Allylic rearrangements, transformation

Allylic rearrangement (Section 10.2) Functional group transformation in which double-bond migration has converted one allylic structural unit to another, as in ... 

However, the proposed in vivo transformation of anhydrovobasinediol to gelsemine might involve a series of more extensive alterations of the molecular skeleton which presumably would include (1) oxidation of the 2,7 double bond, (2) an acid-induced 2,7 shift, and (3) allylic rearrangement of the C-20 ethylidene side chain to a C-20 vinyl, so that a humantenine-type precursor... 

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). 

Cardenolides.1 Digitoxigenin (7) has been synthesized by reaction of a 3/J-benzyloxy a./3-unsaturated 17-ketone (2) with 1, followed by further transformations as shown. Allylic rearrangement of the initial product 3 results in 4. After reduction... 

Oxidation of the allylic carbon of alkenes may lead to allylic alcohols and derivatives or a, 3-unsaturated carbonyl compounds. Selenium dioxide is the reagent of choice to carry out the former transformation. In the latter process, which is more difficult to accomplish, Cr(VI) compounds are usually applied. In certain cases, mixture of products of both types of oxidation, as well as isomeric compounds resulting from allylic rearrangement, may be formed. Oxidation of 2-alkenes to the corresponding cc,p-unsaturated carboxylic acids, particularly the oxidation of propylene to acrolein and acrylic acid, as well as ammoxidation to acrylonitrile, has commercial importance (see Sections 9.5.2 and 9.5.3). 

Chloro compound 112, when treated with sodium methoxide, is methox-ylated at the ring (113) if an ortho methyl group is present, this is methox-ylated to give 114. This transformation is considered to be an allylic rearrangement with participation by 7r-electrons of the heteroaromatic system (84M1171). 

The direct transformation of silyl ethers into esters can also be performed by reaction with an acid chloride in CH2CI2 (0 °C to room temperature) in the presence of FeCls (1.5 equiv.) [18]. The formation of allyl ethers from allylic alcohols and methanol is catalyzed by RUCI3 [19]. The reaction is likely to occur via a jr-allyl Ru intermediate. Allylic rearrangements and racemization of optically active allylic alcohols take place. 

Another fascinating, unique property of allylic systems is their propensity for allylic rearrangement with transfer of functionality from one end of the allylic system to the other and concomitant double bond shift with or without transformation of functional groups. In the latter case (X = Y equation 1) a reaction is synthetically useful only in unsymmetrical cases for a 1,3-transposition of the functional group. 

A general discussion of such inherent problems and the principles of allylic rearrangements will be presented first, followed by specific examples of functional group transformations, which will be systematically surveyed regardless of the mechanism involved. 

A versatile and very powerful strategy for functional group transformation via allyl rearrangement involves intramolecular reactions of the 2,3- and 3,3-sigmatropic type, as exemplified in generalized form in Scheme 8. 

---