Anti addition defined

Structural features of olefins with distorted double bonds have been discussed within the deformation space defined by the eight bond angle deformations. The out-of-plane bond angle distortions are of particular interest because they are involved in addition reactions of the double bond. The symmetrical Blg-type deformation is related to concerted anti-additions, whereas the J3lu-type distortion (cf. Table 1) is appropriate for concerted syn-addition and those reactions that involve three-center intermediates and the formation of transition metal complexes. Twist or torsion is due to the Alu-type oop distortion and may be related to addition reactions, which in principle would lead—in the extreme case of a 90° twist angle—to an eclipsed rather than a staggered arrangement. 

One important stereochemical distinction to be made in additions to alkenes is that syn addition and anti addition to a carbon-carbon double bond may produce stereoisomeric products. The term anti means that one group adds from the top of the molecule (as defined by the plane of the substituents on the double bond), while the other adds from the bottom. In syn addition, both substituents add from the same face. As noted in Figure 9.1, the syn and anti pathways produce different configurations at one of the two tetrahedral centers created by the addition to a carbon-carbon double bond. In older terminology, these processes were referred to as trans or cis additions, but currently it is preferred to use the terms syn and anti for mechanisms and to reserve the terms cis and trans for structures. ... 

A key observation in understanding the difference between these two systems is the stereochemistry of addition. As shown in Figure 10.4, five products are obtained from the reaction of 1,3,3-trideuteriocyclohexene with HCI in acetic acid. One product results from syn addition of HCI, while another from anti addition of HCI. A third product arises from anti addition of acetic acid, and the stereochemistry of addition of the fourth and fifth products cannot be ascertained. Syn addition is defined as the addition of both components (in this case H and Cl or H and OAc) to the same face of the alkene -it bond. Anti addition is addition of the two components to opposite faces of the alkene. 

Out of three phases reported for Ca3N2 [12], to date only the cubic (a) nitride phase is confirmed, crystallizing in the anti-bixbyite type structure. In addition, the carbodiimide nitride Ca4(CN2)N2 [13] may hold for another unconfirmed calcium nitride phase, and Can(CN2)2N6 stands for the ill-defined CauNg [14]. Here we note again that carbon impurities may produce significant difficulties in reactions. 

Now, just the same sort of rationalization can be applied to the radical addition, in that the more favourable secondary radical is predominantly produced. This, in turn, leads to addition of HBr in what is the anti-Markovnikov orientation. The apparent difference is because the electrophile in the ionic mechanism is a proton, and bromide then quenches the resultant cation. In the radical reaction, the attacking species is a bromine atom, and a hydrogen atom is then used to quench the radical. This is effectively a reverse sequence for the addition process but, nevertheless, the stability of the intermediate carbocation or radical is the defining feature. The terminologies Markovnikov or anti-Markovnikov orientation may be confusing and difficult to remember consider the mechanism and it all makes sense. 

In addition to the frequent concerns of sample size and MTX dosing, clinical variation in the use of leucovorin may also confound these analyses. Furthermore, the Shimaski et al. study may be subject to treatment period effects as anti-emetic practices change over treatment eras (12). As noted by Laverdiere, prospective studies evaluating multiple folate pathway polymorphisms will be necessary to define clearly the relationship between the RFC G80A polymorphism and ALL treatment response (24). 

Water can be added indirectly, with anti-Markovnikov orientation, by treatment of the alkene with a 1 1 mixture of PhCH2NEt3+ BH4 and Me3SiCl, followed by addition of an aqueous solution of K2C03.152 For another method of anti-Markovnikov hydration, see 5-12. With substrates of the type C=C—Z (Z is as defined on p. 741) the product is almost always HO—C—CH—Z and the mechanism is usually nucleophilic,153 though electrophilic... 

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