Cycloreversion Subject

Only one exception to the clean production of two monomer molecules from the pyrolysis of dimer has been noted. When a-hydroxydi-Zvxyljlene (9) is subjected to the Gorham process, no polymer is formed, and the 16-carbon aldehyde (10) is the principal product in its stead, isolated in greater than 90% yield. This transformation indicates that, at least in this case, the cleavage of dimer proceeds in stepwise fashion rather than by a concerted process in which both methylene—methylene bonds are broken at the same time. This is consistent with the predictions of Woodward and Hoffmann from orbital symmetry considerations for such [6 + 6] cycloreversion reactions in the ground state (5). 

Not only thermal [2 + 2] cycloaddition reactions but also the corresponding [2 + 2] cycloreversions are subject to large solvent effects. A good example is the thermal decarboxylation of the y9-lactone trani -3- -butyl-4-phenyloxetan-2-one, as described in Eq. (5-36). 

As the focus of this chapter is on the synthetic utility of the rDA reaction, an overview of mechanism is beyond the scope of this review however, the subject has beoi reviewed previously. Structural and medium effects on the rate of the rDA reaction are of prime importance to their synthetic utility, and therefore warrant discussion here. A study of steric effects cm the rate of cycloreversicHi was the focus of early work by Bachmann and later by Vaughan. The effect of both diene and dioiophile substituticHi on Ae rate of the rDA reaction in anthracene cycloadducts has been reported in a study employing 45 different adducts. If both cycloaddition and cycloreversion processes are fast on the time scde of a given experiment, reversibility in the DA reaction is observed. Reversible cycloaddition reactions involving anthracenes, furans, fulvenes and cyclopentadienes are known. Herndon has shown that the well-known exception to the endo rule in tiie DA reaction of furan with maleic anhydride (equation 2) occurs not because exo addition is faster than endo addition (it is not), but because cycloreversion of the endo adduct is about 10 000 times faster than that of the exo adduct. ... 

The reaction types known to produce disilenes are summarized in Chart 1 and apply regardless of the ultimate stability of the product. Historically, thermal 4 + 2 cycloreversion of complex l,2-disilacyclohex-4-enes, i.e. a retro-Diels-Alder fragmentation came first8 -14, followed by silylene dimerization15 -17. This early work produced only indirect evidence for the formation of disilenes as reactive intermediates, but in retrospect it is clear that these species were indeed produced. The early history of the subject is discussed in Reference 1. The first directly observable and isolable disilene resulted from the dimerization of photochemically produced dimesitylsilylene18, ushering in a new era in disilene chemistry. These more recent developments are described in Reference 2. 

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... 

Six articles cover general aspects of heterocyclic chemistry 1,3-dipolar cycloreversions, syntheses with arylnitrenes and a-metallated isocyanides, and photo-oxygenation of nitrogen heterocycles,while others deal with more specialized subjects, i.e. preparation and use of halogeno-lactones, aspects of the chemistry of furan, 1-hydroxy-indoles, ring-opening of azoles by the action of amines, " the use of 2-chlorobenzoxazolium (1) and other heterocyclic onium salts for dehydration and condensation reactions," the synthesis of monosub-stituted tetrathiafulvalenes (2), cycloadditions of azoles containing three heteroatoms,sydnone imines (3), the conversion of acyl-benzofuroxans into nitro-indazoles (cf. p. 199), and advances in the chemistry of pyrrolizidine " and indolizine." ... 

When the tricyclic compounds 60 are subjected to a comparable thermolysis, an initial [3-1-2] -cycloreversion with Hberation of mesitylnitrile oxide (58b) and formation of the bicyclic species 62 occurs with subsequent decomposition into the oxazaphospholes 61 and the oxadiphospholes 63 as indicated in the scheme. Because oxadiphospholes have as yet only been poorly investigated [59,60] this synthetic route has potential for exploitation as a specific access to this novel class of heterodiphospholes. Since control experiments have shown that the oxadiphospholes 63 react with mesitylnitrile oxide (58b) which prevents their isolation, it seems reasonable to remove the inevitably formed 1,3-dipole by means of a trapping reagent. It is well known that phosphaalkenes of the Becker type react with nitrile oxides via cleavage of hexamethyldisiloxane to afford... 

A related reaction type is the high pressure (6.2 kbar) Diels-Alder reaction of 2-pyrones with cyclopropenone ketals 16 to give isolable bicycloadducts 17, which then undergo cycloreversion, electrocyclic rearrangement, and ketal hydrolysis leading to tropones. Tropolones also have been formed in this way. This tropolone annulation chemistry was the partial subject of a review in 1990, and it was exploited in the syntheses of some naturally occurring tropoloisoquinolines 18-20. ... 

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