Rotamers reactions

Acrylic acid, -(3-benzo[f>]thienyl)-a -mercapto-reaction with iodine, 4, 764 Acrylic acid, o -cyano-y3-(2-thienyl)-ring opening, 4, 807 Acrylic acid, -formyl-in pyridazinone synthesis, 3, 46 Acrylic acid, furyl-rotamers, 4, 545 synthesis, 4, 658 Acrylic acid, 2-hydroxybenzoyl-chroman-4-one synthesis from, 3, 850 Acrylic acid, 5-(l-propynyl)-2-thienyl-methyl ester occurrence, 4, 909 Acrylonitrile... 

Furan, 3-acetoxy-2,4,5-triphenyl-synthesis, 4, 659 Furan, 2-acetyl-isopropylation, 4, 607 rotamers, 4, 544 synthesis, 4, 665 toxicity, 1, 136 Furan, 3-acetyl-bromination, 4, 604 Furan, 3-acetyI-2-amino-reactions, 4, 74 Furan, 2-acetyl-3,5-dimethyl-synthesis, 4, 691 Furan, 2-acetyl-5-ethyl-synthesis, 4, 691 Furan, 2-acetyl-3-hydroxy-synthesis, 4, 649... 

N-alkylation, 4, 236 Pyrrole, 2-formyl-3,4-diiodo-synthesis, 4, 216 Pyrrole, 2-formyl-1-methyl-conformation, 4, 193 Pyrrole, 2-formyl-5-nitro-conformation, 4, 193 Pyrrole, furyl-rotamers, 4, 546 Pyrrole, 2-(2-furyl)-conformation, 4, 32 Pyrrole, 2-halo-reactions, 4, 78 Pyrrole, 3-halo-reactions, 4, 78 Pyrrole, 2-halomethyl-nucleophilic substitution, 4, 274 reactions, 4, 275 Pyrrole, hydroxy-synthesis, 4, 97 Pyrrole, 1-hydroxy-cycloaddition reactions, 4, 303 deoxygenation, 4, 304 synthesis, 4, 126, 363 tautomerism, 4, 35, 197 Pyrrole, 2-hydroxy-reactions, 4, 76 tautomerism, 4, 36, 198... 

The reaction of 196 with phenyl chlorocarbene 198 illustrates the synthesis of indolizines by cyclization of pyridinium ylides (Scheme 7). Cyclization of ylide rotamer 199 generates the intermediate product 200, which undergoes elimination of chloride to provide compound 201 <2005EJ01532>. 

In virtually all other W(CHR)(NAr)(OR )2 complexes only the syn alkylidene ro-tamer is observed readily [63]. It was not clear at the time why rotamers could be observed in this particular case and why they interconverted readily. Later it was shown that the reactivities of certain syn and anti species could differ by many orders of magnitude and that the rates of their interconversion also could differ by many orders of magnitude as OR was changed from O-t-Bu to OC-Me(CF3)2. Therefore in any system of this general type two different alkylidene rotamers could be accessible (although both may not be observable), either by rotation about the M=C bond, or as a consequence of the metathesis reaction itself. The presence of syn and anti rotamers further complicates the metathesis reaction at a molecular level, and at least in ROMP reactions (see below) in important ways. The apparent ease of interconversion of syn and anti rotamers in phenoxide complexes could be an important feature of systems in which access to both syn and anti rotamers must be assured (see later). 

Adducts of M(CH-f-Bu)(NAr)(OR)2 complexes were prepared and studied as models for the initial olefin adduct [66] in an olefin metathesis reaction [67]. PMe3 was found to attack the CNO face of yy -M(CH-f-Bu)(NAr)(OR)2 rotamers to give TBP species in which the phosphine is bound in an axial position... 

