Dienophile enamine-containing

The possibilities of the use of enamines containing auxiliaries as dienophiles in tandem [4 + 2] [3 + 2] reactions were not studied in detail. However, the results of the study (117) showed that such processes can occur (Scheme 3.183). 

Intermolecular [4C+2S] cycloaddition reactions where the diene moiety is contained in the carbene complex are less frequent than the [4S+2C] cycloadditions summarised in the previous section. However, 2-butadienylcarbene complexes, generated by a [2+2]/cyclobutene ring opening sequence, undergo Diels-Alder reactions with typical dienophiles [34,35] (Scheme 59). Also, Wulff et al. have described the application of pyranylidene complexes, obtained by a [3+3] cycloaddition reaction (see Sect. 2.8.1), in the inverse-electron-demand Diels-Alder reaction with enol ethers and enamines [87a]. Later, this strategy was applied to the synthesis of steroid-like ring skeletons [87b] (Scheme 59). 

Dienes containing heteroatoms such as N and O undergo inverse electron-demand Diels-Alder reactions with electron-rich dienophiles such as enol ethers and enamines. The relatively low energy of the heteroatom p orbitals dramatically lowers the energy of both the HOMOdiene and LUMOdiene-... 

Over the last twenty years, the cycloaddition reactions of 1,2,4-triazines have been studied intensively. 1,2,4-Triazincs arc clcctron-dcficicnt systems and readily undergo Diels-Alder reactions with inverse electron demand with electron-rich dienophiles, such as enamines and ynamines, or with systems containing strained double bonds. Inter- and intramolecular cycloaddition reactions have been observed, giving pyridines and pyrimidines or condensed pyridines and pyrimidines. 

Boger initially developed the aforementioned methodology to construct penta-substituted pyridine 216 in the formal total synthesis of antitumor antibiotic streptonigrin 217. " The requisite 1,2,4-triazene 214 was generated via a Diels-Alder/retro-Diels-Alder sequence between nitrogen-containing dienophile 212 and 1,2,4,5-tartazine 213. Exposure of enamine 214 to 215 resulted in the formation of 216. Similar chemistry was also applied in the synthesis of the structurally related lavendamycin 217. ... 

An unusual approach to the synthesis of chain-fluorinated diazines relies on the inverse-electron-demand hetero-Zretro-Diels - Alder ihDA/rDA) sequence. The background of this method for the preparation of nitrogen-containing heterocycles in general has been reviewed recently [674], Typical dienes used for the synthesis of chain-fluorinated diazines are given in Fig. 29. Since electron-deficient dienes are necessary for the first step of the sequence - inverse-electron-danand hetero-Diels -Alder reaction, fluoroalkyl substituents of tri- and tetrazines 1111-1112 are favorable for the process. Typical electron-rich dienophiles for the reactions with 1111-1112 are enamines (including amino heterocycles) and alkynes, although other examples are also known. 

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