Trigonal carbon atoms

Fumaric acid is converted to L-malic acid by hydration in the presence of the enzyme fumamse. From the structure of the substrate and the configuration of the product, it is apparent that the hydroxyl group has been added to the si fiice of one of the carbon atoms of the double bond. Each of the trigonal carbon atoms of an alkene has its fiice specified separately. The molecule of fumaric acid shown below is viewed fixjm the re-re fiice. 

The units by which crystallographers describe interatomic distances are Angstrom units (A = 10 8 cm.). Normal values for carbon-carbon interatomic distances are 1.34 A for a double bond (as in ethylene) and 1.54 A (as for-diamond) for a single bond. In a truly aromatic compound (such as benzene) the C-C bond length, as mentioned above, is 1.39 A. C-C-C angles are 109.5° for a tetrahedral carbon atom (sp3) and 120.0° for a trigonal carbon atom (sp2). 

The first asymmetric total synthesis of (+)-lycorine is outlined in Scheme 15. While our earlier applications of the Birch reduction-alkylation of chiral benzamide 5 were focused on target structures with a quaternary stereocenter derived from C(l) of the starting benzoic acid derivative, the synthesis of 64 demonstrates that the method also is applicable to the construction of chiral six-membered rings containing only tertiary and trigonal carbon atoms. s... 

Figure 10. Correlation of proton chemical shifts of methyl groups in carbonium ions with carbon-13 shifts of the adjacent trigonal carbon atoms. See Table 3 for data and references, o Aryldimethylcarbonium ions cycloalkcnyl cations a phenylmethyl-carbonium ions.

Figure 12. Correlation of carbon-13 and proton chemical shifts for trigonal carbon atoms in a number of cations, anions, and neutral molecules.

Trigonal carbon atoms 680, 681 Trigonal prochiral centers 480,481 Triiodothyronine 572 Trimethoprim 805s... 

The two faces of a compound containing a trigonal carbon atom are described as re (rectus) and si (sinister) by a complicated set of rules. Two simple cases are illustrated by the faces presented to the reader by structures 8.6. 

Sum of bond angles around the trigonal carbon atom... 

In contrast to closed shell molecules, free radicals are species which have an odd number of electrons. Simply speaking, all electrons in free radical species are considered to be paired up, except for one orbital which contains the single electron. The molecular orbital which describes the distribution of this odd electron is called the SOMO (singly occupied MO). In the ground state of the radical, the SOMO is also the HOMO. In a carbon-based free radical the SOMO is generally strongly localized to a trigonal carbon atom. 

Remarkably, some hydrazones may be doubly lithiated with the second lithiation occurring a to N at the trigonal carbon atom (101).68 Doubly lithiated hydroazones of a different kind are intermediates in the Shapiro reaction (section 8.1). 

Nucleophiles add to carbonyl groups to give compounds in which the trigonal carbon atom of the carbonyl group has become tetrahedral. 

The number of trigonal carbon atoms In the ring Is Important... 

Six-membered rings with two or more trigonal carbon atoms undergo axial attack in order to form chairs rather than boats. The final product may end up with axial or equatorial substitution, but this is not a consideration in the reaction itself... 

Attack from the apparently more hindered bottom face makes the trigonal carbon atom turn tetrahedral in the opposite sense by forming a vertical bond to the electrophile downwards. The ring goes directly to a chair form with the electrophile in the axial position. 

Just as stereogenic centres can be described as ft or S, it is possible to assign labels to the enantiotopic groups at prochiral tetrahedral carbon atoms or the enantiotopic faces of prochiral trigonal carbon atoms. The basis of the system is the usual ft,S system for stereogenic centres, but j ro-ftand pro-S are used for groups and fte and S/for faces. 

The essence of asymmetric synthesis is the creation of asymmetric centers under the influence of other asymmetric centers in such a way that the resulting enantiomers or diastereoisomers are formed in unequal proportions. Most reactions in asymmetric synthesis that have been described involve the conversion of trigonal carbon atoms into asymmetric, quadrivalent carbon atoms, and this article will be principally concerned with such reactions, although, in many instances, the principles involved may also be applied to asymmetric reactions in which, for example, chiral phosphorus or sulfur atoms are formed. In all reactions in which are formed mixtures of enantiomers having one enantiomer in preponderance, it is possible to describe the stereoselectivity of the reaction in terms of optical yield (optical purity, or enantiomeric yield). The precise significance of these terms has been described in detail elsewhere,1 but, practically, where at a selected wavelength, [a] is the specific rotation of the reaction product and [A] is the specific rotation of a pure enantiomer, the optical yield = [a]/[A]. Thus, the value of the optical yield is a measure of the excess of one enantiomer over the other. 

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