Anionic cycloreversion

Although deprotonation of simple 1,3-dithiolanes at the 2 position is usually accompanied by cycloreversion to the alkene and dithiocarboxylate, this does not occur for the 2-ethoxycarbonyl compound 55. The anion of this is readily generated with LDA and undergoes conjugate addition to a,(3-unsaturated ketones, esters, and lactones to give, after deprotection, the a,8-diketoester products 56 (73TL2599). In this transformation 55 therefore acts as an equivalent of Et02C-C(0) . 

As for other 1,4-diheterocines the cycloreversion ofsyn-bis-cr-homobenzene systems constitutes an easy access also to 1,4-oxazocines from appropriate precursors. At approximately 60 C, the arylsulfonamides 3 readily and quantitatively rearrange to the 4//-1,4-oxazocines 4, which can be used for the preparation of other derivatives via the corresponding anion 6 (M = K) formed on reductive amide cleavage.12... 

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). 

The first example of an oxide-anion accelerated retro Diels Alder reaction was reported by Papies and Grimme [52]. The adduct 19 (Equation 1.23) treated with tetra-w-butylammonium fluoride (TBAF) in THE at room temperature is immediately converted into 20, in contrast to the parent 21 (Equation 1.24) which undergoes cycloreversion into 22 at 100 °C. The dramatic oxide-anion acceleration (> 10 ) was ascribed to the loss of basicity of about 8pK, units in the transformation of alcoholate ion of precursor 19... 

Yeh and Falvey 1991). However, the cyclobutyl dimer of a quinone, and a dithymoquinone, also cleaves on single-electron rednction but much slower than the pyrimidine dimers (Robbins and Falvey 1993). It is truly an unresolved issue as to why the anion-radical cleavage depicted in Scheme 3.72 is so facile. Water participation can probably decrease the barrier of the cycloreversion on physiological conditions (Saettel and Wiest 2001, 2006). Besides, hydration effect is known to increase the electron affinities of nucleic bases (Scheidt et al. 1998, Kim and Schaefer 2007a, 2007b). 

Cycloreversion of oxadiazolines (194) (with an anion stabilizing substituent in the 5-position) with a base affords amide anions, which can be intercepted with electrophiles El—to give (195) (Scheme 85). The sequence can be used to convert an aromatic aldehyde via the imine (193) into an amide (195) <91SL189>. 

Tobe s group also succeeded in applying the [2+2]cycloreversion process to the formation of smaller carbon cages, notably C36 [40]. Macroscopic quantities of C35 have been produced before [41, 42] and were shown to contain carbon cages that are covalently connected to form polymeric clusters of overall D6h-symmetry. In their efforts to obtain C36 from acetylenic precursors, Tobe et al. prepared cyclophynes 19 and 20 [40]. LD time-of-flight mass spectra of 19 depict a signal for the anion of cyclophyne C3(3H8, generated from 19 by four-fold... 

The presence of two O—O bonds renders primary ozonides so unstable that they decompose immediately (Figures 15.47 and 15.48). The decomposition of the permethylated symmetric primary ozonide shown in Figure 15.47 yields acetone and a carbonyl oxide in a one-step reaction. The carbonyl oxide represents a 1,3-dipole of the allyl anion type (Table 15.2). When acetone is viewed as a dipolarophile, then the decomposition of the primary ozonide into acetone and a carbonyl oxide is recognized as the reversion of a 1,3-cycloaddition. Such a reaction is referred to as a 1,3-dipolar cycloreversion. 

These results indicate that the naphthalene radical anion is not stable to the solvent tetrahydrofuran at room temperature on a time scale of 100 hours. Decomposition pathways are alkali metal dependent. Sodium and potassium naphthalene attack THF through a proton abstraction, cycloreversion mechanism, as previously described by Bates for the butyllithium/THF system (27). Lithium naphthalenide attacks the THF not only by the Bates mechanism but also by a nucleophilic ring opening, as is implicit in earlier high temperature work on lithium naphthalenide in THF (28) and in work on the attack of THF by tritylmagnesium bromide (29). The two smaller alkali metals, lithium and sodium, leave behind a... 

Many rDA reactions are carried out at temperatures of 150 C or more in solution phase and often at temperatures of 400-600 C using the flash vapor pyrolysis (FVP) method individual conditions are referenced throughout the text. However, an accelerating effect by anionic, cationic and radical substimtion on either the dienophile or at the termini of the diene fragments has been predicted by Carpenter.Experimentally, this prediction has been substantiated only for anionic substitution. In 1967, Hart reported what is likely the first example of an oxyanion-accelerated rDA reaction. Both oxyanionic " and car-banionic substituents accelerate the cycloreversion reaction such that they proceed rapidly at room temperature (for example, equation 3). In addition, acid-catalyzed rDA reactions have been reported in which protonation effectively makes the dienophile fragment of the adduct more electron deficient. Grieco has utilized a room temperature retro aza DA reactitm useful for the N-methylation of dipeptides and amino acid derivatives (equatitm 4). ... 

Tobe and Wakabayashi utilized, on the other hand, [4.3.2]propella-1,3,11-triene units as the leaving group which would generate a carbon-carbon triple bond in a retro [2-1-2] sense ([2-1-2] cycloreversion), and prepared dehydroannulenes 4a-c and 5a,b as precursors of cyclo[ ]carbons of n=12, 16, 20, 18, and 24, respectively [5]. In their negative mode LD TOF mass spectra, the formation of monocyclic C anions ( = 12, 16, 18,... 

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