In ene reactions

Thermal and photochemical electrocyclic reactions are particularly useful in the synthesis of alkaloids (W. Oppolzer, 1973,1978 B K. Wiesner, 1968). A high degree of regio- and stereoselectivity can be reached, if cyclic olefin or enamine components are used in ene reactions or photochemical [2 + 2]cycloadditions. 

The 27T-electrons of the carbon-nitrogen double bond of 1-azirines can participate in thermal symmetry-allowed [4 + 2] cycloadditions with a variety of substrates such as cyclo-pentadienones, isobenzofurans, triazines and tetrazines 71AHC(13)45). Cycloadditions also occur with heterocumulenes such as ketenes, ketenimines, isocyanates and carbon disulfide. It is also possible for the 27r-electrons of 1-azirines to participate in ene reactions 73HCA1351). 

Calculations of the reactivity of model nitrocarbenes (77a) demonstrated that these species A much more rapidly undergo a rearrangement to give nitrosocar-bonyl intermediates B than are involved in [3+ 2]-cycloaddition with an external trapping agent. Shortly thereafter, this fact was confirmed experimentally (77b) because, at the same time, intermediates B can be detected in ene reactions with certain olefins to give hydroxylamines (25) (Scheme 3.25). 

Diels-Alder adduct from cyclopentadiene, 8 222t Diels-Alder reactions of, 25 488-489 economic aspects of, 25 507-509 electrophilic addition of, 25 490 in ene reactions, 25 490 esterification of, 25 491 free-radical reactions of, 25 491 from butadiene, 4 371 Grignard-type reactions of, 25 491 halogenation of, 15 491—492 health and safety factors related to, 25 510-511... 

The procedure described here provides ready access to both the (+)- and (-) antipodes of trans-2-phenylcyclohexanol, a useful chiral auxiliary in ene reactions of its glyoxylate ester4 and its N-sulfinylcarbamate,5 as well as in cycloaddition reactions... 

Phosphaalkynes have been found to participate readily in ene reactions, both with alkylidenecyclopropanes and with allenes, to form phosphaalkenes and phosphanes,... 

The most reactive electrophiles for the ene reactions are probably the nitroso compounds, as even nonactivated aliphatic nitroso compounds have been reported to undergo the ene reaction at room temperature [55]. Some nitroso compounds which have been applied in ene reactions are depicted in 92-97 [52b, 56]. One of the most reactive electrophiles 97 is normally made in situ because of the extremely high reactivity, but the other nitroso compounds are reasonably stable. 

Outstanding properties are the transformation to 1H- or 2/f-phosphirenes after carbene addition (9->- i0),12b 21 22 [3 + 2]-cycloadditions of 1,3-dipoles leading to a wide variety of heteroatom-substituted phospholes (9 — ll)18 and Diels-Alder reactions (9 - 12) that make not only the phosphinines but also their valence isomers accessible.23,24 In ene reactions phosphaalkynes... 

A recent report details absolute stereochemical control at levels of up to 75 25 in ene reactions, although typical levels are lower35. This selenium reagent has two identical, chiral sulfur atoms and it is these stereogenic centers that provide for reagent-based control of absolute stereochemistry. 

There are a few examples in which imidazolines act as dienophiles in Diels-Alder reactions (81TL2063), while some related compounds can also react with monoalkenes in ene reactions. Nucleophiles may also add across the C=N bond (Schemes 86 and 87) (70JOC3097). 

Surprisingly few examples of the use of allenes as the ene components in ene reactions have been reported. In a series of papers, Taylor and coworkers reported that alkylallenes undergo thermal ene reactions with DMAD, hexafluoro-2-butyne and 3,3,3-trifluoropropyne to give cross-conjugated trienes. Similar reactions occur between allenes and hexafluoroacetone and electron-deficient azo compounds. " ... 

Azodicarboxylates are recognized for their ability to participate as 2ir components of HOMOdi e-con-trolled Diels-Alder reactions with dienes and for their participation in ene reactions with reactive al-kenes. 489 j, addition, electron-rich or reactive simple alkenes that do not contain a reactive allylic hydrogen atom have been shown to participate in competitive [2 + 2] and [4 + 2] cycloaddition reactions with azodicarboxylates in which the observed course of the reaction is dependent upon the solvent polarity and nucleophilic character of the alkene (Table 13). As may be anticipated, alkoxycarbonyl-aroyldiimides (4), diaroyldiimides (5), and aiylaroyldiimides (6) participate with increasing selectivity as 4tr components of [4 + 2] cycloaddition reactions. 

The Lewis acid-catalyzed ene reaction is synthetically useful methodology for forming new carbon-carbon bonds [85]. Ene reactions utilizing reactive enophiles such as formaldehyde and chloral can be promoted by SnCU SnCU also enhances intramolecular ene reactions (Eq. 52) [86,87], Proton scavenging aluminum Lewis acids such as RAICU are most often used in ene reactions to eliminate proton-induced side reactions. 

Reaction of alkenes with carbonyl compounds or carbonyl derivatives in the presence of Lewis acids, the ene reaction, enables the stereoselective preparation of highly functionalized compounds. Copper Lewis acids activate both aldehydes and imines in ene reactions. Evans has reported that Cu(II) Lewis acids catalyze glyoxylates in reactions with alkenes (Sch. 56) [103]. The homoallylic alcohols 257 and 259 are produced in high yield and enantioselectivity. The bis aquo complex 260 is a readily prepared and air-stable catalyst and gave high chemical yield and excellent selectivity in the ene reactions. Another point of note is that catalysts 260 and 261 furnish enantiomeric products even though they differ from each other only by the substituent at the 4-posi-tion of the oxazoline. 

Asymmetric ene Reaction In 1988 Yamamoto and coworkers provided the first indication that asymmetry in ene-reactions could be induced by catalytic amounts of chiral Lewis acids in the presence of 4-A molecular sieves (Scheme 6.64) [88]. They described the first example of asymmetric ene-reaction between prochiral, halogenated aldehydes and alkenes catalyzed by chiral binaphthol-derived aluminum complexes. The hindered 3,3-silyl substituents in the chiral catalyst are essential to achieve good enantioselectivity and high yield. In fact, the use of a catalyst derived from MesAl and 3,3 -biphenylbinaphthol led to the racemic product in a low yield. 

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