Allylic alcohols synthetic utility

The AE reaction has been applied to a large number of diverse allylic alcohols. Illustration of the synthetic utility of substrates with a primary alcohol is presented by substitution pattern on the olefin and will follow the format used in previous reviews by Sharpless but with more current examples. Epoxidation of substrates bearing a chiral secondary alcohol is presented in the context of a kinetic resolution or a match versus mismatch with the chiral ligand. Epoxidation of substrates bearing a tertiary alcohol is not presented, as this class of substrate reacts extremely slowly. 

The Sharpless-Katsuki asymmetric epoxidation (AE) procedure for the enantiose-lective formation of epoxides from allylic alcohols is a milestone in asymmetric catalysis [9]. This classical asymmetric transformation uses TBHP as the terminal oxidant, and the reaction has been widely used in various synthetic applications. There are several excellent reviews covering the scope and utility of the AE reaction... 

The data presented demonstrate that allylic sulfoxides can provide an easy and highly stereoselective route to allylic alcohols taking advantage of the facility of the allylic sulfoxide-sulfenate [2,3]-sigmatropic rearrangement. This is of considerable synthetic utility, since a number of stereoselective and useful transformations of allylic alcohols and their derivatives have become available in recent years107-109. 

Metal-catalyzed C-H bond formation through isomerization, especially asymmetric variant of that, is highly useful in organic synthesis. The most successful example is no doubt the enantioselective isomerization of allylamines catalyzed by Rh(i)/TolBINAP complex, which was applied to the industrial synthesis of (—)-menthol. A highly enantioselective isomerization of allylic alcohols was also developed using Rh(l)/phosphaferrocene complex. Despite these successful examples, an enantioselective isomerization of unfunctionalized alkenes and metal-catalyzed isomerization of acetylenic triple bonds has not been extensively studied. Future developments of new catalysts and ligands for these reactions will enhance the synthetic utility of the metal-catalyzed isomerization reaction. 

Allyltitaniums from Allyl Halides or Allyl Alcohol Derivatives and Ti(ll) and their Synthetic Utility... 

The synthetic utility of the allyl sulfoxide-allyl sulfenate rearrangement is as a method of preparation of allylic alcohols.184 185 The reaction is carried out in the presence of a reagent, such as phenylthiolate or trimethyl phosphite, which reacts with the sulfenate to cleave the... 

Because of the great synthetic utility, asymmetric versions of the epoxidation of allylic alcohols have been developed and will be discussed in the following. Two methods of asymmetric conduction of the reaction are known. The first one is the employment of chiral catalysts and the second possibility is the use of chiral oxidants, which will be presented separately. 

Hi. Epoxidation of allylic alcohols with special synthetic utility or academic interest. In 1999, Adam and coworkers reported on the methyltrioxorhenium (MTO) catalyzed... 

The copper-catalyzed photobicyclization of acyclic 1,6-dienes to bicyclo[3.2,0]heptanes using the bis[copper(l) lrifluoromethanesulfonate]benzene complex has found general and synthetic utility in the conversion of diallyl and homoallyl vinyl ethers to 3- or 2-oxabicyclo[3.2.0]heptanes,5 6 of /V-allyl-A -2-methyl-2-propenecarbamates to iV-carboethoxy-3-azabicyclo[3.2.0]heptanes 7 and of allylic alcohols to the corresponding hydroxy-substituted bicyclo[3.2.0]heptanes.8 9 Examples of such reactions are summarized below. 

The synthesis of a -amino allylic alcohols is particularly difficult, yet this functionality is important in natural products (such as sphingosine) or as a synthon for further elaboration to amino sugars. In synthetic studies of this moiety204, the corresponding enones, with adjacent stereocenters, have been efficiently reduced to the allylic alcohol in quantitative yields and in both a regiocontrolled (1,2) and a stereocontrolled fashion (equation 53). The syn anti ratio of the product depends upon the hydride reductant and solvent being utilized. A 4 1 ratio was obtained with L-selectride and a 1 6 ratio obtained by the use of DIBAL in toluene. 

From the discussion above, the following conclusions can be drawn. Apart from some selected examples, the issue ofchemoselectivity and catalytic activity in iron-catalyzed allylic hydroxylation has not so far been solved. In particular, synthetically useful methods with a broad scope concerning alkene substrates are still lacking. Furthermore, in many cases it seems to be difficult to avoid overoxidation of the allylic alcohol to the corresponding enone. In addition, most published procedures utilize the alkene in a large excess (often as a solvent), thus limiting the use of functionalized alkenes which are not commercially available. 

This reaction, as written, would be catalytic in palladium chloride but in practice it is only partially catalytic because some of the palladium salt is reduced in a side reaction. The side reaction is the arylation of the product allylaromatic compound and this occurs because the product is more reactive towards the "arylpalladium chloride than allyl chloride is. This side reaction, producing 1,3-diaryipropenes, can be minimized by using an excess of the allylic chloride. The allylation and allylic alcohol arylation are both tolerant of the same variations in structure and substituents as is the arylation reaction and therefore are of considerable synthetic utility. 

The next example makes more involved use of these [2,3]-sigmatropic allylic sulfoxide-allylic alcohol rearrangements. It comes from the work of Evans (he of the chiral auxiliary) who, in the early 1970s, first demonstrated the synthetic utility of allylic sulfoxides. Here he is using this chemistry to make precursors of the prostaglandins, a family of compounds that modulate hormone activity within the body. 

The synthetic utility of the oxirane technology was further extended by the development by Sharpless and Katsuki [33, 34] of the titanium(lV) tartrate catalyst for the asymmetric epoxidation of allylic alcohols with TBHP (eq. (16)). 

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