Pseudo axial addition

Several examples exist wherein the chirality of the substrate serves to influence the degree of diastereoselectivity obtained on oxidation with racemic and nonracemic oxaziridines. These examples are different from those above, since an auxiliary whose sole purpose is to direct the stereoselectivity is not present. The majority of these examples rely on cyclic stereocontrol to direct the facial selectivity. For example, in Meyers synthesis of the AB-ring of aklavinone, oxidation of the thermodynamic enolate derived from 33 resulted in the production of tertiary a-hydroxy ketone 34 as a single diastereomer in modest yield.23 The stereoselectivity is rationalized by invoking a transition state wherein pseudo axial addition to... 

As a consequence of the pericyclic reaction path, the addition of a-stereogenic allylmctals to carbonyl compounds is accompanied by an effective 1,3-chirality transfer in the allylic moiety combined with 1,4-chira induction at the prostereogenic carbonyl group3032. The scheme also demonstrates the importance of the orientation of the substituent X in the six-membered transition state. By changing from a pseudo-axial to a pseudo-equatorial position, the cation-induced sy/i-attack addresses opposite faces of both prostereogenic moieties, leading to a Z-and an -isomer, these being enantiomeric in respect to the chiral moiety. 

The neutral 3 dx metallocenes are thus known for x = 3 — 8, but the d9 copper complex has thus far resisted preparation, and the d2 titanocene has been found (54) to be both diamagnetic and dimeric, and is therefore excluded from consideration here. A number of cationic species, corresponding formally to Ti(Cp)2+, and V(Cp)2+, systems are however well known, but it seems very probable that these do not possess pseudo-axial symmetry (see (41) for further discussion), and very recently it has been demonstrated (55) that stable V(Cp)2+ complexes cannot be isolated without the coordination of an additional ligand to the metal. The parent systems are therefore limited to V(Cp)2, Cr(Cp)2, Mn(Cp)2, Fe(Cp)2, Co(Cp)2, and Ni(Cp)2 and the cationic species to Cr(Cp)2+, Fe(Cp)2+, Co(Cp)2+, and Ni (Cp)2+> and the d-d spectra of these systems are now considered individually. 

The role of Ti(OPri)4 in this process is shown in Figure 2-7. The aldehyde is illustrated in two conformations, the solid lines indicating the more favorable orientation. The conformation represented by the dashed line is disfavored by a steric interaction with a pseudo-axial aryl group. Assuming that the attack of a nucleophile comes from the direction of the viewer, this hypothesis accounts for the Sf-face selectivity in all known Ti-TADDOLate-mediated nucleophilic additions to aldehydes. 

Rhodium( I)-catalyzed hydroformylation of cyclic enol acetals 1 leads to acetal-protected syn-3,5-dihydroxyalkanals 2 with extraordinarily high levels (>50 1) of diastereoselectivity (Scheme 5.2) [2]. The diastereoselectivity cannot be ascribed to any obvious steric bias, and serves as a powerful demonstration that the hydroformylation reaction may be subject to exquisite stereoelectronic control. Indeed, while the addition of a pseudo-axial methyl group to the acetal carbon (as in acetonide 3) has a deleterious effect on the rate of the reaction, the sy -diastereomer 4 is still produced selectively, in what is surely a contra-steric hydroformylation reaction. 

Figure 11.13 Comparison of crystal structures of BACE with 15 and 16. Addition of the C6 methyl group in 16 reduces the presumed energetic penalty for the aryl substituent to adopt the pseudo-axial geometry seen in the bound conformation and also makes van der Waals contact with the enzyme.

The authors surmise that the trichlorostannyl intermediate 218 directs a chelation-controlled addition via 220, which may involve pseudo-axial complexation of the carbamate carbonyl. The stereoselectivity of the allylation is significantly altered by the use of 2.0 equivalents of SnCU to produce the corresponding i yn.i yn-isomer of 219 via the antiperiplanar transition state derived from the a-chelation model for addition of y-alkoxyallylstannanes. 

The ESR spectrum observed for polycrystalline samples of poly-DCHBr, is shown in Figure 5. The most important question to be answered about this spectrum is whether it arises from a single species with a pseudo axial g-tensor or from two different radical species. Based on variable temperature and variable microwave power studies of this material we have concluded that the spectrum arises from two different radical species. However, the evidence in that regard leaves some uncertainty. Additional experiments are planned. 

Brereton et al. characterized riser gas mixing with a pseudo-axial dispersion coefficient [96]. They concluded that axial dispersion increases with total pressure drop across the riser (approximately proportional to the suspension density). In addition, they found the smooth exit configuration increased axial dispersion coefficients compared to an abrupt exit. They attributed this phenomenon to a more uniform solids distribution in the case of the abrupt exit which, in turn, corresponds to a decreased irregularity in the upward and downward solids movement primarily responsible for the axial mixing of the gas. The experimental values, reported as DyU L, ranged between 0.01 and 0.18 (D = 6,600 to 119,000 cm /s). 

Mannuronic acid donors also proved very suitable for the stereoselective construction of 1,2-cis-mannosyl linkages, and these donors provide anomeric triflates upon activation with a triflate-based electrophile [10]. Interestingly, the anomeric triflates generated from mannuronic acid donors preferentially reside in a flipped C4 Chair conformation, placing the anomeric triflate in an unfavorable equatorial position (as in structure 21, generated from either 19 or 20, Scheme 4.3) [11]. In addition, this conformer places three of the five ring substituents in sterically unfavorable axial positions. A rationale for this unusual behavior has been forwarded, based on the influence of the orientation of the substituents on the pyranosyl core on the stability of a pyranosyl oxocarbenium ion [12]. In a half-chair oxocar-benium ion, the O-substituents at C-3 and C-4 prefer to take up a pseudo-axial position, which is less electron-withdrawing than the alternative pseudo-equatorial... 

The optically active unsaturated cyclic precursor is hydrogenated from the least hindered side of the double bond in very good yield creating a new asymmetric carbon. The stereochemistry of this reduction is insured by the presence of the bulky pseudo-axial phenyl ring. The chlorohydrate of monomethyl L-aspartate is obtained in 98% optically pure form. One inconvenience to this approach resides in the loss of the starting asymmetric amino alcohol which is transformed to diphenyl ethane. In addition, the extension to the other a-amino acids appears uncertain. 

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