Solvent-free rearrangement

Tetrasubstituted pyrroles were also obtained in a coupled domino process carried out under solvent-free conditions on silica gel (Scheme 9). The process involved the transformation of the alkynoate 27 into the 1,3-oxazoUdine 28 that could be further rearranged (through loss of one molecule of water)... 

The potential for solvent-free synthesis is relatively large, with examples of many well-known reaction types proceeding quite well under this type of regime these include transesterification, condensation and rearrangement reactions. Many workers have moved away from conventional thermal sources for providing the energy needed for these reactions -... 

A solvent-free synthesis of benzo[b]furan derivatives 10-79, a class of compounds which is often found in physiologically active natural products, was described by Shanthan Rao and coworkers. These authors heated phosphorane 10-71 for 8 min in a microwave oven and obtained the benzo[b]furan 10-74 in 73% yield (Scheme 10.18) [25]. The sequence is initiated by an intramolecular Wittig reaction, providing alkyne 10-72 this underwent a subsequent Claisen rearrangement to give the intermediate 10-73. Also in this case, normal oil-bath heating gave much lower yields (5%) of the desired product the authors hypothesize that the micro-... 

The reaction of 1,2,3-triazolium-l-aminides 3 with propiolate esters led to fluorescent 2,5-dihydro-1,2,3-triazine derivatives 4 in one pot, involving a Huisgen cycloaddition followed by a sequence of rearrangements <06JOC5679 06TL1721>. These reactions can be carried out in acetone, in water, or under solvent-free conditions. 

The quest for a solvent-free deprotection procedure has led to the use of relatively benign reagent, ammonium persulfate on silica, for regeneration of carbonyl compounds (Scheme 6.10) [48]. Neat oximes are simply mixed with solid supported reagent and the contents are irradiated in a MW oven to regenerate free aldehydes or ketones in a process that is applicable to both, aldoximes and ketoximes. The critical role of surface needs to be emphasized since the same reagent supported on clay surface delivers predominantly the Beckmann rearrangement products, the amides [49]. 

A solvent-free pinacol-pinacolone rearrangement has been reported using micro-wave irradiation. The process involves the irradiation of the gem diols with Al3+-mon-tmorillonite K 10 clay for 15 min to afford the rearrangement product in excellent yields (Scheme 6.25) [24]. The comparative studies performed by conventional heating in an oil bath showed that the reaction times are too long (15 h). 

Due to its Lewis acidic properties, the use of chloral (trichloroacetaldehyde) in the Beckmann rearrangement was investigated . When a variety of ketoximes is admixed with chloral hydrate and the mixture is heated at low pressure in nitrogen atmosphere, the Beckmann rearrangement afforded the corresponding amides in excellent yields (73-98%). The transformation occurs under neutral, relatively mild and solvent-free conditions. 

Under solvent-free conditions, one-step Beckmann rearrangement of a variety of ketones and aldehydes proceeded in the presence of alumina sulfuric acid 285 (equation 93). Good selectivities were also obtained in the rearrangement of aldoximes to primary amides using zinc oxide as catalyst " (equation 93). 

Activated Fly ash , an industrial waste pollutant, is found to be a good catalyst in Beckmann rearrangement promoted by microwaves under solvent-free conditions. The amides are obtained from the corresponding ketoximes in high yields (75-94%). 

The simplest solvent-free method involves irradiation of neat reactants in an open container. In the absence of reagents or supports, the scope for such processes appears to be limited to relatively straightforward condensations that can be conducted without added catalysts, or to intramolecular thermolytic processes such as rearrangement or elimination. 

Type of reaction rearrangement Reaction condition solvent-free... 

Solvent-Free Rearrangement under Microwave Irradiation 375... 

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