Through Modification of Substituents

Preussomerin I 697 and ( )-preussomerin G 698 were obtained from 620 with a five- and six-steps sequence in 15% and 12% overall yield, respectively, through modifications of substituents of the dioxocin ring. Thus, attack of lithium methoxide from the less hindered face of the enone 620, followed by protection of the phenolic oxygen as its methyl ether provided the methoxy adduct 692. The ketone 693 was obtained through a benzylic bromination-solvolysis-oxidation protocol, which required only a single purification. The C(2)-C(3) olefin was introduced by selective silylation of the C-l carbonyl of diketone 693 and oxidation of the silyl enol ether with Pd(OAc)2. Enone... 

It is well-known that para substituents on the phenyl groups of H2TPP have no influence on the tautomerism rates in the ground state (see Section III,A,1). In the case of PHB, there seems to be only a small substituent effect on <1>phb (the quantum efficiency for hole burning) through modification of the relative energy of Ti (93CM366). 

N-heterocycles are a class of neutral ligands with strong coordination affinity to many metal ions. Since a number of neutral N-donors ligands are available, a wide range of oxo-centered triruthenium complexes with various N-heterocyclic ligands have been prepared through axial ligand substitution. By judicious selection of the N-heterocyclic type and modification of the substituents with different electronic and steric effects, the electronic, redox, and spectroscopic properties in these oxo-centered triruthenium derivatives are controllable. 

Berkessel and co-workers synthesized a library of structurally diverse tertiary amine-functionalized catalyst candidates incorporating a chiral 1,2- or 1,4-diamine chiral backbone [231, 232, 246]. Structure-efficiency studies through sequential modification of the diamine backbone, the tertiary amine functionality, the (thio) urea N-substituents as well as of the amide substituent pattern, exemplarily illustrated a Jacobsen-type 1,2-diamine-based structure (figure 6.24), identified... 

In order to demonstrate the effect, it was felt that cleaner kinetic data was needed. This was secured by modification of the participants in the cycloaddition. A tertiary butyl group was introduced onto the amino nitrogen of each substrate (Fig. 7, R = t-Bu). This renders the reaction noncatalytic in the strict sense. Because the tert-hutyl substituents are too large to pass through the cavity of cucurbituril, the resulting product of cycloaddition is a stable rotaxane, i.e. the triazole cannot dissociate. However, this is desirable in that the chemically meaningful presteady-state phase becomes delineated in a way that could only be incompletely realized in the previous kinetics. As a bonus the undesirable substrate inhibition by propargylammonium ion, which also obscured earlier kinetic measurements, could be avoided as well. 

Through the modification of the known L-type VDCC inhibitors verapamil and its desmethoxy analog emopanil, Eli Lilly identified a novel series of amino acid containing phenylalkylamines which demonstrate submicromolar inhibition of neuronal non-L-type VDCC while showing markedly decreased activity on L-type channels. They initially investigated the modification of the phenethylamine moiety and found that chain elongation and incorporation of a second phenyl substituent (compound 1) markedly shifted activity away from L-towards non-L-type inhibition. 

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