Nomenclature syn/anti

The longest linear chain for the syn-anti nomenclature in this case is defined as the segment that contains both stereogenic centers. 

Syn/anti nomenclature is mainly employed for octahedral complexes when geometric isomerism arises from the presence of a fused ring. Therefore, the syn isomer has adjacent fused rings whereas the anti isomer has opposite fused rings [5]. 

For aldol products many types of nomenclature have been proposed. Throughout this chapter the syn/anti nomenclature of Masamune is adopted and defined as shown in (3) and (4). 

We will use the syn/anti nomenclature [5] to describe the relative configuration of aldol stereoisomers, and the Ik/ul nomenclature [6] to describe the topicity of the reaction. For definitions, see glossary. Section 1.6. 

Heathcock et al. described the process that formed diastereomers in the aldol condensation, from precursors that do not contain a chiral center, as simple diastereoselectivity.209,210 Naming protocols to describe the diastereomers produced in the aldol condensation include the erythro/threo nomenclature, as well as the syn/anti nomenclature was discussed in Section I.4.B. Using this latter convention, diastereomers 340 and 343 were designated as anti and diastereomers 341 and 342 were designated as syn. 

Protonation of the anion [SN2] by acetic acid in diethyl ether produces the thermally unstable sulfur diimide S(NH)2. Like all sulfur diimides, the parent compound S(NH)2 can exist as three isomers (Scheme 5.5). Ab initio molecular orbital calculations indicate that the (cis,cis) configuration is somewhat more stable than the (cis,trans) isomer, while the (trans,trans) isomer is expected to possess considerably higher energy. The alternative syn,anti or E,Z nomenclatures may also be used to describe these isomers. The structures of organic derivatives S(NR)2 (R = alkyl, aryl) are discussed in Section 10.4.2. 

Hantzsch s proposal is consistent with characteristic infrared vibration frequencies of syn- and a fr-diazoates, suitably labeled with 15N, measured by Kiibler and Liittke (1963) and by an X-ray structure determination of a sy -diazoate by Alcock et al. (1980b). It is appropriate, therefore, to replace the syn/anti by the (Z/E) nomenclature for isomers (Sec. 7.1)... 

A later calculation disputed these results in that anti-anti was computed to be the most stable conformer . This work, however, was limited to the SCF level, using a 6-31 -I- -l-G basis set. (If consulting the original paper, it is important to note that this set of workers reversed the standard nomenclature for syn and anti.) Anti-syn was computed to be somewhat less stable, followed by syn-anti and syn-syn. These results were compared to a statistical survey of such interactions in proteins where a marked propensity was observed for the syn-syn structure, the least stable in the gas phase. In contrast, the anti-anti configuration, most stable in the gas phase, is observed rarely in proteins. The authors attributed these discrepancies to crystal packing forces. There also seems to be a preference in the crystals for centrosymmetric (or nearly so) Fl-bonds between carboxyl and carboxy-late, although noncentrosymmetric H-bonds are also present in large numbers. Centrosym-... 

Introduction and stereochemical control syn,anti and E,Z Relationship between enolate geometry and aldol stereochemistry The Zimmerman-Traxler transition state Anti-selective aldols of lithium enolates of hindered aryl esters Syn-selective aldols of boron enolates of PhS-esters Stereochemistry of aldols from enols and enolates of ketones Silyl enol ethers and the open transition state Syn selective aldols with zirconium enolates The synthesis of enones E,Z selectivity in enone formation from aldols Recent developments in stereoselective aldol reactions Stereoselectivity outside the Aldol Relationship A Synthesis ofJuvabione A Note on Stereochemical Nomenclature... 

Concerning the relative configuration of stereocenters in the products, the nomenclature used here is (usually) the syn/anti convention currently used for acyclic aldols and related compounds (6-8). To apply this convention, the carbon backbone is drawn in its longest extended (zigzag) form such as for 1.1 (Scheme 1). If both substituents on the chain project in the same direction from the plane of the carbon backbone as in 1.2 or 1.3, then the descriptor is syn. If the two substituents project in opposite directions from the plane of the carbon backbone as in 1.4 or 1.5 the descriptor is anti.f... 

