Cheletropic extrusion

The methiodide of 2,5-dihydrothiophene (239) is transformed in high yield to Z)-l-(methylthio)buta-l,3-diene (240) on treatment with alkali (81AJC1017). The thermal cheletropic extrusion of sulfur dioxide from both cis and trans isomers of 2,5-dihy-drothiophene 1,1-dioxides is highly stereospecific. For example, c/5-2,5-dimethyl-2,5-dihydrothiophene 1,1-dioxide (241) yields ( , )-hexa-2,4-diene (242) and sulfur dioxide (75JA3666, 75JA3673). 

Since sulfoxides and sulfones are versatile synthetic intermediates, and since in both the thiolene oxide and dioxides the reverse dethionylation114 ( — SO), and cheletropic extrusion of sulfur dioxide296, respectively, readily take place thermally, these cycloadditions are expected to find a useful place in organic synthesis. It should be kept in mind, however, that the retrograde SO-diene reaction and interconversion of the thiolene oxides compete effectively against SO extrusion on heating, and that diene isomerization accompanies the forward reaction (SO + diene). 

An interesting cycloheptatriene (182) synthesis has been described using thiophene 1, 1-dioxides (180) and cyclopropenes 181 (equation 121)ns. Concerted [4 + 2]cycloaddition and subsequent cheletropic extrusion of sulfur dioxide are suggested by the second-order kinetics (first in each reactant), and by the large negative activation entropy. 

Dipolar cycloaddition of pyrido[2,l-A][l,3]thiazinium betaine 507 (R = Me) with 1-diethylamino-l-propyne afforded cycloadduct 508, from which quinolizin-4-one 509 formed by a rapid cheletropic extrusion of COS (Scheme 53) <1995T6651>. 1,4-Dipolar cycloaddition of 507 and 4-phenyl-l,2,4-triazoline-3,5-dione yielded 511 (via 510) <1995H(41)1631> and 512 <1995T6651>. 

Hilvert [33] has recently used this approach to catalyse the Diels-Alder reaction between tetrachlorothiophene dioxide (20) and A(-alkylmaleimides (21). a reaction that takes place in two steps i) initial formation of tricyclic adduct 22 and ii) cheletropic extrusion of sulfur dioxide to give dihydrophtalimide 23, which is spontaneously oxidised under the reaction conditions to 24 (see Scheme 11.6). 

Extrusion (or elimination) of sulfur from thiiranes and thiirenes is a facile process. Virtually all thiiranes and thiirenes, as well as their oxides and dioxides, undergo thermal extrusion of the sulfur moiety with increasing facility according to the trend S < < SO < S02. The thermolytic desulfurization mechanism is more complex than a simple cheletropic extrusion (85TL2789). 

Irradiation of 3,3-diphenylthiooxindole gives 9-phenylthioxanthene (72T597). It is likely that this transformation involves successive cheletropic extrusion of CO, electrocyclization and a photolytic 1,3-sigmatropic shift of hydrogen (Scheme 152). The last step can also be brought about by base catalysis. 

Dipolar cycloaddition of anhydro pyrido[2,l-b][l,3]thiazinium hydroxides (128) with aryl isocyanates and dimethyl acetylenedicarboxylate gave pyrido[l,2]pyrimidines (129) and quinolizine-l,2-dicarboxylates (130), respectively (76CB3668). 1,4-Dipolar cycloaddition of pyrido[2,l-h][l,3]thi-azinium betaine (131, R = Me) with 1-diethylamino-l-propyne afforded cycloadduct 132, from which quinolizin-4-one 133 formed by a rapid cheletropic extrusion of carbonyl sulfide (93TL5405 95T6651). 1,4-Dipolar cycloaddition of anhydro 4-hydroxyl-2-oxo-6,7,8,9-tetrahydro-2//-pyrido-[2,l-b][l,3]thiazinium hydroxides (131) and 4-phenyl-l,2,4-triazoline-3,5-dione yielded 135 via 134 [94H(39)219 95H(41)1631] and 136 (95T6651). 

Hypervalent three-membered thiaheterocycles have been claimed as intermediates which undergo cheletropic extrusion of thiones352. This approach, however, has found scant application as a synthetic method. 

Paquette and co-workers have reported on the facile cheletropic extrusion of sulfur dioxide from the heterocyclic annulated semibullvalene 134 which furnishes 135.206) This tetraene is seen to be a double-bond isomer of 1,6-dimethylpentalene. 

Several cases of photochemical reactions, for which the thermal equivalents were forbidden, are shown below. In some cases the reactions simply did not occur thermally, like the [2 +2] and [4 +4] cycloadditions, and the 1,3- and 1,7-suprafacial sigmatropic rearrangements. In others, the photochemical reactions show different stereochemistry, as in the antarafacial cheletropic extrusion of sulfur dioxide, and in the electrocyclic reactions, where the 4-electron processes are now disrotatory and the 6-electron processes conrotatory. In each case,... 

