Allylic rearrangement, also

Allylic rearrangement also provides a means of interconverting propargylic and allenic boranes. ... 

Dramatic rate accelerations of [4 + 2]cycloadditions were observed in an inert, extremely polar solvent, namely in5 M solutions oflithium perchlorate in diethyl ether(s 532 g LiC104 per litre ). Diels-Alder additions requiring several days, 10—20 kbar of pressure, and/ or elevated temperatures in apolar solvents are achieved in high yields in some hours at ambient pressure and temperature in this solvent (P.A. Grieco, 1990). Also several other reactions, e.g, allylic rearrangements and Michael additions, can be drastically accelerated by this magic solvent. The diastereoselectivities of the reactions in apolar solvents and in LiClO EtjO are often different or even complementary and become thus steerable. 

Carboxylic acids react with butadiene as alkali metal carboxylates. A mixture of isomeric 1- and 3-acetoxyoctadienes (39 and 40) is formed by the reaction of acetic acid[13]. The reaction is very slow in acetic acid alone. It is accelerated by forming acetate by the addition of a base[40]. Addition of an equal amount of triethylamine achieved complete conversion at 80 C after 2 h. AcONa or AcOK also can be used as a base. Trimethylolpropane phosphite (TMPP) completely eliminates the formation of 1,3,7-octatriene, and the acetoxyocta-dienes 39 and 40 are obtained in 81% and 9% yields by using N.N.N M -tetramethyl-l,3-diaminobutane at 50 in a 2 h reaction. These two isomers undergo Pd-catalyzed allylic rearrangement with each other. 

Ether groups in the benzene ring of quinazoline behave as in ethers of homocyclic aromatic compounds, e.g., they can be demethylated with anhydrous aluminum chloride. Allyl ethers also undergo a Claisen rearrangement/ ... 

Substituted TMM complexes also cycloadd to aldehydes in the presence of a tin cocatalyst such as MesSnOAc and MesSnOTs [31]. Reaction of 2-heptenal with methyl precursor (6) gave a mixture of methylenetetrahydrofurans (68) and (69). This regioselectivity is reversed with 10-undecenal and methyl precursor (5), where adduct (70) now predominates over (71). As in the carbocyclic system, the phenylthio group also functions as a regiocontrol element in reaction with cyclohexyl aldehyde. The initially formed adduct (72) eliminates the element of thio-phenol on attempted allyl rearrangement, and the overall process becomes a cycloaddition approach to furans (Scheme 2.21) [20]. 

The study by Baechler and coworkers31, cited above, also provided data on the (1,3)-allylic rearrangement in /J-methylallyl phenyl sulfoxide. Using the same approach as was used in reinterpreting the sulfone data, the activation energy is estimated to be 151 kj mol- and AfH°(PhSO) = 45kJmol-1. 

The 1,3-allylic rearrangement has also been observed with several cyclic sulfoxides. For example, the interesting thermal interconversion of the bicyclic stereoisomeric pair 141... 

Nucleophilic substitution at an allylic carbon can also take place by an Sn2 mechanism, in which case no allylic rearrangement usually takes place. However, allylic rearrangements can also take place under Sn2 conditions, by the following mechanism, in which the nucleophile attacks at the y carbon rather than the usual... 

Allylic rearrangements have also been demonstrated in propargyl systems, for example,... 

LiAlH4. When the substrate is an allylic alcohol, the reaction is not regiospecilic, but a mixture of normal coupling and allylically rearranged products is found. A free-radical mechanism is involved. The TiCla—LiAlUj reagent can also convert... 

Double-bond isomerization can also take place in other ways. Nucleophilic allylic rearrangements were discussed in Chapter 10 (p. 421). Electrocyclic and sigmatropic rearrangements are treated at 18-27-18-35. Double-bond migrations have also been accomplished photochemically, and by means of metallic ion (most often complex ions containing Pt, Rh, or Ru) or metal carbonyl catalysts. In the latter case there are at least two possible mechanisms. One of these, which requires external hydrogen, is called the nwtal hydride addition-elimination mechanism ... 

Treatment of double-bond compounds with selenium dioxide introduces an OH group into the allylic position (see also 19-14). This reaction also produces conjugated aldehydes in some cases.Allylic rearrangements are common. There is evidence that the mechanism does not involve free radicals but includes two pericyclic steps (a and... 

A recent study has also found evidence of reductive elimination of 2-propenylimidazolium salt from a [Pd(7i-allyl)(NHC)]" complex, probably following allyl rearrangement (Scheme 13.3) [29], The elimination reaction generated L-Pd(O) in situ, which could then catalyse a novel C-C couphng process. 

It has also been reported" " that rearrangements of both a- and y-propylallyl triflinates on heating in acetonitrile yield the same sulfone y-propylallyl triflone. Although the possibility of an ion-pair mechanism may be responsible for the lack of allylic rearrangement in one case as a result of the better leaving group ability of the triflinate anion as compared with the arenesulfmate anion, it is just as likely a consequence of the unbuffered conditions in which these reactions were performed. In this respect this is reminiscent of the results observed by Cope and coworkers" mentioned above which were also performed under nonbuffered conditions, and which could be simply corrected by the addition of 2,6-lutidine . 

Dihydro-2H-pyran-2-ones (e. g., 4-195) are valuable intermediates in the synthesis of several natural products [67]. Hattori, Miyano and coworkers [68] have recently shown that these compounds can be easily obtained in high yield by a Pd2+-catalyzed [2+2] cycloaddition of ct, 3-unsaturated aldehydes 4-192 with ketene 4-193, followed by an allylic rearrangement of the intermediate 4-194 (Scheme 4.42). In this reaction the Pd2+-compound acts as a mild Lewis acid. a,(3-unsaturated ketones can also be used, but the yields are below 20%. 

Tochtermann reported the addition of dichlorocarbene to the racemic glycal 52, whose cyclopropyl-allyl rearrangement leads to the 2//-pyran. The synthesis of the optically pure (+)-(2S,3R,7S) and (-)-(2f ,3S,7f )-glycal precursors has also been achieved. As pointed out, optically pure glycals are versatile precursors for carbohydrate synthesis <00EJO1741>. 

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