Z/E isomerism is not limited to true double bonds and may be used when sp2 electrons of a heteroatom are conjugated with a 7r-system to form a planar pseudo double bond. In particular, in the case of amides, the cis isomer is called E. Although the general tendency now is to use the E/Z nomenclature in chemistry, despite their inaccuracy cis and trans are still utilized by biochemists because they give a more readily understandable description of molecular shape, in particular for amides in peptides and proteins. When the chains are connected through a motif containing more than three dihedral angles (i.e. carbamates), the syn-anti... 

Reaction of an a-substituted enolate with an aldehyde or ketone can give two pairs of aldol dia-stereomers, which are conveniently designated as the syn form (17) and the anti form (18), where is part of the parent chain in lUPAC nomenclature (equation 29). For simplicity, only one enantiomer of each pair will usually be shown throughout this section. The syn/anti notation for aldol diastereomers has been described in detail by Heathcock. ... 

In a strict sense, the prefix "iso" can only be applied to aldehydes showii meth. -brandui. It should be remembered that also odier internal aldehydes nmybe chiraL Ihis is an important reason why the description n/iso is more and more repbced by Hb. But both descriptions are useless when the ratio of two chiral centers has to be described. In diis situation, odier nomenclatures Imve to be applied, like a/p, syn/anti, or the Cahn—Ii old—Prelog (CIP) s tem. 

More complex is the problem of syn and anti eliminations (cis and trans in the older nomenclature), i.e. whether the substituents on Ca and leave the parent compound from synperiplanar or from antiperiplanar positions, viz. 

The hypothesis was discussed by K. Auwers and V. Meyer, H. Goldschmidt, E. Muller, A. Hantzsch, E. Beckmann, etc. In order to distinguish between these isomeric forms of, say, the benzaldoxim.es, A. Hantzsch employs the term syn as a prefix to indicate that the hydroxyl group lies on the same side of the mol. as the hydrogen atom of the aldehyde, and anti when the hydrogen and hydroxyl lie on opposite sides of the mol. The same nomenclature is applied to other compounds, e.g. the potassium benzoylphenyldiazotates, etc. Thus ... 

The following nomenclature rules apply to describing the transition state structures. The terms exo/endo refer to orientation of the chain starting from the enophile and proceeding to the ene. Endo means that the chain points in the direction of the ene, whereas in an exo form it points away from the ene. E and Z describe the geometry of the double bond of the ene. Syn and anti characterize the positions of the vinylic hydrogen atoms relative to each other. 

The stereochemistry of the cycloadducts in intramolecular Diels-Alder reactions depends upon the different geometry of the possible transition states 37—40 whose nomenclature can be explained as follows The orientation with the chain connecting the diene and dienophile lying under or above the diene is called endo. The opposite means exo. E and Z mark the geometry of the diene double bond which is connected with the chain. Syn and anti describe the arrangement of the hydrogen atoms (or substituents) at the prestereogenic centers which are involved in the C-C bond formation.12... 

Beware The terms cis and frans do not always translste directly into Z and E, Consider the preparation of an enamine from cyclohexanone, which forms a double bond that you d probably call cis (it s in a ring), But applying the rigorous rules laid down for E/Z nomenclature (p. 487), It is E, As for the useful terms syn and anti (Chapter 16), there are no rigid rules for deciding whether a double bond is cis or trans,... 

The addition of hydroxy groups to the carbon-caibon double bond of an alkene (equation 1) is classed under the lUPAC nomenclature for transformations as a dihydroxy addition. Many reagents can bring about this transformation,which can proceed in a syn or anti manner, as shown in equation (2), and the type of addition which occurs depends on the reagent. 

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