Alkyl sigmatropic

An interesting application of this chemistry involves cyclopropylcarbinyl systems that generate cyclo-pentenes via a [l,3]-alkyl sigmatropic rearrangement. The prototype examples involves heating a vinyl-... 

Sigmatropic migration involving alkyl group shifts can also occur ... 

Fig. 11.7. Classification of sigmatropic shifts of alkyl groups with respect to basis set orbitals.

When an alkyl group migrates, there is an additional stereochemical feature to consider. The shift can occur with retention or inversion at the migrating center. The analysis of sigmatropic shifts of alkyl groups is illustrated in Fig. 11.7. The allowed processes include the suprafacial 1,3-shift with inversion and the suprafacial 1,5-shift with retention. Sigmatropic rearrangements of order [3,3] are very common ... 

These are suprafacial sigmatropic shifts of order [1,5] and should occur with retention of configuration at the migrating carbon. This stereochemical course has been established for the 1,5-alkyl shift that converts 16 to 17. The product which is isolated, 18, results from a subsequent 1,5-hydrogen shift, but this does not alter the stereochemistry at the migrating... 

Sigmatropic migrations of alkyl or aryl groups" are less common than the corres-... 

Thia-[2,3]-Wittig sigmatropic rearrangement of lithiated carbanions 47, obtained by deprotonation of the S-allylic sulfides 46, affords the thiols 48 or their alkylated derivatives 49. The corresponding sulfonium ylides 51, prepared by deprotonation of the sulfonium salts 50 also undergoes a [2,3]-sigmatropic shift leading to the same sulfides 49 [36,38] (Scheme 13). As far as stereochemistry is concerned, with crotyl (R R =H,R =Me) and cinnamyl (R, R =H,R =Ph) derivatives, it has been shown that the diastereoselectivity depends on the nature of the R substituent and on the use of a carbanion or an ylide as intermediate. 

Deprotonation of allylic aryl sulfoxides leads to allylic carbanions which react with aldehyde electrophiles at the carbon atom a and also y to sulfur . With benzaldehyde at — 10 °C y-alkylation predominates , whereas with aliphatic aldehydes at — 78 °C in the presence of HMPA a-alkylation predominates . When the a-alkylated products, which themselves are allylic sulfoxides, undergo 2,3-sigmatropic rearrangement, the rearranged compounds (i.e., allylic sulfenate esters) can be trapped with thiophiles to produce overall ( )-l,4-dihydroxyalkenes (equation 24). When a-substituted aldehydes are used as electrophiles, formation of syn-diols 27 occurs in 40-67% yields with diastereoselectivities ranging from 2-28 1 (equation 24) . ... 

A useful method for ortho-alkylation of aromatic amines is based on [2,3]-sigmatropic rearrangement of S-anilinosulfonium ylides. These ylides are generated from anilinosulfonium ions, which can be prepared from iV-chloroanilines and sulfides.289... 

The cycle size plays an important role in the transition state during elimination of simple molecules (HHal, H20, H2S) from the M+ of monofunctional alkyl derivatives. Five-member transition state (1,4-sigmatropic shift of a hydrogen atom) is preferential for the elimination of HC1 and HBr. For thiols this process gives 40% of the eliminating H2S, while the preferential one (60%) involves 1,4-elimination (1,5-sigmatropic shift). The latter process (1,4-elimination) completely dominates in the case of alcohols [14]. Another example of steric effects involves a McLafferty-type... 

When N-alkyl-N-allenylaniline 311 was treated with magnesium monoperoxy-phthalate in aqueous MeOH, the corresponding N-oxides 312 were formed initially, which would then undergo sequential [2,3]- and [3,3]-sigmatropic rearrangement and aromatization to afford N-alkyl-2-ethenylindoles 316 [152],... 

Thiosulfonium ions where one of the alkyl groups is an allyl group can undergo a different sort of rearrangement (50), a [2,3] sigmatropic shift of the... 

In the solution proposed by Evans [25], however, the unsaturated ketone is condensed with the anion of an alkyl allyl ether, followed by a [3,3]-sigmatropic oxy-Cope type rearrangement (Scheme 5.22). In the retrosynthetic sense this means a [3,3]-sigmatropic rearrangement of the bis-enol form of the 1,6-dicarbonyl system. 

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