Wittig rearrangement Woodward-Hoffmann rules

Wheland intermediates, 41, 131,151 Wittig reaction, 233 Wittig rearrangement, 293 Wolff rearrangement, 119 Woodward-Hoffmann rules, 344 Wurtz reaction, 289... 

Hauser noted that diallyl ether (8) also undergoes Wittig rearrangement upon base treatment and suggested that product formation could involve either a 1,2-shift or a cyclic mechanism (equation 3). Later studies by Schdllkopf and Makisumi with substituted allylic ethers (10,11 and 14-16 equations 4 and 5) pointed to a cyclic (SnO mechanism a process allowed by the Woodward-Hoffmann rules. The diastereoselectivity of the reaction was not determined in these cases, but Schollkopf subsequently found that benzyl rrans-crotyl ether (20 equation 6) affords mainly the anti products upon rearrangement of ether (20) with BuLi in THF. Rautenstrauch observed a 1 1 mixture of syn and anti products upon rearrangement of ether (20) in the presence of TMEDA, whereas the cis isomer (23) gave only the syn product (22 equation 7). ... 

Problem 4.18. Examples of the Stevens rearrangement and the nonallylic Wittig rearrangement are shown. What do the Woodward-Hoffmann rules say about the nature of these reactions Offer two explanations (not necessarily mechanisms) of why these reactions proceed. (Hint Think of the conditions on the applicability of the Woodward-Hoffmann rules.)... 

The Stevens rearrangement and the Wittig rearrangement (nonallylic version) (Chapter 4) can be classified as four-electron [1,2] sigmatropic rearrangements. The Woodward-Hoffmann rules state that for a four-electron sigmatropic rearrangement to be allowed, one of the components must be antarafacial, yet it is... 

---