Retro Ene reactions

The retro-ene reaction also is of synthetic importance. While the application of high pressure facilitates the ene reaction, the retro-ene reaction is favored at higher temperatures. Furthermore small-ring strain can shift the equilibrium towards the side of the dienes. The vinylcyclopropane 11 rearranges by a synchronous process to the open-chain diene 12. Formally this process is the reverse of an intramolecular ene reaction ... 

Hydroxyalkenes are especially suitable starting materials for the retro-ene reaction since a stable carbonyl compound is then released as product. The retro-ene reaction of /3-hydroxyalkynes, e.g. 13 14, can be used for the preparation of... 

With a substitution pattern as found in l-vinyl-2-methylcyclopropane 7, a retro-ene reaction (see Ene reaction) may take place to yield hexa-1,4-diene 8 ... 

Retro-ene reaction of an N-allylsilanamine under flash vacuum thermolysis (FVT) conditions. The iminosilane has been identified not only by reaction with the trapping agent /-butyl alcohol, but also directly by coupling the oven with a high-resolution mass spectrometer.7... 

In this section are described those domino reactions which start with a retro-pericy-clic reaction. This may be a retro-Diels-Alder reaction, a retro-l,3-dipolar cycloaddition, or a retro-ene reaction, which is then usually followed by a pericyclic reaction as the second step. However, a combination is also possible with another type of transformation as, for example, an aldol reaction. 

DFT studies of the intramolecular ene-like (or the so-called 1,3-dipolar ene) reaction between nitrile oxides and alkenes show that this reaction is a three-step process involving a stepwise carbenoid addition of nitrile oxide to form a bicyclic nitroso compound, followed by a retro-ene reaction of the nitrosocyclopropane intermediate. The competitive reactions, either the intramolecular [3 + 2] cycloaddition between nitrile oxides and alkenes or the intermolecular dimerization of nitrile oxides to form furoxans, can overwhelm the intramolecular 1,3-dipolar ene reaction if the tether joining the nitrile oxide and alkene is elongated, or if substituents such as trimethylsilyl are absent (425). 

An interesting pericyclic-anionic-pericyclic domino reaction showing a high stereoselectivity is the cycloaddition-aldol-retro-ene process depicted in scheme 20.1581 The procedure presumably starts with a [4+2]-cycloaddition of diene 98 and S02 in presence of a Lewis acid. After opening of the formed adduct reaction with (Z)-silyl vinyl ether 99 leads to a mixture of alk-2-enesulfinic acids 101. It follows a retro-ene reaction which affords a 7 3 mixture of the products 102 and 103. The reaction described by Vogel et al is a nice example for the efficient generation of polypropionate chains with the stereoselective formation of three stereogenic centers and one (0-double bond in a three-component domino reaction in its strict definition. 

Retro-Diels-Alder strategy, 21 148 Retro-ene reactions, 21 150 Retrofit (revamp) design, 20 724 Retrofitted passive heat-transfer... 

While there seem to exist only a few examples for Reaction C in Scheme 1.1 [585], which mostly are photochemical [586-595], these retro-ene reactions can be considered as the purely intramolecular case of a substitution described in Reaction G. The thermolysis of 177 delivers 178 by such a retro-ene reaction [596] (Scheme 1.79). 

The cyclobutanone 105, on thermolysis at 190°C, undergoes a retro-ene reaction yielding the crystalline 0,y-unsaturated ketone 115. This product in the solid regenerates 105 photochemically, in almost quantitative yield, whereas in solution the diketone 116 is the exclusive photoproduct. The ketone 115 is found in the solid to have the conformation 115a, a conformation that results, presumably, from the requirement for the bulky methyl at C-7 to adopt the pseudo-equatorial rather than the pseudoaxial position. This molecule in fact has a shape... 

Alternatively, a retro-ene reaction cleaves the O10-C11 bond and gives a highly unsaturated ketene. The ketene can undergo cycloaromatization to give a diradical intermediate. H- abstraction and radical-radical recombination then give the product. 

The first of these reactions requires a y-hydrogen, and thus at least one Cs-alkyl substitutent. It may either be regarded as the even-electron analogy of the McLaf-ferty rearrangement (McL, Chap. 6.7) as it will be treated here, [85,86,142] or alternatively, as a retro-ene reaction. [143,144]... 

Soiling, T.I. Hammerum, S. The Retro-Ene Reaction of Gaseous Immonium Ions Revisited. J. Chem. Soc., Perkin Trans. 2 2001, 2324-2428. 

Allenic esters (165) have been found to undergo a retro-ene reaction on flash vacuum thermolysis above 800 °C to give unsubstituted vinylketene together with formaldehyde or acetaldehyde (see Scheme 59). 

Superscript symbols are provided to facilitate an understanding of the reaction. The reverse of an ene reaction is often referred to as the retro-ene reaction. 

While a hydroxymethyl group has been succesfully used to temporarily block the lactam nitrogen of halopyridazin-3(2//)-ones such as 191 for Suzuki reactions <2003TL4459>, and the resulting aryl-2-hydroxymethylpyridazin-3(2//)-ones have been immediately deprotected in situ via a base-induced or thermal retro-ene reaction (Equation 39), two recent reports show that Suzuki reaction on unprotected substrates like 5-chloropyridazin-3(2//)-one 192 is feasible (Scheme 47) <2005JME6004, 2006JME2600>. 

When heated at 230 °C, tetrahydro-l,3-oxazines 173, bearing an a,/3-unsaturated ester substituent at position 2, underwent a retro-ene reaction and yielded 5,6-dihydro-477-l,3-oxazines 174 together with methyl 4-methyl-3-pentenoate 175 (Equation 16). For the methyl-substituted oxazine 173 (R = Me), the yields of both products were somewhat higher than those in the reaction of the unsubstituted analog 173 (R = H) <1997JHC501>. 

Ramberg-Backlund reaction 128 Raney nickel 75,78,81,95,122,146 Reformatsky reaction of thiocarbonyl compounds 142 Retro-ene reaction 161... 

Reviews of the ene reactions with thiocarbonyl compounds have been published [120, 203], The sulfur retro-ene reactions are well covered in a general review on the synthetic applications of the retro-ene reaction [513],... 

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