Rearrangement with Stereochemical Control

This study was made in the context of the antithrombotic activity of unsaturated compounds isolated from garhc, such as ajoene [107]. 

The electron richness of the sulfur atom can be further decreased by introduction of a higher oxidation state. The thermolysis of allyl vinyl sulfone, at rather high temperature, was shown to give the corresponding sulfene, which was trapped by appropriate reagents [113]. 

Very little information is available about the effect of catalysts towards the thio-Claisen transposition. Meyers and his group had noticed this, and screened a variety of additives. Palladium(II) and nickel(II) complexes (10mol%) significantly accelerated the rearrangement of hindered keteneaminothioacetals, which could be performed at 25 °C, instead of heating to 140 °C without catalyst [114]. Some variation of the diastereoselectivity was observed. The authors proposed that palladium(II) coordinates to the allylic double bond. The effect of CeCh has also been reported [115]. 

The thio-Claisen rearrangement has been successfully used for stereoselective carbon-carbon bond formation. Depending on the substitution of the pericycHc nucleus, two types of stereocontrol can be involved an internal control, closely related to the configurations elements of the heterodiene, or an external control by 

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This reaction was initiaiiy reported by Perrier in 1979. It is a mercury (II) salt -induced or promoted conversion of 5-enopyranosides into cyclohexanones with stereochemical control, by which substituents at positions 3 and 5 are predominantly in a trans relationship. To differentiate from another reaction also discovered by Perrier (called the Perrier Reaction), this reaction is known as the Ferrier-II rearrangement, " Ferrier-II carbocyclization, Perrier carbocyclization, or Ferrier-II reaction. Occasionally, it is also referred to as the Perrier Reaction. Therefore, it is called the Ferrier-II rearrangement in this book. It is useful in the conversion of carbohydrates into carbosugars, myo-inositols, and other natural products. ... 

Concerted mechanisms have also been considered to justify the high degree of stereoselectivity observed in many instances as, for example, in the cases shown in Scheme 3 [13,18-21], However, the high stereochemical control often observed in many ODPM rearrangements does not necessary imply that the reaction is taking place via concerted mechanisms. A stepwise process is also consistent with the stereochemical outcome of the reaction, where there are conformational or configurational restrictions to rapid C—C rotation. This subject has been extensively discussed and reviewed by Schuster [16]. 

The Lewis acid trimethylsilyl triflate brings about the rearrangement of 2,3-epoxyamines to the corresponding 2-trimethylsilyloxymethylaziridinium ions.38 Such intermediates react regiospecifically with nitrogen nucleophiles to form 1-substituted 2,3-amino alcohols with full stereochemical control. 

As illustrated with the above example, a notable feature of this annulation method is its stereoselectivity. As in the earlier version of the method, the overall annulation process can be viewed as accomplishing the effective suprafacial exo cycloaddition of a substituted carbene to the conjugated diene (Scheme 11). However, the generality of this stereochemical control does not appear to be as impressive as in the previous oxyanion-based strategy. Thus, whereas rearrangement of the (vinylcyclopropyl)methyl sul-fones derived from ( )-l,3-pen iene affords almost exclusively the predicted tra/is-substimt cy-clopentene, annulation employing (Z)-l,3-pentadiene produces a mixture of cyclopentenes in which the expected cu-substituted isomer is the minor product ( heme 13). 

A stereochemical control of the Ugi reaction can be effected with carbohydrates as chiral templates (e.g. tetrakis(O-pivaloyl)galactosylamine), which gives rise to easily separable amides. From these a variety of non-natural amino acids can be derived after acidic hydrolysis. The Passerini reaction, related to the Ugi rearrangement, gives a-hydroxyamides. A modification of this reaction using titanium tetrachloride gives a-branched amides in high yields via C-metalated imidoyl chlorides (equation 38). 

Besides the high stereochemical control of 2,3- and 3,3-sigmatropic rearrangements it is their obvious generality with respect to hetereoatom variation which has led to numerous synthetically useful applications. Even though not all possibilities have been examined yet, an impressive number of combinations of heteroatoms X and Y in the 2,3- and X, Y and Z in the 3,3-sigmatropic reactions have been realized. 

Another unprecedented stereochemical control in a Claisen rearrangement was reported recently by Yamamoto using organoaluminum reagents (Scheme 59). Sometimes (e.g. with R = Cu2CH==CH— CsHi i) complete reversal of double bond geometry could be achieved simply by modifying the bulky aluminum catalyst. 

Still-Wittig rearrangement of the ( )- and (Z)-17-ethylidene-16a ethers (93) and (94) afforded the 20a- and 20p-methyl steroids (95) and (96) with complete stereochemical control (equation 23). As in other examples, sp stereochemistry depends upon double bond geometry in the starting allylic ether. 

The Simmons-Smith cyclopropanation reaction Stereochemically controlled epoxidations Regio- and Stereocontrolled Reactions with Nucleophiles Claisen-Cope rearrangements Stereochemistry in the Claisen-Cope rearrangement The Claisen-Ireland rearrangement Pd-catalysed reactions of allylic alcohols Pd-allyl acetate complexes Stereochemistry of Pd-allyl cation complexes Pd and monoepoxides of dienes The control of remote chirality Recent developments Summary... 

The chemistry of allylic alcohols and their derivatives is remarkably rich. They can be used to add allyl groups to other molecules with good regio- and stereochemical control. The methods in this chapter, particularly the palladium-catalysed reactions and the sigmatropic rearrangements, should allow you to make many otherwise difficult target molecules. 

Further reactions on these compounds lead to other oxidised products in which the lack of stereochemical control in the epoxidation is unimportant, so, for example isophorone oxide rearranges with various catalysts to the cyclopentanone 182 (80% yield) while both isomers of pulegone oxide 179 gives the cycloheptadione29 183 (78% yield). Exhaustive methylation of the extended enolate produced by reduction of 181 gives 184 in good yield.28... 

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