Rearrangements of acetals

Interestingly, the Ireland-Claisen rearrangement of acetate 3a or propionate 3b docs not give the desired acids 4.40... 

Condensation of /3-(hydroxyalkyl)cyclopropanols with aldehydes, followed by rearrangement of acetals formed (Scheme 27), is proposed as simple and convenient method for the synthesis of c -2,7-disubstituted oxepan-4-ones <20050L515>. Though the intermediate acetal could be isolated in some cases, the reaction is usually carried out as one-pot procedure with sequential addition of two Lewis acids, Al(OTf)2 and TiCl4. The overall yields range from 50% to 70%. 

The rearrangement of acetals of 2-haloalkyl aryl ketones is a well-documented process yielding esters of 2-arylalkanoic acids by 1,2-aryl shift (equation 7). The mechanism of this rearrangement is reminiscent of other semipinacol rearrangements. Loss of the halogen (usually assisted by Lewis acid), yields a carbocation (4), which then undergoes a 1,2-aryl shift with carbonyl group formation. 

Oxonium intermediates resulting from the pinacol rearrangement of a-sulfonate acetal derivatives can be intercepted with nucleophilic reagents, affording protected ketones or aldehydes. Rearrangement of acetals in the presence of excess bis(2-methylpropyl)aluminum hydride affords the acetal products31. 

Algarra, F., A. Corma, V Fomes, H. Garcia, A. Martinez and J. Primo, 1993, Rearrangement of acetals of 2-bromopropiophenone as a test reaction to characterize the Lewis sites in large pore zeolites, in Heterogeneous Catalysis and Fine Chemicals III, eds M. Guisnet, J. Barbier, J. Barrault, C. Bouchoule, D. Duprez, G. Perot and C. Montassier, Vol. 78 of Studies in Surface Science and Catalysis (Elsevier, Amsterdam) pp. 653-660. 

Rearrangement of Acetals of 2-Bromopropiophenone as a Test Reaction to Characterize the Lewis Sites in Large Pore Zeolites... 

In the present work, we have carried out the rearrangement of acetals of a-bromopropiophenone to 2-phenylpropanoates catalyzed by large pore zeolites containing Lewis metal ions. We have found that our results can be interpreted taking into account the Lewis nature of the active sites but also assuming that their softness-hardness is modified by the zeolite framework. 

One of the most general approaches to the synthesis of this type of compounds involves the Lewis acid promoted rearrangement of acetals of a-substituted propiophenones [4,5]. Besides silver salts, a large variety of soft and borderline Lewis acids have been found to be convenient catalysts for the 1,2-aryl shift, noticeably zinc halides in substoichiometric amounts [6]. As final products need to be free of metal traces for human consume, the use of Lewis acids supported on microporous solids can be advantageous since a better recovering of the catalyst can be anticipated. 

Repeated crystallization from methanol raises the optical purity to 98%. Rearrangement of acetal 93 with zinc chloride followed by ester hydrolysis furnishes (R)-ibuprofen (94) (82% optical purity) [32] (Scheme 14). 

Wittig rearrangement of acetals has been probed.However, the reaction of benzyl Y-(trimethylsilyl)propargyl ether with Bu"Li has been shown to yield the ortho-12,3 ]- rather than the [1,2]-Wittig product. The enantioseleetive... 

Rearrangements of acetals require substitution at the internal alkene carbon. 

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