Stereochemical implications

A related feature is the ability of asymmetric complexes to induce chiral reactions, e.g., in catalytic hydrogenation (reviewed by Birch and Williamson, 1976). Asymmetric synthesis using homogeneous transition metal catalysts has recently been reviewed (Bogdanovii, 1973). 

FIGURE 9.6 SnI reactions may produce diastereoisomers in unequal amounts. 

TABLE 9.1 Outcomes of Various Possible Stereochemistries for 5 2 Reactions  

FIGURE 9.8 S, j2 substitution goes with inversion of stereochemistry. 

The optically pure tosylate of 2,2-dimethyU-phenylpropan-l-ol ([a]o = -30.3 °) was heated in acetic acid to give as the major product an acetate with [a]n = +5.3 °. If the optically pure acetate has [aJo = +53.6 , what percentage racemization and what percentage inversion have occurred Explain this in terms of the mechanismfs) involved  

We should consider both S l and 5 2 mechanisms for the substitution. We might have some preference for S l because of the steric hindrance in the molecule (Sn2 reactions are more sterically demanding because we need to crowd more groups around a single carbon atom) and the fact that the cation is benzylic and hence stabilized by resonance. The product is approximately 10 % optically pure. This means we have 55 % of one enantiomer and 45 % of the other. If we assume that the inverted product comes from an 8 2 reaction, and the race-mized product from an S l reaction, we would conclude that our reaction is 90 % 8 1 and 10 % 8 2. However, we should also consider the possibility that the reaction is entirely 8 1, but that there is a small ion pair effect. 

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Among the newer reactions we have chosen those that are not only synthetically useful but at first glance not immediately obvious transformations. Another criterion was the stereochemical implications of the process. Yet, we admit our own bias in choosing from the plethora of novel transformations that have appeared in the literature over the past 30 years or so. Space limitation was by necessity a criterion. Nevertheless we have included approximately 450 name reactions and 2100 references. We sincerely apologize if we have inadvertedly omitted important reactions. 

However, it can be expected that anions of allyl sulfoxides maintain, at low temperature, the chiral integrity of the sulfoxide group and that reactions with electrophiles could have regio-and stereochemical implications. 

Cleavage at any of the three cyclopropyl bonds (a, b, or c) could potentially account for the isomerization.178 However, cleavage at a or b, although both yielding cis product, have different stereochemical implications since the products obtained are enantiomers ... 

The utility of small ring conjunctive reagents in total syntheses appears to be very great. The convulated structural correlations between the target and a starting material hamper its ready adaptation. The facility and stereochemical implications make the rethinking of retrosynthetie analysis well worth the effort. To illustrate some of this potential, several applications in the total synthesis of natural and unnatural products are analyzed. 

This reaction has been extensively studied " and I discuss it here merely to point out certain pitfalls. The conversion of a Ni(0) bis (butadiene) cranplex XIX to a Ni(II) bis- r-allylic complex XX (the structures are drawn withmit stereochemical implication) by reductive coupling appears at first sight to be an allowed process each ligand contributes four electrons and the metal contributes two, making ten in all. Thus Eq. (1) is satisfied, provided the metal itself introduces no discontinuity ... 

Functionalization of organic substituents adjacent to the sulfoxide moiety constitutes an important method of synthesis of a variety of sulfoxides which are required for special synthetic purposes or serve as a source of many sulfur-free organic compounds. This section will be mainly devoted to the use of alkylation reactions of sulfinvl anions in stereoselective synthesis and will not review the tremendous amount of work described in the chemical literature without any stereochemical implication. 

Traub, W. Some stereochemical implications of the molecular conformation of collagen. Israel J. Chem. 12, 435 (1974)... 

Although all alkaloids can be named by the principles already outlined in this article, the cumbersome nature of such names for complex ring systems makes it desirable to use trivial parent names for some large heterocyclic skeletons. It is preferable for such trivial names to refer to skeletons with no substituents (or very few), and it is often convenient for them to carry inherent stereochemical implications. The most extensive source of these names is the Chemical Abstracts Index Guide (or the Ninth Collective Index Nomenclature Manual), but the names given here do not correspond, in many cases, to those in common use, and IUPAC recommendations, when they appear, may well differ in some respects. Some of the principal skeletons listed by Chemical Abstracts are illustrated (122-130). 

The stereochemical implication for the ejection of a leaving group in the B-position of a carbonyl group which yields an a,B-unsaturated system has been treated elsewhere (see p. 233). It may be pointed out here that double bond formation through the opening of a cyclopropane ring should also take place following the same stereoelectronic principle. One example of such a reaction is the transformation of B,r-cyclopropyl-s-hydroxyketone 299 which is smoothly converted into the dienone 300 (85) under acid conditions. 

Kobayashi S, Ashizawa S, Takahashi Y, Sugiura Y, Nagaoka M, Lear MJ, Hirama M (2001) The First Total Synthesis of N1999-A2 Absolute Stereochemistry and Stereochemical Implications into DNA Cleavage. J Am Chem Soc 123 11294... 

The stereochemical implications of this and related reactions are considered in Section IV,E. 

Beware of thinking that the occurrence of the lithiobetaines A and B must have stereochemical implications. Until fairly recently, lithium /ree betaines were incorrectly considered intermediates in the Wittig reaction. Today, it is known that lithima-containing betaines are formed in a dead-end side reaction. They must revert back to an oxaphosphetane—which occurs with retention of the configuration—before the actual Wittig reaction can continue. 

As these arrows suggest, carbenes that have heavily electron-donating substituents are less electrophilic than other carbenes indeed, diamino carbenes can be quite nucleophilic. The division of carbenes into two types explains their structure. It also helps to explain some of their reactions, especially those that have a stereochemical implication. We will spend the rest of this chapter discussing how carbenes react. 

This book attempts to give a comprehensive account of all aspects of molecule structure and the stereochemical implications of the dynamics of the most important enzymic reactions. The editor hopes that the volume will not only be of interest to specialists, but will also provide general information useful to organic chemists, biochemists and molecular biologists. Future problems can only be resolved by close interdisciplinary collaboration of scientists in these various fields. 

The very interesting stereochemical arguments based on the chemical properties of strychnine and its degradation products, that were elaborated by Holmes and published in 1952 (38), did not prove the complete stereochemistry of the molecule, but they did show that all the previous observations that had stereochemical implications were in accord with the X-ray structure. 

The stereochemical implications of this ring-contraction have recently been studied in the 1,3-dithian series K. J. S. Beer, D. Harris, and D. J. Royall, Tetrahedron Letters 24, 1531 (1964). 

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