Jensen, Allan Rostrup Forup; (1999) New Chiral Auxiliaries For The [3+2]-Cycloaddition Of Nonstabilised Azomethine Ylides. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The [3+2]-cycloaddition reaction of nonstabilised azomethine ylides to alkenes is a valuable synthetic method for the assembly of functionalised pyrrolidines. However, there are only a few examples of such eye lo add it ions being successfully performed with an unstabilised azomethine ylide that has been tethered to a removable chiral auxiliary. Most of the reactions studied so far have exhibited only modest levels of diastereoselectivity (ca. 60 % d.e.), and in every case, destruction of the chiral auxiliary has proven necessary before the newly fashioned chiral pyrrolidine cycloadduct could be liberated. In the first part of this thesis, the potential utility of optically pure 1,1-dialkylhydrazines as chiral auxiliaries for nonstabilised azomethine ylide cycloadditions to alkenes has been investigated. While the preparation of several chiral 1,1-dialkylhydrazines was carried out successfully, the formation of the N-amino azomethine ylide precursors from these hydrazines failed, occasionally giving interesting unwanted and unexpected by-products. The second part of this thesis focuses on the evaluation of several new chiral auxiliaries as stereochemical control elements for 1,3-dipolar cycloadditions of azomethine ylides to alkenes. This work failed with some auxiliaries and was partially successful with others. The successful [3+2]-cycloaddition reactions were all performed with dimethyl fumarate as the dipolarophile, but afforded little or no stereoselectivity. The cleavage of the newly-created pyrrolidine systems from these auxiliaries under various conditions also proved to be unsuccessful. During this work a new method for the preparation of 1,1-dialkyIhydrazines by the reduction of N-nitroso precursors was discovered. The potential utility of this method has been evaluated, and the results are discussed in Chapter 4.

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