Trigonal carbon atoms

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

The number of trigonal carbon atoms In the ring Is Important 

Sum of bond angles around the trigonal carbon atom 

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

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. 

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. 

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

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). 

Figure 11. Correlation of carbon-13 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.

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 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 

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