Steric requirements, definition

R3 R2 and R2 Ri gauche interactions however, for the same set of substituents, an increase in the steric requirements of either Rj or R3 will influence only one set of vicinal steric interactions (Rj R2 or R3 R2). Some support for these conclusions has been cited (eqs. [6] and [7]). These qualitative arguments may also be relevant to the observed populations of hydrogen- and nonhydrogen-bonded populations of the aldol adducts as well (see Table 1, entries K, L). Unfortunately, little detailed information exists on the solution geometries of these metal chelates. Furthermore, in many studies it is impossible to ascertain whether the aldol condensations between metal enolates and aldehydes were carried out under kinetic or thermodynamic conditions. Consequently, the importance of metal structure and enolate geometry in the definition of product stereochemistry remains ill defined. This is particularly true in the numerous studies reported on the Reformatsky reaction (20) and related variants (21). 

Bulky substituents in 10-positions decrease the androgenic and anabolic activities to a large extent. This definitely points to the /8-face attachment at C-10. Results obtained with 9a,10a-testosterone derivatives confirm the importance of /3-face attachment. However the steric requirement of the methyl group, as measured by the 2.0 A van der Waals radius [222], is no bar to /3-face attachment to the receptor at C-10. The steric requirements at C-10 can accommodate 10-methyl substitution (the presence of the 19-methyl group). 

The DlOP-rhodium(I) complex attached to organic polymers , e.g., polystyrene resin and poly(methyl vinyl alcohol), exhibits good catalytic activity as a chiral catalyst comparable to the corresponding homogeneous catalyst. In contrast, the rhodium(I) complexes anchored on inorganic supports display only a low efficiency . Studies show that the steric requirements for a match of the chiral ligand, a hydrosilane and a ketone are of definite importance in bringing about effective asymmetric induction. 

All the facts described here clearly indicate that the steric requirement for a match of the chiral ligand, a hydrosilane and a ketone is of definite importance in order to bring about an effective asymmetric induction. However, it is not understandable why the two catalysts, (BMPP)2Rh(SX l (8) and (DIOP)Rh(SX l (6), behave differently in activating monohydrosilanes. 

Anticipating the discussion in subsequent sections, the Brjzinsted definitions have proven to be capable of a quantitative thermodynamic development and the acidity functions thus arising (pH, Ho, and Hr ) have been much used. This development owes much to the rapidity of proton transfers which allows the equilibria to be easily attained experimentally. It is also aided by the low steric requirement of hydrogen which minimizes the role of nonbonding repulsions so that the position of equilibria is governed primarily by bond enei ies and electronic factors. 

A different procedure was required for the definition of steric parameters of OZ, SZ, and NZ Z2 groups as no MCD or NCD groups were available to make possible the use of the method used above. Following the treatment of Charton M), we may write any group X in the form M2Z where M1 is that atom of the group X which is bonded to the skeletal group G, and Z is the remainder of X. Then we assume that the steric parameter ux is given by the equation... 

Allyltin reagents were suggested to add to ir-allylpalladium complexes directly on the allyl ligand,137138 although a definitive stereochemical experiment could not be successfully carried out. The fact that these reactions proceed with allyl inversion of the stannane (equation 168) would require an unlikely, sterically unfavorable allylpalladium-R intermediate if metal attack was followed. 

Formation of the sterically unfavourable (Z)-alkadienes on dehydrosulfonylation of a, a-dialkylated (/i)-allylic sulfones promoted by Bu OK/Bu OH has been attributed to a bvn-cffect which, by definition, stabilizes the. vy -conformation required on approach to the transition state.13 The. syn-effect of substituents at the -position of the (C)-allylic sulfones decreases in the order RO— CH3— > RS— > — CFb— > (CH3)2CH—> (CH3)3C—> C6H5— and in accord with previous observations of isomerization of (/i)-vinylie to allylic sulfones. 

Steric Relations. Some interesting specificity has been documented that reflects a requirement for a definite steric approach of an inhibitor to the receptor in the chloroplasts. 

As mentioned above, the photooxidation was discovered by exposure of compound 22 to sunlight. The reaction proved to be of great value for angucycline synthesis because the -hydroxy group present in most natural products which is easily eliminated under basic or acidic conditions (see Scheme 2) and the carbonyl group at C-1 can thus be introduced under mild neutral conditions. We assume that the reaction is initiated by Norrish type II y-hydrogen abstraction of the excited carbonyl in 25 to yield a diradical 26 as shown in Scheme 8 with 1-deoxyrabelomycin (25) as the example [39]. The H-abstraction requires a very definite steric environment in which the benzylic protons have to be in proximity of the excited carbonyl group. Subsequent addition of the diradical 26 with... 

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