Steric Course

The Pd-catalyzed allylic subshtution with soft nucleophiles proceeds via two substitutions with inversion-that is, with a net retention of configuration. By using standard tests, the same steric course was found for the Ir-catalyzed alkylation 

Asymmetric Catalysis The Beginnings with Phosphinooxazolines as Chiral Ligands 

Unfortunately, further investigation of the PHOX-lr-catalysts gave disappointing results in particular, the regioselectivities were low with alkyl-subshtuted substrates (R = Alkyl). 

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Proton-catalyzed olefin cyclizations of open-chain educts may give tri- or tetracyclic products but low yields are typical (E.E. van Tamelen, 1968, 1977 see p. 91). More useful are cyclizations of monocyclic educts with appropriate side-chains. The chiral centre to which the chain is attached may direct the steric course of the cyclization, and several asymmetric centres may be formed stereoselectively since the cyclizations usually lead to traas-fused rings. 

The steric course of the alkylation of 17-ketones is not influenced by an 11-keto group. However, the presence of a 12a-OH group as well as a 16a-acetoxy group renders a-side approach to C-17 more difficult, and alkylation leads to epimeric mixtures. 

The chemical reduction of enamines by hydride again depends upon the prior generation of an imonium salt (111,225). Thus an equivalent of acid, such as perchloric acid, must be added to the enamine in reductions with lithium aluminum hydride. Studies of the steric course (537) of lithium aluminum hydride reductions of imonium salts indicate less stereoselectivity in comparison with the analogous carbonyl compounds, where an equatorial alcohol usually predominates in the reduction products of six-membered ring ketones. 

After recrystallization from hexane, the major diastereomer is obtained in a 71 % yield. Although interpretation of the steric course of the reaction is difficult25, the preferred formation of the (6S )-diastereomer may be rationalized in terms of an imine conformation which is favored according to the Felkin-Anh model (vicinal C-O orthogonal to C = N)26 and by chelation21. 

Alkylation occurs predominantly or exclusively at the more substituted end of the allylic system regardless of the nucleophile. The steric course of the reactions is the same as that observed with palladium88 and molybdenum89 catalysts. 

Change in the designation due to CIP rules and not the steric course of the reaction. 

The addition of lithium dimethyl-, dibutyl-, diphenyl- or 1-butenylcuprate to 2d produced (JiS) 94% ee, ifiS) 95% ee, (jiR) 96% ec, and ifiR) 90% ee, respectively. In this case the difference between S and R results from the CIP selection rules and not to the steric course of the reactions. The conformation of the chiral auxiliary in 1 d is such that one IV-methyl group is axially, the other equatorially, arranged on the bicyclic structure. The reagents attack the double bond from the side opposite to the equatorial iV-methyl group. Other chiral auxiliaries such as a-c14 were less effective15. 

Change of base (pyridine, aniline, water) had no obvious effect on the steric course of the reaction (59). Rather surprisingly, it appears that the reactions of the cis and trans isomers of 1-bromopropene with the hydride [Co(CN)5H] give different products this eliminates an initial addition of Co—H to the double bond, since this would lead to the same product for the two isomers 105). Vinyl chloride apparently reacts only with [Co salen] , and not with the hydride, to form the CH2=CHCo complex 43) the same may be the case with the BAE complexes 40). 

It is not easy to control the steric course of photoreactions in solution. Since molelcules are ordered regularly in a crystal, it is rather easy to control the reaction by carrying out the photoreaction in a crystal. However, molecules are not always arranged at an appropriate position for efficient and stereoselective reaction in their crystals. In these cases inclusion chemistry is a useful technique, as it can be employed to position molecules appropriately in the host-guest structure. Chiral host compounds are especially useful in placing prochiral and achiral molecules in suitable positions to yield the desired product upon photoirradiation. Some controls of the steric course of intramolecular and intermolelcular photoreactions in inclusion complexes with a host compound are described. 

Brunow, G. Karlsson, O. Lundquist, K Sipila, J. On the distribution of the diaster-eomers of the structural elements in lignins the steric course of reactions mimicking lignin biosynthesis. Wood Sci. Technol. 1993, 27, 281-286. 

The frontier-electron density was used for discussing the reactivity within a molecule, while the superdelocalizability was employed in comparing the reactivity of different molecules 44>. Afterwards, the applicability of the frontier-electron theory was extended to saturated compounds 50>. The new theoretical quantity "delocalizability was introduced for discussing the reactivity of saturated molecules 60>. These indices satisfactorily reflected experimental results of various chemical reactions. In addition to this, the conspicuous behavior of HO and LU in determining the steric course of organic reactions was disclosed 44.51). 

