Cyclo-reversion

Fragmentation Group-transfers, retro-ene, cyclo-reversions. 

I.3.4.2.5. Carbonyl and Thiocarbonyl Compounds a-(Hydroxyimino)phenyl-acetonitrile oxide (generated in situ at room temperature from PhC( NOH)C ( NOH)Cl in the presence of NaHC03 or Et3N) reacts with simple aldehydes and ketones R1R2CO to give 1,4,2-dioxazoles 180 (347). Related dioxazoles, formed by cycloaddition of benzonitrile oxide to aromatic aldehydes, upon treatment with I-BuOK, undergo cyclo-reversion, allowing direct conversion to substituted benzoic acids or their esters (348). 

Cyclo-reversions proceed readily in reactions of enantiopure D7i and D7i ent with cyclic (551) and acyclic (585) nitrones. The sulfinyl group in lactones D7i and D7i ent controls the Jt-facial selectivity and is also controller of the endo/exo selectivity (Scheme 2.274) (788). 

Recently, dipolarophile 1)13 (fumaronitrile) (777) has been used in the synthesis of indolizine lactone (677). Both, intermolecular and intramolecular cycloadditions were studied. Intermolecular 1,3-cycloaddition of nitrone (671) to D13 led to the formation of isoxazolidine (672). Subsequent deprotection and esterification of the obtained alcohol (673) with (674) gave isoxazolidine (675) in 65% yield. Ester (675), when refluxed in xylene for 10 min, after elimination of fumaronitrile by cyclo-reversion, underwent spontaneously intramolecular cycloaddition to give the tricyclic cycloadduct (676) in 84% yield (Scheme 2.291). 

Diels-Alder reaction of 3-vinylindole 131 with aryne in the presence of air gives, besides primary Diels-Alder product 132, the methyl 12-methyl-12H-[3]-benzoxepino[l,2-l7]indole-5-carboxylate 135. This can be explained by the formation of 1,2-dioxetane 133, its cyclo reversion and final intramolecular cyclization of dienol 134 or its tautomers (Scheme 26 (1996JCS(P1)1767)). 

Other hand, would require substantial steric distortion in the transition state. Moreover, while the [2+2] dimer can undergo cycloreversion directly to form the corresponding larger monocyclic ring, conversion of the [4+2] dimer into the monocyclic ring would require two steps a 1,3-shift followed by a [4+2] cyclo-reversion. 

This cyclo reversion reaction has been utilized to generate 34 and study its reactions with olefins in a FTICR spectrometer . 34 has been found to react with various olefins essentially along two competing channels H atom abstraction leading to the immonium ion m/z 44 (Scheme 36) and regioselective cycloaddition-cycloreversion. The latter reaction opens up a new possible access to ionized primary enamines in the gas phase. 

As far as the mechanistic aspects are concerned, there is no difference between cycloaddition reactions and cycloreversion reactions, and therefore the latter have been included in this review. The question of whether cycloaddition or cycloreversion takes place depends on the relative free energies of reactants and products. Although most (2 + 2)-cyclo-reversions are achieved thermally, several examples are known of... 

There exists some confusion about the reported (2 + 2)-cyclo-reversion of 2,3,4,5,6-pentaphenyl-l,7-diazabicyclo[3,2.0]hepta- 2,6-diene (264), a compound that has been proposed as a thermal isomer of... 

Possible okaramine precursors, i.e. cyclo (Trp-Trp) (20), cyclo (2-(reverse-prenyl)-Trp-Trp) (21), cyclo ( -(reverse-prenyty-Trp-Trp) (22), cyclo (2-(reverse-prenyl)-Trp-2 -(reverse-prenyl)-Trp) (23), and cyclo (7V1-(reverse-prenyl)-Trp-2 -(reverse-prenyl)-Trp) (24), were isolated during the course of the investigation of okaramine congeners [21]. Three of these compounds, 21, 22, and 24, are new compounds. cyclo(2-(Reverse-prenyl)-Trp-2 -(reverse-prenyl)-Trp) (23) was synthesized by Schkeryantz and coworkers as a precursor of gypsetin [24], but it was isolated for the first time from natural sources in our study. One year after our findings, Kozlovsky and coworkers reported the isolation of fellutanines A, B, C, and D from Penicillium fellutanum [25], which were found identical to the compounds 20, 22, and 23, respectively. 

The scope of the microwave technique in the preparation of fullerene derivatives was assessed by investigating the well known Diels-Alder reaction of C o with anthracene (4) [19], which has been reported to proceed under thermal conditions (13% reflux, toluene, 3 days [19aj 25%, reflux, benzene, 12 h [19b]) (Scheme 21.4). In addition to 5, multiply-substituted adducts were formed these underwent cyclo-reversion to the starting materials. 

The first examples of the 1,3-oxazete system (44) have been prepared by the sequence shown in Scheme 8. The first stable 4-imino-l,2-oxazetidines (45) have been obtained from [3 + 1] cycloaddition of isocyanides to N-neopent-ylidene-t-butylamine AT-oxide. Their chemistry is dominated by O—N bond cleavage for example, acid brings about rearrangement to amino-acid derivatives, and, in the presence of tetrahydroisoquinoline, cyclo-reversion to give imine and isocyanate can be detected. An unstable 1,3-oxazetidine is involved in the reaction of the imine (46) with the aldehyde (47) to give the imine (48). ... 

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