Carbon-heteroatom

Figure 10.3-40. The rating for the disconnection strategy carbon-heteroatom bonds is illustrated, Please focus on the nitrogen atom of the tertiary amino group. It is surrounded by three strategic bonds with different values. The low value of 9 for one ofthese bonds arises because this bond leads to a chiral center. Since its formation requires a stereospecific reaction the strategic weight of this bond has been devalued. In contrast to that, the value of the bond connecting the exocyclic rest has been increased to 85, which may be compared with its basic value as an amine bond.

The disconnection strategy Carbon-Heteroatom Bonds gives only one strategic bond, which is rated with a value of 100. 

You have already seen that a carbon-heteroatom bond is easy to make, since we used such bonds as natural places for disconnections (frames 234 ft). It is good strategy therefore to make a carbon-heteroatom bond and then to transform it into a carbon-earbon bond. The Claisen rearrangement is one way to do this an ortho allyl phenol (B) made from an allyl ether (A) ... 

Reductive Cleavage of Carbon-Heteroatom and HeCeroatom-Oxygen Bonds... 

T boron Carbon Heteroatom Extra hydrogens Charge Total Ref. 

The isomerization of vinyl- or ethynyl-oxiranes provides a frequently exploited source of dihydrofurans or furans, but analogous conversions of vinylaziridines have not been applied so often. While most of the examples in Scheme 87 entail cleavage of the carbon-heteroatom bond of the original heterocycle, the last two cases exemplify a growing number of such rearrangements in which initial carbon-carbon bond cleavage occurs. 

The advantage of starting with a ring of -1 members lies in the nature of the rearrangements, which proceed through cyclic transition states, so that the system never becomes open-chain — the carbon-carbon bond is broken only while the carbon-heteroatom bond is being made. 

The chiral naphthyloxazoline substrates can also be employed in asymmetric carbon-heteroatom bond-forming reactions with lithium amides, which provide unusual... 

Aromatic carbon-heteroatom coupling reactions with participation and formation of heterocycles 98JCS(P1)2615. 

Eormation in saturated 0-heterocycles via carbon-heteroatom bond-forming reductive elimination 98ACR852. 

Synthesis of A-arylpyrroles andA-arylpyrrolidines via carbon-heteroatom bondforming reductive elimination 98ACR852. 

Functionalization of pyridines via formation of carbon-heteroatom bond with elements of groups IV, V, and VI 99KGS437. 

Superacid media, HF/AsF5 and HF/SbF5, caused perfluorinated 1,2-oxazetidines to ring open by breaking the carbon heteroatom in preference to the N—O bond (89CJC1724). 

Cyclic systems have frequently been used in studies of chemical bonding and reactivity, reaction mechanisms and a variety of other problems of interest to chemists3. Their utility depends on the changes in the carbon-carbon and the carbon-heteroatom bonds as well as on steric and electronic effects that result from the introduction of heteroatoms into the system. Indeed, the carbon-heteroatom bond length in small rings shows an effective increase with increasing heteroatom electronegativity4, in line with a... 

Furan, Pyrrole, and Thiophene.-—The carbon-heteroatom distances found in furan, pyrrole, and thiophene correspond to 5 = = 5%, 12 = = 6%, and 17 = = 10% double-bond character, respectively. Resonance of the normal structure I with structures of the types II and III (X = O, NH, S) is assumed to be responsible for this double-bond character, while excited structures characteristic... 

The Diels-Alder cycloaddition is the best-known organic reaction that is widely used to construct, in a regio- and stereo-controlled way, a six-membered ring with up to four stereogenic centers. With the potential of forming carbon-carbon, carbon-heteroatom and heteroatom-heteroatom bonds, the reaction is a versatile synthetic tool for constructing simple and complex molecules [1], Scheme 1.1 illustrates two examples the synthesis of a small molecule such as the tricyclic compound 1 by intermolecular Diels-Alder reaction [2] and the construction of a complex compound, like 2, which is the key intermediate in the synthesis of (-)chlorothricolide 3, by a combination of an intermolecular and an intramolecular Diels-Alder cycloaddition [3]. 

The Diels-Alder reaction is the most widely used carbon-carbon, carbon-heteroatom and heteroatom-heteroatom bond-forming reaction for the construction of six-membered rings therefore it is not surprising that many methods have been used to accelerate the reaction and to improve its selectivity. Chapters 2, 3 and 5 illustrate the effects of temperature, Lewis acids and pressure, respectively this chapter provides a survey of other physical and chemical methods by which the Diels-Alder reaction can be profitably carried out. 

Transition-Metal-Based Carbon-Carbon and Carbon-Heteroatom Bond Formation for the Synthesis and Decoration of Heterocycles... 

Perhaps the largest class of nonaromatic unsaturated systems is that of the carbon-heteroatom double bonds, including as it does the carbonyl compounds. 

There is only one type of compound in this category, substituted nitriles. It will be useful to divide the data sets for substituent effects at carbon-heteroatom triple bonds into two classes, substituted nitriles and heteroethynylene sets. 

Results of Correlations for Carbon-Heteroatom Triple-Bond Sets... 

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