The steric course of electrophilic additions of BrN3, IN3, and iV-bromosuccinimide (NBS) to the tetrahydropyridazine ring of benzo[g]pyridazino[l,2- ]phthalazine-6,13-dione system, for example, 130, has been studied and the results are shown in Scheme 12 <1997CJC348>. 

The literature contains a few examples of increased selectivity [56-58] in which the steric course and the chemo- or regioselectivity of reactions can be altered under the action of microwave irradiation compared with conventional heating. 

Also, Bose et al. [76] have shown that the steric course of /J-lactam formation can be influenced by the MW heating rate. For example, in the reaction of the benzoylox-yacetyl chloride 53 with the Schiff base 54 (Scheme 4.28) the cis adduct 55 is the main product at low irradiation power whereas high power favors the formation of the trans adduct 56. Lactams of this type can serve as intermediates for the side chain oftaxol and its analogs. 

Two possible roles for the metal ion in a template reaction have been delineated (Thompson Busch, 1964). First, the metal ion may sequester the cyclic product from an equilibrium mixture such as, for example, between products and reactants. In this manner the formation of the macrocycle is promoted as its metal complex. The metal ion is thus instrumental in shifting the position of an equilibrium - such a process has been termed a thermodynamic template effect. Secondly, the metal ion may direct the steric course of a condensation such that formation of the required cyclic product is facilitated. This process has been called the kinetic template effect. 

Third, it is acknowledged that the metathesis step itself is responsible for the formation of both cis and trans vinylene configurations (18). The steric course in olefin metathesis has been scrutinized by several researchers. Two basic phenomena are recognized ... 

Any acceptable account of the steric course of olefin metathesis must also take into consideration the following experimentally observed facts ... 

Soluble metathesis catalysts yield trans products in reactions with // / v-2-pentene, but generally are not very stereospecific with c/.v-2-pen-tene. In the latter case, the initially formed butenes and hexenes are typically about 60 and 50% cis, respectively. Basset noted (19) that widely diverse catalyst systems behaved similarily, and so it was suggested that the ligand composition about the transition metal was not a significant factor in the steric course of these reactions. Subsequently, various schemes to portray the stereochemistry have been proposed which deal only with interactions involving alkyl substituents on the reacting olefin or on the presumed metallocyclobutane intermediate. 

Fortunately, steric control arising from interactions of alkyl moieties derived from reacting olefins can be enhanced and observed by selection of appropriate reactants. This effect was demonstrated in the work of Lawrence and Ofstead (76), who studied the metathesis of 4-methyl-2-pentene induced by a WCl6Et2OBu4Sn catalyst. This catalyst is not particularly unique, for the steric course of the metathesis of m-2-pen-tene with this system was found to be essentially equivalent to that previously observed (18) with a conventional catalyst employing an or-ganoaluminum cocatalyst. 

The diverse steric course of the metathesis of cyclic vs. acyclic olefins poses a dilemma. It stands to reason that once a cycloolefin monomer... 

The cyclization of fully conjugated polyenes containing 2n + 2 jr-electrons (equation 1) was termed electrocydie by Woodward and Hoffmann, who showed that the steric course of such reactions was governed by the rules of orbital symmetryI. 3. 

Ionization of p-nitrobenzenesulphonate esters of dienols generates carbocations which undergo cyclization. Thus fraws-5,9-decadienyl p-nitrobenzenesulphonate (157) (R = O2SC6H4NO2-/O reacts with formic acid, followed by hydrolysis, to yield the butenylcy-clohexanol 158, together with the decalinols 159 and 160 (equation 77)87. The c/s-ester 161 affords the c -products 162-164 (equation 78)88. The steric course of these reactions is consistent with a concerted mechanism (equation 79). 

Toromanoff, E., Steric Course of the Kinetic 1,2 Addition of Anions to Conjugated Cyclohexenones, 2, 157. 

It is always advisable to examine the complete molecular topology in the neighborhood of the chiral carbon atom and to confirm the results by employing another analytical method before the final assignment. In conclusion, Prelog s rule does predict the steric course of an asymmetric synthesis carried out with a chiral a-keto ester, and the predictions have been found to be correct in most cases. Indeed, this method has been widely used for determining the absolute configuration of secondary alcohols. 

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