Wedged bonds

Similarly the stereobonds" can be defined and added to the bond list in the fourth column of the CT. A single bond acquires the value of 0 if it is not a "stereobond, 1 for np (a wedged bond). 4 for either up or down, and 6 for down (a basbed bond), The cisjtrans or E[Z configuration of a double bond is determined by the x,y.2 coordinates of the atom block if the value is 0, Tf it is 3, the double bond is either cis or tmns. In the bond block of our example (Figure 2-76), the stereocenter is set to 1 (up) at atom 6 (row 6, column 4 in the bond block), whereas the configurations of the double bonds are determined by the x,y coordinates of the atom block. 

Fischer projection formulas can be used to represent molecules with several stereogenic centers and are commonly used for caibohydrates. For other types of structures, a more common practice is to draw the molecule in an extended conformation witii the main chain horizontal. In this arrangement, each tetrahedral caibon has two additional substituents, one facing out and one in. The orientation is specified widi solid wedged bonds for substituents facing out and with dashed bonds for substituents that point in. 

In these views, the ring is roughly in the plane of the page, a wedged bond protrudes out of the page, and a dashed bond recedes into the page. Two substituents are cis if they are both out of or both into the page, and they are trans if one is out of and one is into. 

Figure 1.19 The structure and bond angles of ethene. The plane of the atoms is perpendicular to the paper. The dashed edge bonds project behind the plane of the paper, and the solid wedge bonds project in front of the paper.

In the trans isomer, one methyl is written down (dotted bond) whilst the other is written up (wedged bond). If we transform this to a chair conformation, as shown in the left-hand structure, the down methyl will be equatorial and the up methyl will also be equatorial. With ring flip, both of these substituents then become axial as in the right-hand conformer. From what we have learned about monosubstituted cyclohexanes, it is now easily predicted that the diequatorial conformer will be very much favoured over the diaxial conformer. 

It is also surprisingly easy to assign R ov S configurations to chiral carbons in the Fischer projections but, because horizontal lines imply wedged bonds (towards you) and vertical lines imply dotted bonds (away from you), there are important guidelines to remember ... 

Drawing a chiral molecule (enantiomer) On a plane paper, chiral molecules can be drawn using wedge bonds. There are also a few other methods that use horizontal bonds representing bonds pointing out of the paper and vertical bonds pointing into the paper. Some examples are given below. 

The geometry is tetrahedral at carbon. It does not matter whether we draw the wedge bonds to the right or to the left of the carbon, or indeed up or down . 

In the formula shown below for rodorubicin (a cytostatic agent) convert the groups shown in the Haworth projection into a planar projection of the rings in which the substituents are shown with wedged bonds. 

Dizocilpine is a chiral compound with two chirality centres. The methylated centre has the S configuration, whilst the other bridgehead atom is -configured. For clarification it is helpful to add hydrogen atoms or wedged bonds. Two other possible formulae for the structure are shown below, the last one of which, however, although frequently used is not recommended. 

RS Designations for Alanine and Lactate Draw (using wedge-bond notation) and label the (R) and (S) isomers of 2-aminopropanoic acid (alanine) and 2-hydroxypropanoic acid (lactic acid). 

Bond information includes the bond order, that is, single, double, or triple bond, and bond style, such as simple straight-line bond, wedged bond, dashed bond, wavy bond, broken-line bond, bold bond, and so on. Bond information can also include some special bond types representing the aromatic bonds or bond stereochemistry, for example, wedged or dashed bond. Bond labels are also sometimes found in structures. Different types of bond information are illustrated in Figure 4.2. 

The pixels in the original image of a line indicate the line width. The line width at the end points of a line is used to determine whether it is a wedged bond. This is done when a new connection is made between nodes. 

Strategy Two substituents are cis if they both have either dashed or wedged bonds. The substituents are trans if one has a wedged bond and the other has a dashed bond. 

Figure 2.1.2. The structure of methane, CH4. The straight-line bonds indicate hydrogens in the plane of the paper. The solid-wedge bond indicates a hydrogen coming out of the plane of the paper and the dashed-wedge bond indicates a hydrogen on the back side of the plane of the paper.

The wedge bonds represent bonds coming out of the plane of the paper... 

Reminder about Fischer projections, first introduced in Chapter 5, section 5-10 vertical bonds are equivalent to dashed bonds, going behind the plane of the paper, and horizontal bonds are equivalent to wedge bonds, coming toward the viewer. ... 

Figure 1. The structure of strychnine. A strychnine molecule shown (A) in line form and (B) as a space-filling representation. In this line diagram and others in this chapter, wedged bonds project above the plane of the paper and dashed bonds project into it.

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