Abstract
Asymmetric [3+2] cycloadditions of azomethine imines comprise a useful synthetic tool for the construction of pyrazole derivatives with a variable degree of saturation and up to three stereogenic centers. As analogues of pyrrolidines and imidazolidines that are abundant among natural products, pyrazoline and pyrazolidine derivatives represent attractive synthetic targets due to their extensive applications in the chemical and medicinal industries. Following the increased understanding of the mechanistic aspect of metal-catalyzed and organocatalyzed [3+2] cycloadditions of 1,3-dipoles gained over recent years, significant strides have been taken to design and develop new protocols that proceed efficiently under mild synthetic conditions and duly benefit from superior functional group tolerance and selectivity. In this review, we represent the current state of the art in this field and detailed methods for the synthesis of non-racemic pyrazolines and pyrazolidines via [3+2] metal and organocatalyzed transformations reported since the seminal work of Kobayashi et al. and Fu et al. in 2002 and 2003 up to the end of year 2017.
Highlights
Asymmetric [3+2] cycloadditions of azomethine imines comprise a useful synthetic tool for the construction of pyrazole derivatives with a variable degree of saturation and up to three stereogenic centers
Less than three dozen reports on the synthesis of non-racemic pyrazolines and pyrazolidines by [3+2] cycloaddition of azomethine imines can be found in the literature
This review covers the synthesis of non-racemic pyrazolines and pyrazolidines by [3+2]
Summary
[3+2] Cycloadditions comprise a powerful tool for the construction of five-membered heterocycles [1,2,3,4]. The advent of asymmetric catalysis, which took place in the field of [3+2] cycloaddition chemistry, nowadays enables the preparation of various types of non-racemic saturated heterocycles in a highly efficient manner. Within this context, the use of nitrile oxides, nitrones and azomethine ylides is well established [7,8], whereas asymmetric cycloadditions of azomethine imines remain much less explored [8,9,10,11]. Reactions with acetylenes provide access to pyrazolines, whereas whereas cycloadditions to olefins are employed to obtain the fully-saturated pyrazolidines. Fu Fu have published the first example ofcatalyst asymmetric [3+2] cycloadditions of azomethine imines to acetylenes in. In the reactions of Ag(I)olefins,and asymmetric organocatalysts comprise the amenable in cycloadditions to acetylenes [11,15].
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