Stereogenic centers definition

This procedure is not a domino process in its strictest definition, but since the oxidant tert-butyl hydroperoxide is added after allylation is complete, it is a very impressive and useful transformation for the rapid assembly of three contiguous stereogenic centers, including a tertiary alcohol moiety. 

An interesting pericyclic-anionic-pericyclic domino reaction showing a high stereoselectivity is the cycloaddition-aldol-retro-ene process depicted in scheme 20.1581 The procedure presumably starts with a [4+2]-cycloaddition of diene 98 and S02 in presence of a Lewis acid. After opening of the formed adduct reaction with (Z)-silyl vinyl ether 99 leads to a mixture of alk-2-enesulfinic acids 101. It follows a retro-ene reaction which affords a 7 3 mixture of the products 102 and 103. The reaction described by Vogel et al is a nice example for the efficient generation of polypropionate chains with the stereoselective formation of three stereogenic centers and one (0-double bond in a three-component domino reaction in its strict definition. 

In addition to this nonequivalence, the imprecise definition of the chirality axis and difficulties in dealing with stereogenic centers in polycyclic structures led to confusion in the case of certain adamantanes, cyclohexanes and spiranes (see Section 1.1.5.3.3.). These problems were solved in the late 1970s and it was recommended1 that the PjM rather than the obsolete aR/aS description be used. This use is recommended as it leads to a considerable simplification of static Stereochemistry. 

The second description allows a decision to be made for any atom, whether it is a stereogenic center or not but it is not adapted to specification of configuration. The deceptively simple first definition, unfortunately, contains two serious problems, solutions to which required almost 30 years. c... 

Definition) All atoms of the ligands to be compared arc ordered according to their topologic distance (number of bonds to the stereogenic center). All atoms of equal topologic distance are said to form a sphere, The atoms in the first sphere are called proximal atoms (p). The atoms of spheres II, III,. . N.. are bound to the stereogenic center via 2, 3. , N.. bonds. 

Table 10. Definitions of the Descriptors c-erythrojc-threo and synjanti for Models with Two Stereogenic Centers...

Apart from cyclic or acyclic transition state geometry further distinctions of diastereoselec-tion have to be made with respect to the way in which the chiral center is attached to the reactive site. The term auxiliary control is used if a chiral subunit, e.g., an alcohol or an amine, is fixed covalently to the unsaturated substrate and then removed by bond cleavage after the addition. In contrast, if the stereogenic center remains part of the molecule after the addition, the term substrate control is applied (these definitions are given in Section A. 1.). 

Cydization leads to fonnation ot a new stereogenic center However, since the radical-bearing side chain can attack the double bond only from above as a result of its p linkage with the bicyclic system, the stereochemistry of the new stereogenic center is definitively established. This new stereogenic center is thus introduced with substrate control. 

The preceding discussion has demonstrated how the specificity inherent in the cyclic transition state may be used to fix structural elements in Cope products, establishing double bonds and other functional groups in definite relationships in a variety of acyclic and cyclic skeletons. This final section treats the remarkable consequences of cyclic transition states in stereochemical control, including alkene configuration and both relative and al olute configuration at stereogenic centers. ... 

Certain carbon-carbon double bonds can also be stereogenic centers. With reference to the definition in Section 5.3, explain how this can occur, and then locate the three additional stereogenic centers in discodermolide. 

A source of potential difficulty in evaluating the electrochemical and photophysical characteristics of the transition-metal-centered dendrimers is the presence of stereogenic centers undefined in their nature. While the potential difficulties for interpreting their characteristics are assumed to be relatively minor, detailed knowledge of their structures is severely hindered by the lack of definition in this matter. 

In the Cope rearrangements of terminal substituted substrates, new stereogenic centers are created at positions 1 or 6. The configuration of these stereocenters is often regulated by a stereocenter which is immediately attached to the new one or by a remote stereocenter. This section discusses stereochemical aspects which concern relative asymmetric induction in Cope rearrangements. For the definition of the term relative asymmetric induction see also Section 1.6.3.1.1.5. p 3481. 

Chiral, definition of, 260 Chiral axis. See Stereogenic axis Chiral center. See Stereogenic center Chiral drugs, 273 Chiral molecules, 259—263, 290 absolute configuration, 267, 292 Fischer projection formulas, 271-272, 278, 280, 292-293... 

The discovery that -imines obtained from a variety of isopropylidene protected sugar open-chain aldehydes and ketenes, or ketene equivalents afford cz5-substituted 8-lactam adducts usually with a high asymmetric induction and definite relative geometry depending on the absolute configuration of the stereogenic center next to the imine carbon atom (Scheme 1) [21-24], prompted many laboratories to exploit this reaction in a number of syntheses. Imines derived from both easily available enantiomeric forms of 2,3-0-isopropylidene-glyceraldehyde are particularly attractive. 

The field of natural product synthesis is definitely one of the most challenging and attractive areas of organic chemistry and numerous contributions focusing on the development of synthesis routes for natural products are reported constantly (2 ). Among the different types of transformations that are necessary to successfully achieve a complex total synthesis, those enabling the stereoselective introduction of a stereogenic center have attracted special interest. 

WL Interactive versions of these problems are assignable in OWL. Stereochemistry Definitions and Stereogenic Centers 5.26 Define or describe the following terms. 

---