Reactions sigmatropic

The [3,3] sigmatropic reaction pattern is quite general for other systems that incorporate one or more heteroatoms in place of carbon in the 1,5-hexadiene unit. The... 

Further examples of synthesis using [2,3] sigmatropic reactions are given in Section 6.6 of Part B. 

Problem 30.7 i Classify the following sigmatropic reaction by order Ja J J, and tell whether it will proceed with suprafacial or antarafacial stereochemistry ... 

Two other important sigmatropic reactions are the Claisen rearrangement of an allyl aryl ether discussed in Section 18.4 and the Cope rearrangement of a 1,5-hexadiene. These two, along with the Diels-Alder reaction, are the most useful pericyclic reactions for organic synthesis many thousands of examples of all three are known. Note that the Claisen rearrangement occurs with both allylic aryl ethers and allylic vinylic ethers. 

Sigmatropic reaction (Section 30.8) A pericyclic reaction that involves the migration of a group from one end of a tt electron system to the other. 

Note that the same holds true for [ 1, j] sigmatropic reactions of symmetrical substrates... 

Shishido, K., Shitara, E., Komatsu, H., Hiroya, K., Fukumoto, K., Kametani, T. (1986) Total Syntheses of ( )-Physovenine and ( )-Physostigmine. An Application of Tandem ElectrocycUc-[3,3]Sigmatropic Reaction of Benzocyclobutenes. Journal of Organic Chemistry, 51, 3007-3011. 

Various types of photochemically induced 1,3-shifts have been observed in nitrogen containing heterocycles. Concerted [1,3] suprafacial sigmatropic reactions are photochemically allowed processes, but many of the reported transformations especially those which arise by n->n excitation un-... 

Rearrangement reactions have provided access to some interesting azepinone or azepine dione systems. Curtius rearrangement followed by a [3,3] sigmatropic reaction on intermediate carbonyl azides gave azepin-2-one derivatives, for example 2, in fair to moderate yield. The precursor intermediates for this sequence were made, in turn, by treatment of 2-siloxysubstituted 2-alkenylcyclopropanecarboxylic acids (for example, 1) (Scheme 1) with diphenylphosphorazidate and triethylamine <00SL725>. 

Let us see the migration of a hydrogen atom and how the selection rules have been put forward to explain the formation of an imaginary transition state in a sigmatropic reaction. 

Not only propargyl precursors but also acceptor-substituted 1-methyleneallyl compounds such as 67, 71 or 74 can be used to produce the target allenes by sigmatropic reactions (Scheme 7.10). After oxidation of selenium compounds 67 followed by equilibration of the resulting selenoxides 68 and selenenic esters 69, hydrolysis... 

This aggregation of organic molecules in water actually leads to accelerated rates for some organic reactions in water. Diels-Alder and other sigmatropic reactions work very well in water, despite not being soluble or miscible in that solvent. This effect is discussed further in Chapter 7. 

The 1,3-hydrogen shift in propene is one of the model reactions which, despite the fact that it does not occur, is used to illustrate the principles involved in the W.-H. treatment of sigmatropic reactions. The situation for the neutral system... 

Thus, as predicted by the orbital symmetry rules, this thermal suprafacial [1,3] sigmatropic reaction took place with complete inversion at C-7. Similar results have been obtained in a number of other cases.426 However, similar studies of the pyrolysis of the parent hydrocarbon of 103, labeled with D at C-6 and C-7, showed that while most of the product was formed with inversion at C-7, a significant fraction (11 to 29%) was formed with retention.427 Other cases of lack of complete inversion are also known.428 A diradical mechanism has been invoked to explain such cases.429 There is strong evidence for a radical mechanism for some [1,3] sigmatropic rearrangements.430 Photochemical suprafacial [1,3] migrations of carbon have been shown to proceed with retention, as predicted.431... 

All reactions are classified as [3,3]-sigmatropic reactions. They are orbital symmetry regulated processes. 

The Cope and Claisen rearrangements are important sigmatropic reactions. Both... 

Selective polymerization, enantiomers, 185 Semico rrin-copper complexes, 199 Sharpless epoxidation, racemic alcohols, 45 Side-chain units, prostaglandins, 310 Sigmatropic reactions, 222 Silanes, oxidative addition, 126 Silica gel, 285, 352... 

The major reaction in the thermal decomposition of 2,3-dihydrofuran (9) is a unimolecular isomerization to cyclopropanecarboxaldehyde (89JPC-1139). In an analogous [1,3] sigmatropic reaction, the isomerization of 2-methyl-4,5-dihydrofuran (10) leads to acetylcyclopropane, which can rearrange to 3-penten-2-one (94JPC2341). The latter product may also be formed directly from 10. 

There is another class of isomerization reaction which proceed through a cyclic transition state, known as a sigmatropic reaction. This is formally the shifting of a single (a) bond between the terminal carbon atoms of a linear conjugated molecule, according to the example drawn in Figure 4.40. 

Classify as sigmatropic reactions and as cycloadditions each of the following, and predict stereochemistry at the migrating center for the ground-state transformation. 

The pericyclic theory has proved enormously fruitful for the field of sigmatropic reactions. In addition to providing a deeper understanding of previously known phenomena, it has led to the discovery of a variety of new rearrangements. Our treatment here, as in the previous sections, is not intended to be comprehensive. 

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