Double bonds defined

In this situation the two end-ring double bonds define a positive chirality. Application of the twisted diene rule gives a positive sign for the lowest-energy 1A - 1B transition, and in the case of 15-cA-(3R,3 R)-zeaxanthin this is confirmed by experiment. 

This is an example of a Diels-Alder reaction. This is an electrocyclic reaction where no charges are involved. While no charges are involved, electron pairs do move and their movement can be illustrated using arrow pushing. The mechanism, illustrated below, involves aligning cyclopentadiene (a diene) with methyl vinyl ketone (a dienophile) such that all three double bonds define a six-membered... 

The parameter K shown in the figure represents the reactivity of the terminal double bonds, defined by K=k lk, and k is the rate constant of TDBP. The... 

Defined against appropriate number of non-interacting double bonds < ) Defined against corresponding ionic linear polyene 2>3). 

Once the atoms arc defined, the bonds between them arc specified in a bond block. Each line of this block specifies which two atoms are bonded, the multiplicity of the bond (the bond type entry) and the stereo configuration of the bond (there arc also three additional fields that arc unused in Molfiles and usually set to 0). The indices of the atoms reflect the order of their appearance in the atom block. In the example analyzed, V relates to the first carbon atom (see also Figure 2-24). "2" to the second one, 3" to oxygen atom, etc. Then the two first lines of the bond block of the analyzed file (Figure 2-29) describe the single bond between the two carbon atoms C1-C2 and the double bond C2=0-5, respectively. 

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. 

Here i is now a double bond up to the seventh sphere (D,7 ) of non-rotatablc bonds centered on the proton, Td is the distance of the proton from the center of the double bond, and is the angle, in radians, between the plane defined by the bond and the distance q, (Figure 10.2-6a). 

The properties of these new materials are being studied. Hydroboration is also appHed for the conversion of double bonds in polymers into hydroxyl groups (450—454). Well-defined copolymers of ethylene—vinyl alcohol can be prepared (455). 

Most parent structures consist essentially of an assembly of rings and/or chains, the degree of hydrogenation of which is defined (usually completely saturated or containing the maximum number of non-cumulative double bonds in cyclic portions), and having no attached functional substituents (the carbohydrates are a notable exception to this). The stereochemistry at all (or most) chiral centres is defined thus such parent structures are sometimes referred to as stereoparents . Some examples are shown (77)-(83). 

The NMR spectra of the product do not show these features. The highest C shift value is Sc = 160.9 and indicates a conjugated carboxy-C atom instead of the keto carbonyl function of an isoflavone (5c =175). On the other hand, a deshielded CH fragment at 5c/<5 = 138.7/7.i52 appears in the C NMR spectrum, which belongs to a CC double bond polarised by a -A/effect. The two together point to a coumarin 4 with the substitution pattern defined by the reagents. 

Unsaturated -lactone 34 adopts a well-defined conformation and provides a suitable platform for the introduction of the stereogenic center at C-24 (monensin numbering). Catalytic hydrogenation of the carbon-carbon double bond in 34 takes place preferentially from the less hindered side of the molecule and provides an 8 1 mixture of stereoisomers in favor of 35 (100% yield). Cleavage of -lactone 35 with concentrated hydriodic acid at 130°C, followed by treatment of the resultant iodide 36 with triphenylphosphine, completes the synthesis of intermediate 19. 

Methodology for the preparation of preformed enolates with well-defined double bond configuration was developed and it was shown that simple diastereoselection can be controlled, in many instances, through the use of these enolates 3. 

Initiation is defined as the series of reactions that commences with generation of primary radicals and culminates in addition to the carbon-carbon double bond of the monomer so as to form initiating radicals (Scheme 3.1... 

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