Suprafacial rearrangements

Both suprafacial and antarafacial sigmatropic rearrangements are symmetry-allowed, but suprafacial rearrangements are often easier for geometric reasons. The rules for sigmatropic rearrangements are identical to those for cycJoaddition reactions (Table 30.3). 

Both these 1,51 hydrogen shifts occur by a symmetry-allowed suprafacial rearrangement, as illustrated in Figure 30.12. In contrast with these thermal [L,51 sigmatropic hydrogen shifts, however, thermal [1,3 hydrogen shifts are unknown. Were they to occur, they would have to proceed by a strained antarafacial reaction pathway. 

One of the first uses of the allylic sulfoxide-sulfenate interconversion was made by Jones and coworkers64, who reported exclusive suprafacial rearrangement of the allyl group in the steroidal sulfoxide 17 shown in equation 13. Two other examples are shown in equations 1465 and 1566. Evans and coworkers have demonstrated the utility of the suprafacial allylic sulfoxide-sulfenate rearrangement in a new synthesis of the tetracyclic alcohol 24 (equation 16)67, as well as in a synthesis of prostaglandin intermediates as shown in equation 1768. The stereospecific rearrangement of the unstable sulfenate intermediate obtained from the cis diol 25 indicates the suprafacial nature of this process. 

Notice that the new (black) a bond forms on the same face of the ring as the old (green) a bond this is, as you should expect, a suprafacial rearrangement. 

Suprafacial rearrangement of hydrogen across the top face of the tt system gives 6-12. 

For example, on heating at 160 °C in 1,2-dichlorobenzene, the allylic trichloroacetimidate 84 smoothly rearranges into the corresponding 2-amino-2-deoxy sugar 85 [81]. The suprafacial rearrangement from C2 to C4 is similarly performed to obtain the 4-amino-4-deoxy sugar derivative 87 [82] (O Scheme 15). 

Both of these [1,51 hydrogen shifts occur by a symmetry-allowed suprafacial rearrangement, as illustrated in Figure 30.14. 

Evans rather general route to y-substituted allyl alcohols is depicted in a specific example, which demonstrates the stereochemical outcome of the suprafacial rearrangements in cyclic systems (Scheme 14).65... 

Problem 4.17. A small amount of 4-allylphenol is often obtained from the Claisen rearrangement of allyl phenyl ether. Draw a concerted mechanism for this reaction, name the mechanism, and determine whether the suprafacial-suprafacial rearrangement is thermally allowed or disallowed by the Woodward-Hoffmann rules. If it is not allowed, draw a multistep mechanism for the reaction. 

If the reaction requires a suprafacial rearrangement, carbon will migrate using one of its lobes if the HOMO is symmetric and will migrate using both of its lobes if the HOMO is asymmetric. 

Bis(acetonitrile)dichloropalladiuin(ll). 14, 35-36 15, 28-29 16, 25-26 17, 30-31 Allylic rearrangement. This suprafacial rearrangement finds use in the preparation of 4-acetoxy-2-alkenonitriles and 6-hydroxy allylic phosphine oxides. ... 

The fact that AH is so much lower than that for the corresponding rearrangement of pentadiene, for which the suprafacial pathway is geometrically convenient, supports our conclusion that the suprafacial rearrangement of the latter, though it is undoubtedly the preferred pathway at elevated temperatures - and is probably concerted though perhaps not synchronous - is inconsistent with orbital symmetry conservation. 

Hydrogen shifts can take place if the reaction is carried out under photochemical conditions because the HOMO would be symmetric, so suprafacial rearrangement can occur (Table 28.4). 

Migration across the same face is a suprafacial rearrangement. 

We noted above that small amounts of ultraviolet light are required for the activation of vitamin D. This interesting reaction occurs by a [1,7] suprafacial rearrangement. Vitamin D is a necessary part of the human diet for bone growth. Inadequate amounts of this vitamin result in inadequate calcification of bones. This condition in children is called rickets. The disease in adults is called osteomalacia. In spite of a dietary vitamin D deficiency, it is known that an individual may generate vitamin D since sunlight can activate a Vitamin D precursor. But in northern climates where the days are short in the winter, and one s skin is covered, it is not possible to produce enough vitamin D to make up for dietary deficiencies. 

---