Cycloreversion, , 4-membered rings

The tendency to undergo valence isomerization is generally of fundamental importance regarding the stability of the compounds. In the case where an equilibrium exists between the eight-membered ring and the bicyclo[4.2.0]octatriene, decomposition may readily occur by a [2 + 2] cycloreversion process, particularly if molecular nitrogen or a cyano compound can be eliminated. 

The retro Diels-Alder reaction is strongly accelerated when an oxide anion substituent is incorporated at positions 1 and 2 of the six-membered ring which has to be cycloreversed, namely at one terminus carbon of the original diene or at one sp carbon of the dienophile [51] (Equation 1.22). 

If the cycloaddition and cycloreversion steps occurred under the same conditions, an equilibrium would establish and a mixture of reactant and product olefins be obtained, which is a severe limitation to its synthetic use. In many cases, however, the two steps can very well be separated, with the cycloreversion under totally different conditions often showing pronounced regioselectivity, e.g. for thermodynamic reasons (product vs. reactant stability), and this type of olefin metathesis has been successfully applied to organic synthesis. In fact, this aspect of the synthetic application of four-membered ring compounds has recently aroused considerable attention, as it leads the way to their transformation into other useful intermediates. For example aza[18]annulene (371) could be synthesized utilizing a sequence of [2 + 2] cycloaddition and cycloreversion. (369), one of the dimers obtained from cyclooctatetraene upon heating to 100 °C, was transformed by carbethoxycarbene addition to two tetracyclic carboxylates, which subsequently lead to the isomeric azides (368) and (370). Upon direct photolysis of these, (371) was obtained in 25 and 28% yield, respectively 127). Aza[14]annulene could be synthesized in a similar fashion I28). 

Since the norcarene intermediate 34 has a double bond in the 6-membered ring, a Diels-Alder cycloreversion leading to cyclopropene (35) and butadiene is also a possible disconnection. The corresponding synthetic sequence has been carried out in the laboratory in 37% yield [32] ... 

Besides the bond-pair cheletropic disconnection of oxiranes and aziridines to an alkene and "atomic oxygen" (from a carboxylic peracid) or a nitrene, respectively, and the hetero-Diels-Alder cycloreversion, of special interest are the 1,3-dipolar cycloeliminations of five-membered rings [-(34-2)] leading to 1,3-dipoles and an unsaturated acceptor or dipolarophile. So large is the number of different five-membered heterocyclic systems resulting from 1,3-dipolar... 

Four-membered rings and their heterocyclic analogs that contain one or two carbonyl functions can follow three principal reaction paths when photolyzed in solution. Depending on the nature of the substituents, the mode of reaction can be (1) decarbonylation, (2) cycloreversion, or (3) ring expansion. 

On the other hand, stabilized ylides react with aldehydes almost exclusively via trans-oxaphosphetanes. Initially, a small portion of the cw-isomer may still be produced. However, all the heterocyclic material isomerizes very rapidly to the fnms-configured, four-membered ring through an especially pronounced stereochemical drift. Only after this point does the [2+2]-cycloreversion start. It leads to triphenylphosphine oxide and an acceptor-substituted fnms-configured olefin. This frara-selectivity can be used, for example, in the C2 extension of aldehydes to /ran.v-con figured aj8-unsaturated esters (Figure 9.11) or in the fnms-selective synthesis of polyenes such as /1-carotene (Figure 9.12). 

There are abundant examples in the literature which reveal that the thermal electrocyclization of 8e systems to form eight-membered rings proceeds with lower activation energies than for the lower vinyl-og, (Z)-hexatrienes. The helical geometry of the transition state leads to less steric congestion about Ae reacting termini of the octatetraene and this likely accounts for the facility of the thermal 8e conrotatory process. Minor structural differences can induce cycloreversion of the process. ... 

For the reaction of labile ylides with aldehydes, the formation of cis-oxaphosphetanes is favored, unless lithium ion-containing bases are used. The four-membered ring system of cis-39 dissociates in a [2+2]-cycloreversion and forms triphenyl phosphonium oxide and the desired olefin Z-13. Using labile ylides in Wittig reactions that contain no electron-withdrawing group except the phosphonium moiety, leads predominantly to the formation of c/ -oxaphosphetanes. Hence, Z-olefins are typically formed with > 90 % selectivity. ... 

Direct photolysis of cis- and rraH5-bicyclo[5.2.0]non-8-ene (47) in hydrocarbon solution with monochromatic far-ultraviolet (185-214 nm) light sources afforded (Z,Z)- and (Z, )-cy-clonona-1,3-dienes via formal electrocyclic ring-opening, cycloheptene via formal a2s + a2s) cycloreversion and only a small amount of methylenecyclopropane derivative 44. The formation of this three-membered ring has been proposed to arise from a cyclobutylidene, formed as a result of [1,2]-hydrogen migration in the 71,R(3a) state. ... 

I- -f, 2J Cycloreversions involving the opening of two three-membered and one four-membered ring were also observed for cis- and tran.s-tris-[2.1.1]-<7-homobenzene derivatives leading to (Z,Z,Z)-cyclodeca-2,5,8-trien-l-one and (Z, , )-cyclodeca-2,5,8-trien-l-one, respectively. ... 

Eight-membered rings can be obtained by [4+4]-cycloadditions of 1,3-dienes [1] via diradicals or other intermediates. Synthesis of such compounds has been achieved by thermal, [2] photochemical, [3] and by metal-catalyzed [4] processes these reactions have been the subject of extensive mechanistic [5] and theoretical [5c] studies. Their strategic applications in natural product synthesis have been reviewed. [5d] The thermal version has generated little interest, except in orthoquino-dimethane dimerizations and in cycloreversions the Cope rearrangement of 1,2-divinyl-cyclobutanes [3] is more commonly used. [4+4]-Cycloadditions are also used with 1,3-dipoles or mesoionic heterocycles for the synthesis of six- and seven-membered rings. Sometimes also [6+4]-cycloadditions are... 

Scheme 6.6 Cycloaddition/cycloreversion cascade followed by an unusual three-membered ring formation developed by Wegner et al.

As predicted, both (644) and (645) undergo thermal rearrangement much more slowly than does [2,2,2]propellane, suffering cycloreversion of the four-membered ring to give dienes (646) and (647), respectively. Reactions of (644) and (645) with ADE were reported to proceed by cleavage of both the three- and four-membered rings, but details of the mechanisms of these latter reactions remain to be uncovered. ... 

Four-membered Rings. The ratio of cracking to geometric isomerization for cis-1,2-diphenylcyclobutane (Scheme 50) is 2.0 0.3 thermally, 7.1 0.7 on direct irradiation, and 2.6 0.2 on acetone-sensitized photolysis, indicating that there may be a significant concerted component to the cycloreversion reaction in the singlet excited state. ... 

Across other double bonds Triarylketenimines react with nitrosoarenes via a [2-1-2] cycloaddition reaction across the N=0 bond to give the four-membered ring cycloadduct 64 Some cycloadducts are only stable in solution and they rapidly undergo a cycloreversion reaction to form diphenylcarbodiimide and diphenylketone. 

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