Molybdenum catalysts that contain enantiomerically pure diolates are prime targets for asymmetric RCM (ARCM). Enantiomerically pure molybdenum catalysts have been prepared that contain a tartrate-based diolate [86], a binaph-tholate [87], or a diolate derived from a traris-1,2-disubstituted cyclopentane [89, 90], as mentioned in an earlier section. A catalyst that contains the diolate derived from a traris-1,2-disubstituted cyclopentane has been employed in an attempt to form cyclic alkenes asymmetrically via kinetic resolution (inter alia) of substrates A and B (Eqs. 45,46) where OR is acetate or a siloxide [89,90]. Reactions taken to -50% consumption yielded unreacted substrate that had an ee between 20% and 40%. When A (OR=acetate) was taken to 90% conversion, the ee of residual A was 84%. The relatively low enantioselectivity might be ascribed to the slow interconversion of syn and anti rotamers of the intermediates or to the relatively floppy nature of the diolate that forms a pseudo nine-membered ring containing the metal. 

N-Nitroso compounds have been found to exist as syn and anti rotamers [30, 31] due to restricted rotation of the N-N bond resulting from nitrogen lone-pair delocalization (Fig. 3.2). This delocalization causes the hydrogens at the a-carbons to become acidic as evident by their base-catalyzed reactions, such as exchange with deuterium... 

Radical reactions are known to proceed by an early transition state, which structurally resembles the starting complex. Here, it is assumed that the hydrogen atom transfer occurs rapidly as compared to any rotamer interconver-... 

The absolute stereochemistry for 150 (entries 2 and 3) was determined by hydrolysis and conversion to known compounds. Assuming a tetrahedral or cis octahedral geometry for the magnesium [110], the product stereochemistry is consistent with si face radical addition to an s-cis conformer of the substrate. This is the same sense of selectivity as that obtained with oxazo-lidinone crotonates or cinnamates suggesting that the rotamer geometry of the differentially substituted enoates is the same. The need for stoichiometric amount of the chiral Lewis acid to obtain high selectivity with 148 in contrast to successful catalytic reactions with crotonates is most likely a reflection of the additional donor atom present in the substrate. 

There are few addition reactions to a,/J-disubstituted enoyl systems 151 that proceed in good yield and are able to control the absolute and relative stereochemistry of both new stereocenters. This is a consequence of problematic A1,3 interactions in either rotamer when traditional templates such as oxazolidinone are used to relieve A1,3 strain the C - C bond of the enoyl group twists, breaking conjugation which results in diminished reactivity and selectivity [111-124], Sibi et al. recently demonstrated that intermolecular radical addition to a,/J-disubstituted substrates followed by hydrogen atom transfer proceeds with high diastereo- and enantioselectivity (151 -> 152 or 153, Scheme 40). 

Two structurally unrelated immunosuppressant drugs, cyclosporin A and FK506, have been shown to bind to separate proteins, which have in common the ability to catalyse the interconversion (8) of the cis and trans rotamers of peptidyl-proline bonds of peptide substrates. A profound change in the conformation, and hence the shape and binding properties of the protein, may result. The mechanism of this isomerization appears, on the basis of recent work (Rosen et al., 1990 Van Duyne et al., 1993 Albers et al., 1990), to involve simple twisting about the amide bond, rather than such alternatives as conversion to a C-N single bond by addition of a nucleophile to C=0.y The proteins which catalyse the reaction may be... 

It is well known that we usually deal with a mixture of rotamers in chemical reactions. If they are interconverted rapidly at a given temperature (eq. [6]), then the Curtin-Hammett relation (171) (eq. [7]) will explain the product dis-... 

The foregoing examples of differential reactivities of rotamers may be summarized by saying that the reactivity is controlled by the steric factor. The difference in the reactivities of rotamers of 9-(2-bromomethyl-6-methyl-phenyl)fluorene (56) in SN2 type reactions falls in the same category (176). However, the substituent effect is not limited to a steric one there can be conformation-dependent electronic effects of substituents as well. A pertinent example is found in the reactivity of the bromomethyl compound (56) when the rotamers are heated in a trifluoroacetic acid solution (Scheme 10). The ap form gives rise to a cyclized product, whereas the sp form remains intact (176). The former must be reacting by participation of the it system of the fluorene ring. 

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