Slgmatropic Rearrangement

Slgmatropic rearrangement with H-transler and C-C txjrxJ formation (inter or intramolecular) and chiral induction... 

B. The Reversible [2,3]-Slgmatropic Rearrangement of Allylic Sulfenates to Sulfoxides... 

Since a [1,5] Slgmatropic rearrangement involves three electron pairs (two 7T bonds and one cr bond), the orbital-symmetry rules in Table 30.3 predict a suprafacial reaction. In fact, the fl,51 suprafacial shift of a hydrogen atom across two double bonds of a tt system is one of the most commonly observed of all slgmatropic rearrangements. For example, 5-methylcyclopentadiene rapidly rearranges at room temperature to 3rield a mixture of 1-methyl-, 2-methyl-, and 5-methyI-substituted products. 

Chemists wishing to study the thermal rearrangement of (1) - it gives C2) by a [3,3] slgmatropic shift (p T 288) - decided it could be made by a double Wlttlg reaction on (3). 

Two other important slgmatropic reactions are the Cope rearrangement of a 1,5-hexadiene and the Claisen rearrangement of an allyl aryl ether (Section 18.6). These two, along with the Diels-Alder reaction, are the most useful pericyclic reactions for organic sjmthesis many thousands of examples of all three are known. Note that the Claisen rearrangement works well with both allyl aryl ethers and with allyl vinylic ethers. 

---