Side-chain 3,3 Sigmatropic rearrangement

Sesquiterpenoid. 203, 1071 Sex hormone, 1082-1083 Sharpless, K. Barry. 734 Sharpless epoxidation, 735 Shell (electron), 5 capacity of, 5 Shielding (NMR). 442 Si prochirality, 315-316 Sialic acid. 997 Side chain (amino acid), 1020 Sigma (cr) bond, 11 symmetry of, 11 Sigmatropic rearrangement, 1191-1195... 

N,O-acetal intermediate 172, y,<5-unsaturated amide 171. It is important to note that there is a correspondence between the stereochemistry at C-41 of the allylic alcohol substrate 173 and at C-37 of the amide product 171. Provided that the configuration of the hydroxyl-bearing carbon in 173 can be established as shown, then the subsequent suprafacial [3,3] sigmatropic rearrangement would ensure the stereospecific introduction of the C-37 side chain during the course of the Eschenmoser-Claisen rearrangement, stereochemistry is transferred from C-41 to C-37. Ketone 174, a potential intermediate for a synthesis of 173, could conceivably be fashioned in short order from epoxide 175. 

A radical pair pathway has been identified for the conversion of 29 into 30 as indicated by the E/A multiplet due to the A2B2 system in the amine side chain at S 2-5-3-0 ppm (Baldwin and Brown, 1969 for CIDNP in the formation of a methylenecyclohexadiene, see Bethell et al., 1972c). The low enhancement factor (5-20) at maximum polarization suggests that a concerted [1,3] sigmatropic rearrangement may occur concurrently. 

Efficient synthesis of the mycotoxin asteltoxin 189 was accomplished beginning with the cycloaddition between 3,4-dimethylfuran and 3-benzyloxypropanal, which furnished pho-toaldol 183 in 63% yield (Scheme 42)84. Epoxidation from the convex face of this adduct, with subsequent epoxide opening, afforded 184, which was then elaborated through a series of steps to 185. The side chain was introduced via lithiosulfoxide 186 to furnish, after double sigmatropic rearrangement, 187. Hydrolysis of this afforded 188, which was oxidized and elaborated to 189 in two steps. 

The [2,3]- or [3,3]-sigmatropic rearrangements (Scheme 24) provide a means to introduce either the protected amine or the carbon atom which will become the carboxylic acid, while also positioning the double bond in the correct position for the alkene isosteres. Moreover, when starting from homochiral allyl alcohols, a very effective chirality transfer assures the stereospecific construction of the R1 and R2 side-chain stereochemistries. 

The approach of the two orbitals on the same side of the surface is called supra , that on opposite sides is called antara . The above rules, 1 and 2, also apply in the case of sigmatropic rearrangements, since the changes of orbital symmetries are the same on going from a HOMO to a LUMO or from a chain of 4n to a chain of 4n + 2 tt electrons. 

This compound rearranges with migration of the acetate group to the side chain and the restoration of aromaticity. This may be an ionic reaction or a [3,3]-sigmatropic rearrangement. 

A sequence for assembling the cholestane side chain from 20-oxopregnanes19 involved as the last step the stereoselective formation of the dihydroxy functionality by a sigmatropic rearrangement. Rearrangement of the (22 ,24S)-stereoisomer 4 gave exclusively the (22R,2iE)-stereoisomer 5. 

A modification called tandem [2,3] sigmatropic rearrangement of sulfonium ylide—bromine allylic rearrangement has been reported (88JOC5149). Thus, reaction of the C5 brominated 2-pyrone 224 with ethyl diazoacetate under rhodium catalysis results not only in transfer of the ester moiety to C5, as described earlier, but also in the transfer of the bromine atom from C5 to the side chain at C6 in such a way that the functional group remaining at that side chain, as in 225, can be further elaborated (89JHC1205). 

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