Selected stereochemical investigations

The stereochemical course of enzymatic substitution at phosphorus has been determined for over thirty enzymes. While most of these studies have been completed 

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Maleimides and maleic anhydride have been most frequently employed as cyclic cis-olefin dipolarophiles in the stereochemical investigation of 1,3-dipolar cycloadditions, especially on the endo and exo selection of the reaction. They are one of the most reactive dipolarophiles toward many kinds of azomethine ylide 1,3-dipoles. Because of their structural simplicity, the only stereochemical variation possible in cycloadditions is an endo and exo selection. If a strong attractive interaction exists between the extended conjugation of these dipolarophiles and azomethine ylides, an endo-selective cycloaddition results. 

In this work, we showed the usefulness of cm-selective hydrogenation of aromatic rings in pheromone synthesis, and also illustrated the usefulness of GC comparison in stereochemical investigation. 

Traditional models for diastereoface selectivity were first advanced by Cram and later by Felkin for predicting the stereochemical outcome of aldol reactions occurring between an enolate and a chiral aldehyde. [37] During our investigations directed toward a practical synthesis of dEpoB, we were pleased to discover an unanticipated bias in the relative diastereoface selectivity observed in the aldol condensation between the Z-lithium enolate B and aldehyde C, Scheme 2.6. The aldol reaction proceeds with the expected simple diastereoselectivity with the major product displaying the C6-C7 syn relationship shown in Scheme 2.7 (by ul addition) however, the C7-C8 relationship of the principal product was anti (by Ik addition). [38] Thus, the observed symanti relationship between C6-C7 C7-C8 in the aldol reaction between the Z-lithium enolate of 62 and aldehyde 63 was wholly unanticipated. These fortuitous results prompted us to investigate the cause for this unanticipated but fortunate occurrence. 

One of the most actively investigated aspects of the biohydrolysis of carboxylic acid esters is enantioselectivity (for a definition of the various stereochemical terms used here, see [7], particularly its Sect. 1.5) for two reasons, one practical (preparation of pure enantiomers for various applications) and one fundamental (investigations on the structure and function of hydrolases). The synthetic and preparative aspects of enantioselective biocatalysis by hydrolases have been extensively investigated for biotechnology applications but are of only secondary interest in our context (e.g., [16-18], see Sect. 7.3.5). In contrast, the fundamental aspects of enantioselectivity in particular and of structure-metabolism relationships in general are central to our approach and are illustrated here with a number of selected examples. 

This chapter is divided into four major sections. The first (Section 2.1) will deal with the structure of both alkoxy and silyl nitronates. Specifically, this section will include physical, structural, and spectroscopic properties of nitronates. The next section (Section 2.2) describes the mechanistic aspects of the dipolar cycloaddition including both experimental and theoretical investigations. Also discussed in this section are the regio- and stereochemical features of the process. Finally, the remaining sections will cover the preparation, reaction, and subsequent functionalization of silyl nitronates (Section 2.3) and alkyl nitronates (Section 2.4), respectively. This will include discussion of facial selectivity in the case of chiral nitronates and the application of this process to combinatorial and natural product synthesis. 

Reactions of the enantiopure substrates In order to gain more information about the origin of the apparently low selectivity of the kinetic resolution of rac-ldb, the stereochemical course of the reactions of both the matched acetate Idb and the mismatched acetate ent-ldb was investigated. 

Isolated butyl branches in low-density polyethylene are formed by an intrachain radical rearrangement that is followed by repeated addition of ethylene without further rearrangement. Here, stereochemical selectivity during the formation of CH2R—CH2-CHR—CH2-branches in the free radical initiated polymerization of monosubstituted vinyl monomers is Investigated. The configuration partition functions are denoted by Zm and Zn respectively. They can be written as 2 Um Up Iv1 v2 v3]T and Zr = U U2 Up Ur Up [v3 v2 v3lT. Numerical values... 

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