Via sigmatropic

In comparison, irradiation of 33 in benzene resulted in a transient spectra with Amax = 390 nm. The decay of transient absorption was resolved into a slow and a fast component (t= 10 ms and 225 ns), which were assigned to E-34 and Z-34, respectively. Enol Z-34 is short-lived in benzene because it reketonizes efficiently via sigmatropic 1,5-H atom shift, which is not retarded with H-atom bonding, as occurs in methanol. Addition of triplet quenchers in benzene solutions did not completely quench the formation of Z-34 thus, the authors concluded that both triplet and singlet pathways were prevalent in benzene. However, the low quantum yield, 0 11,150,152 j-Qj. formation from 33 in benzene suggests that only E-34 elim-... 

It is known that 5-acyloxyoxazoles 132 rearrange to 4-acyl-5(4/l/)-oxazolones 133 in the presence of 4-(dimethylamino)pyridme or 4-(pyrrohdino)pyridine. Recently, an asymmetric variant of this nucleophUe-catalyzed rearrangement that employs a chiral derivative of 4-(pyrrolidino)pyridine has been described. This procedure allows the construction of quaternary stereocenters with high levels of enantioselectivity (Scheme 7.38). Representative examples of saturated 5(4//)-oxazolones prepared via sigmatropic rearrangements are shown in Table 7.16 (Fig. 7.18). 

TABLE 7.16. SYNTHESIS OF SATURATED 5(4H)-OXAZOLONES VIA SIGMATROPIC REARRANGEMENT OF 5-ALKOXYOXAZOLES... 

Review on chirality transfer via sigmatropic rearrangements R. K. Hill in Asymmetric Synthesis, J. D. Mosher, Ed., Vol. 3, p 503, Academic, Orlando 1984. 

The non-planar polyene nature of azepines renders them susceptible to a variety of intra-and inter-molecular pericyclic processes. The azepine-benzeneimine valence isomerization has been discussed in Section 5.16.2.4, and the ring contractions of azepines to benzenoid compounds in the presence of electrophiles is covered in Section 5.16.3.3. In this section the thermal and photochemical ring contractions of azepines to bicyclic systems, their dimerizations and their isomerizations via sigmatropic hydrogen shifts are discussed. Noteworthy is a recent comprehensive review which compares and contrasts the many and varied valence isomerizations, dimerizations and cycloadditions of heteroepins (conjugated seven-membered heterocycles) containing one, two and three heteroatoms (81H(15)1569). 

Ring Expansion/Con traction via Sigmatropic Rearrangements - Cope Rearrangement... 

R. K. Hill, Chirality Transfer via Sigmatropic Rearrangements, in Asymmetric Synthesis. 

Examples of transpositions of hydride via sigmatropic reactions (equation 4) will be restricted to one special example, which will be discussed in Section I.3.3.5.3. 

Reaction 3 Isomerization of i-,trani-9,ll-octadecadienoyl soap to trans-,cis-8,10-octadecadienoyl soap via sigmatropic rearrangement. 

Hill, R. K. Chirality transfer via sigmatropic rearrangements. Asymmetric Synth. 1984, 3, 503-572. 

Scheme 2.16S Synthesis of a fluorinated natural product analog via sigmatropic rearrangement of a divinyl gem-difluorocyclopropane [43].

The generation of allylic bonds with (or without) concomitant formation of skeletal branching may be readily achieved via sigmatropic rearrangements such as Cope or Claisen rearrangements (Scheme 3.19). These oe-cupy a favored position among the carbon-earbon bond forming reactions, because they allow control of stereochemistry as well. 

Q 11. Explain the formation of citral through the following reaction sequence via sigmatropic shifts. 

SCHEME 21.9 C-N bond-forming and -breaking reaction via sigmatropic rearrangement. 

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