Allyl retro-allylation

From a stereochemical point of view, compound 35 is rather complex, for it possesses four contiguous oxygen-bearing stereocenters. Nonetheless, compound 35 is amenable to a very productive retro-synthetic maneuver. Indeed, removal of the epoxide oxygen from 35 furnishes trans allylic alcohol 36 as a potential precursor. In the synthetic direction, SAE of 36 with the (+)-dialkyl tartrate ligand would be expected to afford epoxy alcohol 35, thus introducing two of the four contiguous stereocenters in one step. 

Removal of the carbonate ring from 7 (Scheme 1) and further functional group manipulations lead to allylic alcohol 8 which can be dissected, as shown, via a retro-Shapiro reaction to give vinyl-lithium 9 and aldehyde 10 as precursors. Vinyllithium 9 can be derived from sulfonyl hydrazone 11, which in turn can be traced back to unsaturated compounds 13 and 14 via a retro-Diels-Alder reaction. In keeping with the Diels-Alder theme, the cyclohexene aldehyde 10 can be traced to compounds 16 and 17 via sequential retrosynthetic manipulations which defined compounds 12 and 15 as possible key intermediates. In both Diels-Alder reactions, the regiochemical outcome is important, and special considerations had to be taken into account for the desired outcome to. prevail. These and other regio- and stereochemical issues will be discussed in more detail in the following section. 

Hydrocyanation of butadiene is more complicated than that of ethene it requires two hydrocyanation steps and several isomers can be observed. The isomers obtained in the first step of the HCN addition to butadiene are shown in Figure 11.3. The addition first leads to compounds 1 and 2, in a 1 2 ratio, but they equilibrate to a favourable 1 9 ratio via the retro-reaction. The retro reaction involves a C-C bond breaking reaction, which is rare, but in this case the intermediate is a Tt-allyl species and a stable, anionic cyanide group. Electron-rich nickel species (Ni-dippe) can cleave aromatic nitrile C-C bonds... 

However, if only two carbon atoms are present (15) they may be disconnected to give an allylic alcohol 16 and the acetoacetic ester, through a retro-Carroll rearrangement [6] (Scheme 7.6). 

The retrosynthesis involves the following transformations i) isomerisation of the endocyclic doble bond to the exo position ii) substitution of the terminal methylene group by a more stable carbonyl group (retro-Wittig reaction) iii) nucleophilic retro-Michael addition iv) reductive allylic rearrangement v) dealkylation of tertiary alcohol vi) homolytic cleavage and functionalisation vii) dehydroiodination viii) conversion of ethynyl ketone to carboxylic acid derivative ix) homolytic cleavage and functionalisation x) 3-bromo-debutylation xi) conversion of vinyl trimethylstannane to methyl 2-oxocyclopentanecarboxylate (67). 

Base removes bepzylic H, retro- allyl anion+alkene, all-s gives trans-cyclooctene retro [2+2], s on both a-bonds makes trani-double bond. This is an exercise in drawing believable transition structures like 6.42. 

It is well known that thermal decomposition of allyl-substituted silanes proceeds by retro-ene reaction with formation of transient species having a Si=C bond, such as silaben-zene, silatoluene and dimethylsilaethylene4b e. The kinetic data on the gas-phase pyrolysis of a similar allyloxysilane derivative, (l,l-dimethylallyloxy)dimethylsilane (16), and the results on thermolysis of allyloxydimethylsilane (17) in a flow system both indicate the participation of an intermediate silanone, (CH3)2Si=0 (10), as shown in Scheme 523. 

Thioformyl cyanide was generated under FVT conditions by retro-ene cleavage of allyl cyanomethyl sulfide as shown in equation 99. Spectroscopic properties of this compound are discussed in Section II. Note that thioformyl cyanide had already been prepared by retro-Diels-Alder reaction (see Section III F 4)... 

In similar way, the simplest a-oxothione was obtained by retro-ene reaction of the corresponding allyl sulfide379 (equation 100) and characterized by IR spectroscopy. Upon FVT at higher temperatures (R = Me, 900-1000 °C) subsequent loss of carbon monoxide took place yielding thioacetone379. 

A homoallyl alcohol has been used to generate an allylrhodium species via retro-allylation.173 Subsequent reaction with an aldehyde (RCHO) yields the corresponding secondary alcohol, R-CH(OH)CH2-C(Me)=CH2, in situ. This can be isomerized in the same pot to yield saturated ketone, R-C(=0)CH2-CHMe2. 

A procedure has been developed for the palladium-catalysed a-arylation of amides by aryl bromides using the zinc enolates of the amides. The reaction works well with bromoarenes carrying a variety of ring substituents and with bromopyridine. In addition, the reaction has been shown to be effective with morpholine amides to give products which are precursors for aldehydes and ketones.39 A new method has been reported for the allylation of aryl halides using homoallyl alcohols as the allyl source the palladium-catalysed reaction, which may be both stereo- and regio-sepecific, uses a retro-allylation reaction to form a a-allyl(aryl)palladium intermediate.40... 

Treatment of silyl ethers of allyl alcohol, and some simple derivatives, with t-BuLi in THF-HMPA, followed by a suitable electrophile, has been shown to give the 1,4-retro-Brook rearrangement products, which are ethers, carbonates or carbamates of... 

In a gallium-mediated allyl transfer process, bulky gallium homoallylic alkoxides have been retro-allylated to generate (Z)- and ( )-crotylgallium reagents stereo- (g) specifically.195 Immediate reaction with aromatic aldehydes gives erythro- and threo-homoallylic alcohols. 

The mechanism and kinetic aspects of the retro-ene reaction of the allyl n-propyl sulfide and its deuterated derivatives have been studied using four different types of DFT methods with eight different levels of the basis sets.7 The mechanistic studies revealed that the reaction proceeds through an asynchronous concerted mechanism. Theoretical calculations have indicated that the reaction displays a kinetic isotope effect of 2.86 at 550.65 K. 

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