Cycloalkanes heteroaromatics

In the Diels-Alder reaction with inverse electron demand, the overlap of the LUMO of the 1-oxa-l,3-butadiene with the HOMO of the dienophile is dominant. Since the electron-withdrawing group at the oxabutadiene at the 3-position lowers its LUMO dramatically, the cycloaddition as well as the condensation usually take place at room or slightly elevated temperature. There is actually no restriction for the aldehydes. Thus, aromatic, heteroaromatic, saturated aliphatic and unsaturated aliphatic aldehydes may be used. For example, a-oxocarbocylic esters or 1,2-dike-tones for instance have been employed as ketones. Furthermore, 1,3-dicarbonyl compounds cyclic and acyclic substances such as Meldmm s acid, barbituric acid and derivates, coumarins, any type of cycloalkane-1,3-dione, (1-ketoesters, and 1,3-diones as well as their phosphorus, nitrogen and sulfur analogues, can also be ap-... 

Few examples of preparatively useful intermolecular C-H insertions of electrophilic carbene complexes have been reported. Because of the high reactivity of complexes capable of inserting into C-H bonds, the intermolecular reaction is limited to simple substrates (Table 4.9). From the results reported to date it seems that cycloalkanes and electron-rich heteroaromatics are suitable substrates for intermolecular alkylation by carbene complexes [1165]. The examples in Table 4.9 show that intermolecular C-H insertion enables highly convergent syntheses. Elaborate structures can be constructed in a single step from readily available starting materials. Enantioselective, intermolecular C-H insertions with simple cycloalkenes can be realized with up to 93% ee by use of enantiomerically pure rhodium(II) carboxylates [1093]. 

Empirical additive substituent increments obtained by analysis of substituted alkanes, alkenes, cycloalkanes, aromatic and heteroaromatic compounds have proved to be useful for the prediction of 13C chemical shifts. These substituent increments will be tabulated for the various classes of organic compounds in Section 4.13. 

Recently, the C-H bond of nitrogen heteroaromatics has been shown to become activated upon the conversion of the heteroaromatic to its corresponding /V-oxide derivative (for recent examples of direct functionalization of pyridine /V-oxides and /V-iminopyridiniuin ylides, see [63-67]). Li and co-workers demonstrated that tert-butyl peroxides can mediate the oxidative coupling between pyridine /V-oxides 67 and cycloalkanes 62 (Scheme 36) [68]. 

Qu and Guo applied TBHP-promoted CDC to the reaction of cycloalkanes and various heteroaromatics such as purines (nucleosides), benzothiazoles, and benzoxazoles (Scheme 8.26). The reaction shows broad substrate scope, which is represented by the reaction with purine 69. Cycloall lated products such as 70 were obtained in good yields. 

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