Thermal rearrangements, See

The chemical reactivity of sulfoxides as compared with sulfides is much greater. The effect of the sulfinyl group on adjacent methylene protons allows chlorination and the Pummerer rearrangement to take place. The chlorination is stereospecific, resulting in cis products. The Pummerer rearrangement results in two possible isomers (Scheme 2) (55). The Nuphar sulfoxides can be epimerized on carbon C 7 by thermal rearrangement (see Section IV, Scheme 1). 

There are correlations between mass spectral fragmentations and thermal and photochemical fragmentations and rearrangements see Sections 4.02.1.2.1 and 4.02.1.2.2. 

The azo compounds A and B were prepared and the thermal and photochemical behavior of these materials was investigated. The results are summarized in the equations below. Discuss how these results m relate to the photochemical di-rc-methane rearrangement. (See Section 12.1.4 for some indications of the reactivity of... 

The 3//-azepines obtained by cycloaddition of azirines to cyclopentadienones (see Section 3.1.1.1.2.) are thought to arise from the initially formed 2/7-azepines by [1,5]-H suprafacial sigmatropic shifts.31-108 In contrast, 1/Z-azepine 9 results from the thermal rearrangement of the nonisolable 2//-azepine-2-carboxylate 8.13 Presumably, the 1 //-azepine is stabilized, relative to the 3//-isomer, by intramolecular hydrogen bonding between the NH and the adjacent ester group. 

There are several reactions in which the sulfoxy oxygen exhibits its nucleophilicity, the most noticeable being the thermal rearrangement of thiirane oxides (in the presence of a suitable disposed /J-hydrogen) to allylic sulfenic acids2,63,105 (see equation 9 in Section III.C.l). 

In the presence of a suitably disposed /i-hydrogen—as in alkyl-substituted thiirane oxides such as 16c—an alternative, more facile pathway for thermal fragmentation is available . In such cases the thiirene oxides are thermally rearranged to the allylic sulfenic acid, 37, similarly to the thermolysis of larger cyclic and acyclic sulfoxides (see equation 9). In sharp contrast to this type of thiirane oxide, mono- and cis-disubstituted ones have no available hydrogen for abstraction and afford on thermolysis only olefins and sulfur monoxide . However, rapid thermolysis of thiirane oxides of type 16c at high temperatures (200-340 °C), rather than at room temperature or lower, afforded mixtures of cis- and trans-olefins with the concomitant extrusion of sulfur monoxide . The rationale proposed for all these observations is that thiirane oxides may thermally... 

The photoprocess could be an example of a 2S + x2, reaction. Explain the fact that (a) optically pure dinitrile (A) upon photolysis gives (B) with 83% retention and 1770 inversion of configuration (b) optically pure (A) upon thermal rearrangement gives (B) with 95.5% retention of configuration. See Refs. 190 and 191. 

The thermal rearrangements of vinylcyclopropanes to form cyclopentenes as well as 1,4-hexadienes by homodienyl [l,5]-shift are well-known16,49-51 and even described in textbooks (see, e.g., Chapter 18 in Reference 4). However, the heteroanalogous transformations are less known. 

It has been established that the course of the sequential pericyclic reaction of cyclopentadienones with acyclic conjugated alkadienes depends on the reaction temperature, thermal treatment at low temperatures affording 3a,4,7,7a-tetrahydroinden-l-one derivatives by way of a Cope rearrangement (see Scheme 38). Roman et al have developed an efficient stereoselective synthesis of enantiomerically pure i-nitrotricyclo[5.2.2.0 ]undeca-3,8-dienes via a tandem consecutive asymmetric Diels-Alder-Cope rearrangement (see Scheme 39). Adducts... 

UV irradiation of the naphthalen-l,4-imines 104 and 105 promotes their isomerization to 3-benzazepine derivatives 139 and 140. Although no direct evidence was obtained to confirm the formation of azaquad-ricyclanes (138) as intermediates (see Section II,F), the extra strain associated with structure 138 and the extra benzenoid stabilization of the products 139 and 140 make it understandable that the thermal rearrangement of 138 should occur faster than that of 76 or 78. Analogous photochemical transformations are those of compound 106 to trimethyl 3-benzazepin-l,3,5-tricarboxylate, and of 1,4-epoxynaphthalenes to benzoxepin derivatives. ... 

Other possible carbene precursors (vicinal dihalides, peresters, ketenes, carbene adducts with stable hydrocarbons, etc.) cannot generally be used for in situ generation of carbenes, because the fragments are likely to recombine. However, they can be used, for example, in experiments involving pyrolysis or other forms of external carbene generation where the fragments get a chance to separate in the gas phase and become trapped in distant matrix sites. AU conceivable halo- and dihalocar-benes were made and smdied in this way (see, e.g., the 1993 review by Sander et al." ) However, such methods can only be applied to carbenes which resist thermal rearrangement to more stable products. 

Examples of the synthesis of fused-ring azetidines by ring closure onto a preformed azetidine are uncommon, in contrast to approaches used for the preparation of fused ring azetidinones (see Sections 5.12.3, 5.11.4.4 and 5.10.4.3). The l,3-diazabicyclo[3.2.0]hep-tane derivative (48) was prepared (72JOC516) by cyclization with DCC of the carbamoyl-azetidine followed by thermal rearrangement of the intermediate thiazolidinone (Scheme 6). 

Polyalkylated 2- and 3-pyrrolenines (2H- and 3H-pyrroles) and 3-indolenines (3H-indoles) are formed by methylation of the parent 1H heterocycles (see Section 3.05.1.2.7), but the acid-catalyzed or thermal rearrangement of 2,2,5-trisubstituted 2H-pyrroles and of 3,3-disubstituted 3//-indoles yields the corresponding 2,3,5-trisubstituted IH-pyrroles and the 2,3-disubstituted l//-indoles, respectively (see Section 3.05.1.1). 

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