Cyclo-elimination

The UV irradiation of the 4,5-dihydro-1,3,5-oxazaphosph(V)olene 39 leads to a cyclic elimination. Analogously to the thermal [5 + 3 + 2] cyclo elimination, the photochemical reaction generated nitrile ylides reacted with alkynes and alkenes to yield the 2/f-pyrrole 40 and the pyrrol-l-ine 41, respectively (equation 17)71. 

An effect of the ester spatial conformation on the cyclo-elimination process has been observed [88] the isobutene elimination is assumed to... 

The activation energy of the cyclo-elimination has been estimated at 3 kcal mole-1 below Tg [88]. Since the isobutene yield decreases in the presence of naphthalene, benzophenone and oxygen, the reaction is thought to proceed through the excited triplet state of the ester side group [88]. 

The decomposition of intermediate 13 to the isolated alcohol 14 can occur by one of two possible pathways. The first is a net cyclo-elimination that regenerates the catalyst and forms boronate 10. 

The role of the remaining acetoxy group is not clear. They may cleave homolitically as mentioned above or give acetic acid and double bond by a cyclo elimination reaction (38). 

Immediately after the discovery of the advantageous properties of permethric acid, many research laboratories initiated enhanced efforts to find the most convenient synthesis ofthis compound. The Japanese Sagami Research Institute developed a synthetic route based on a Claisen rearrangement and a 1,3-cyclo-elimination, for which FMC took out a licence. A few weeks later, two other firms, Sankyo and Kmaray, filed patent applications for similar syntheses. 

Preparation of Isothiocyanates.—A conventional synthetic method is illustrated in the preparation of AT-(isothiocyanato-acyl)-amino-acids, e.g. S C N (CH2) C0 NHCHR C02H, starting from the peptide trimethyl-silyl ester treatment with CSg and an alkyl chloroformate gives an alkoxycarbonyl dithiocarbamate which readily cyclo-eliminates COS and alkanol. Acylation of an amino-acid trimethylsilyl ester with SCN--(CH2) C0 C1 provides an alternative route cyclization of an a-isothio-cyanato-acid (158) to a 2-thio-oxazolidone (159) occurs readily in solution, ... 

Cyclo elimination is also utilized by nature to synthesize enzymatically the cyclopropylic precursor of chrysanthemic acid from isopentenyl pyrophosphate. According to Crombie, this chemical process in the flowerhead of Chrysanthemum cinerafolis can be rationahzed as shown in reaction scheme 9a [927]. The role of the leaving group Y is taken by a bionucleophile Nu ... 

Hexenoic esters 124 suitable for 1,3-cyclo elimination can be obtained, if the allyl alcohol component for making the vinyl-enolether system for Claisen rearrangements already contains the chlorine atoms, thus avoiding the introduction of carbon tetrachloride at a later stage [179] (Reaction scheme 79). 

An adapted version of the Sagami-synthesis (Reaction scheme 65) allows the use of the interesting 3,3-dimethylpentenic ester IIL If one chlorine atom in carbonte-trachloride is substituted by CF3 or CH3, or if 1,1-dibromotetrafluoroethane is used, the radical addition to this olefinic bond is still feasible. As in the case of 1,1,1-trichloroethane, more drastic conditions are required. l,l,l-trichloro-2-tri-fluoroethane, on the other hand, behaves almost normally, thus providing an efficient starting point to produce the desired acid as shown in Reaction scheme 144, particularly if the 1,3-cyclo-elimination is carried out with potassium carbonate in dimethylacetamide at 140 °C, giving predominantly the cis-Z-isomer [411]. 

As outlined previously (Section 3,2), reactions here considered as cyclo-eliminations are those in which n a-bonds plus m tt-bonds are transformed into (n — I) (j-bonds plus (m + 1) Tr-bonds. Most of the reactions in this category have elsewhere in the literature been defined as cyclo-additions and sigmatropic rearrangements. It is felt that the particular classification adopted here is more rigorous and less confusing. 

Reaction (6.136) is the prototype for cyclo-eliminations in this category it is closely related to a [1,3] sigmatropic shift, and the reverse reaction is basically i[ 2 + 2] cyclo-addition. In the thermochemical reactions the [2g + 2,] interaction should be observed if concerted, and this topology has been shown... 

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