The reverse reaction is a linear cheletropic extrusion of SO2 with conrotatory twisting of the terminal methylenes. With cyclic triene SO2 forms only the 1,4 adduct. In this case the antarafacial addition to the triene is geometrically impossible and the alternative non-linear n s + (O s process does not compete with the concerted 71 5 + (O s linear cheletropic addition to a diene component. So a nonlinear cheletropic addition containing eight electrons becomes highly unfavorable. This is illustrated by the fact that SO2 is eliminated 60,000 times more slowly (even at 180°) from (c) than from (b). In (b) + (ip s, retro process is allowed. 

The reverse of this process is known as cheletropic extrusion (or cheletropic elimination). 

The only direct study of the chemical reactivity of the thiaziridine system was conducted on the thiaziridine 1,1-dioxide system 19, the only thus far isolable compounds of the thiaziridine series. As could be predicted and in line with theoretical considerations, thermolysis of alkyl-substituted thiaziridine 1,1-dioxides 19a,d,e afforded aldimines 60a,d,e as a result of the cheletropic extrusion of the sulfur dioxide moiety (Scheme 9) <1981CB774>. Treatment of these thiaziridine 1,1-dioxides with perchloric acid has also resulted in the loss of sulfur dioxide with the concomitant formation of the corresponding ammonium perchlorate salts 61 <1981CB774>. 

In a related study Gaioni generated the divinyl dichloride by cheletropic extrusion of sulfur dioxide from the dichlorocarbene/3-sulfolene adduct (49). Under the reaction conditions the divinyl dichloride solvolyzes to the cyclopentenone as outlined previously (equation 32). 

Thermolysis of thiiranes causes extrusion of elemental sulfur, with the formation of alkenes <64HC(19/1)576, 66CRV297, 76RCR25) however, the desulfurization mechanism by thermolysis is not clear at present. It cannot be explained by a simple cheletropic extrusion, but involving a more complex reaction scheme <85TL2789>. 

The theimolysis of 2-substituted 2,5-dihydrothiophene 1,1-dioxides leads to ( ) conjugated dienes via cycloieversion followed by the concerted cheletropic extrusion of sulfur dioxide. The thermolysis of the a, -alkylated sulfone (96) gives the intennediate, which loses SO2 to give 1-dodecadien-l-yl acetate (98), a component of the sex pheromone of the red bollwoim moth. Tliis procediue has been extended to the thermolysis of a,3-dialkylated sulfones in order to obtain ( , )-1,4-disubstituted-1,3-dienes (equation 46). Similar processes have been used for the syntheses of alkaloids. The synthesis of an Elaeocarpus alkaloid, elaeokwine A (100), makes use of the retrodiene extrusion of sulfur dioxide to give the 1,3-diene intermediate (99) that is subsequently consumed by an intramolecular imino DA reaction (equation 47). o-Xylylene (102) has been generated by rDA expulsion of SO2 from benzo-fiised 3,6-dihy(ho-l,2-oxathiin 2-oxide (101). ... 

If the fragmentation of the two atoms from the group, to which they were bonded, takes place in a concerted process resulting in the formation of a conjugated system, this is a cheletropic extrusion. Thus, the two possibilities can be found in the case of the loss of a SO2 group SO2 extrusion from an episulfone, as in the Ramberg-Backlund reaction, is an extrusion reaction, but SO2 extrusion from a cyclic sulfolene is a cheletropic reaction. ... 

One of the best routes to o-quinodimethanes involves cycloreversion with elimination of a small, thermodynamically stable, molecule. The most important example is the loss of sulfur dioxide in the widely used cheletropic extrusion from sulfolenes (Section 2.4). In this allowed lc 2 + bond order of the "backbone" sulfolene double bond is ideally 2. If the bond order is lowered by bond fixation this renders the extrusion more difficult and the... 

The cheletropic extrusion of SO2 from heterocyclic sulfones is now the most widely used and flexible route to the r)-quinodimethanes. The sulfones are stable, easily handled and readily synthesised. The ease of this thermal elimination depends on the bond order of the sulfolene 3,4-bond. It is normally in the range 150-200 C for a typical aromatic heterocyclic fused system but can be much higher for 2,3-naphtho fused and other systems, e.g. 11, where this bond order is reduced by bond fixation. The extrusion temperature for the 2-substituted pyrazoles 29 is significantly higher than for the 1-substituted isomers 30 and, in contrast to the... 

One of the few problems with the stilfoiie route to o-quinodiinethanes is the relatively high temperature required for cheletropic extrusion of SO2. Loss of SO2 from the benzosultine 47... 

Mori et al. have studied the decarboxylation of (39) under a variety of conditions and have found that the conversion to (40) occurs without the involvement of radicals. They suggest that the process is a concerted cheletropic extrusion via the s-cis conformation. A further study has examined the photodecarboxylation of the (5)-ester (39) in unstretched-polyethylene films. The decarboxylation affords (40) with complete retention of the stereochemistry. The yield of product is 98% and the ee is >98%. The photochemical behaviour of the ester in other confining media such as cyclodextrins indicates that cage-escape products are also formed. The irradiation of grandifloric acid (41) at 254 nm in acetonitrile brings about decarboxylation with the formation of epimers. In methanol a different reaction occurs that results in the conversion of the C-methyl group into a carbomethoxy substituent. ... 

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