Alkyl groups sigmatropic rearrangements

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 ... 

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... 

The reaction of aliphatic and aromatic ketone oximes 97 with a dialkyl carbonate 98 in the presence of K2CO3 at 180-190 °C yields 3-alkyl-4,5-disubstituted-2(3//)-oxazolones 104 in 22-48% yields. Mechanistically, it is proposed that N-alkylation of the initially formed oxime O-carbonate 99 yields 100, which affords the enamine 101 in the presence of base. A [3,3] sigmatropic rearrangement ensues to produce 102, which then cyclizes to 104. In cases where 97 contains two methylene groups in proximity to the C=N bond, one of which is benzylic, the above reaction sequence is regioselective for the benzylic methylene group (Fig. 5.25 Table 5.6, Fig. 5.26). ... 

The possibility of rearrangement in pentadienyl anions must be borne in mind when they are employed synthetically. When 1- or 5-alkyl groups are present, intramolecular 1,6-sigmatropic hydrogen shifts are possible and the stereochemistry follows Woodward-Hoffmann rules, being thermally antara-facial but photochemically suprafacial. Bates, for example, showed that the same equilibrium mixture of isomers results at 40°C from the deprotonation of either 5-methyl-1,4-hexadiene or 2-methyl-1,4-hexadiene (79). The tendency is to form isomers with fewer alkyl groups in the 1,3, and 5 positions of the delocalized system (50). 

The ability of sulfur to stabilize carbanions is exemplified in the following cases. The pentadienyldi-thiocarbamate (144 Scheme 11) can be alkylated at the methylene group to afford the methylated product (145). At 110 C, the dithiocarbamate unit walks its way to the other end of the pentadienyl chain via tandem [3,3] sigmatropic rearrangements. The formation of (14d) is thermodynamically controlled as the more-substituted pentadienyl unit is formed. Dithiocarbamate (146) can be alkylated and eventually transformed into sulfur-free products the sequence giving (148) from (146) is but one of these processes. 

Piers and Ruediger have smartly used this basic reaction (equation 168) for a synthesis of the sesquiterpenoid -himachalene ". However, loading the systems to be rearranged with alkyl groups can considerably decelerate the Cope process with the result of competing homo[l, 5]-sigmatropic hydrogen shift in the trans compounds (equation... 

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