absorption spectrum of Z-2,5-dimethyl-1,3,5-hexatriene 1 is typical of a cZt triene, but the long wavelength tail indicates the presence of the (minor) cZc rotamer.The sudden change in the photoreactivity of 1 around 290 nm is in agreement with selective excitation of these rotamers.No indication is obtained for an intrinsic excitation energy dependence of the photoreactivity of the individual rotamers.
- Brouwer, A. M.,Cornelisse, J.,Jacobs, H. J. C
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p. 435 - 438
(2007/10/02)
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