Cycloaddition, reductive

The cycloaddition, reduction and oxidation reactions emanating from a,/J-unsatu-rated nitroalkenes provide easy access to a vast array of functionalities that include nitroalkanes, N-substituted hydroxylamines, amines, ketones, oximes, and a-substi-tuted oximes and ketones [73-75], Consequently, there are numerous possibilities of using these in situ generated nitroalkenes for the preparation of valuable building blocks and synthetic precursors. 

This cycloaddition-reduction-hydrolysis sequence was also used in an approach to butyrolactones related to ribonolactone (71). These compounds are inducing agents of hunger and satiety in mammalians. Here, a subsequent aldol 1,3-diol reduction was used, and the required carboxy function was established by oxidation of the aromatic ring with ruthenium tetroxide. Cycloaddition of benzonitrile oxide to allyl alcohol afforded an enantiomeric mixture of isoxazolines 55 and 56, which were treated with sodium hydride and methyl iodide to achieve separation by chromatography on cellulose triacetate (71). 0-Demethylation, followed by... 

Isoxazolines are used in organic synthesis because they mask a number of functionalities such as p-hydroxyketones or y- or P-amino alcohols. While they are not affected by the reaction conditions of many organic transformations, they can be easily unmasked in later steps of the synthesis. In fact, because of its wide applicability, this cycloaddition-reductive opening procedure has been termed the "isoxazoline route". 

Cravotto, G., Giovenzana, G. B., Pilati, T., Sisti, M., Palmisano, G. Azomethine Ylide Cycloaddition/Reductive Heterocyclization Approach to Oxindole Alkaloids Asymmetric Synthesis of (-)-Horsfiline. J. Org. Chem. 2001, 66, 8447-8453. 

More recently an alternative biosynthetic pathway has been proposed by Townsend and Basak (Scheme 22) [83]. This alternative model involves the iterative syn-oxidative cyclization of a (Z,Z,Z)-triene intermediate 44 in contrast to the (E,E,E)-triene intermediate 42 of the epoxide cascade mechanism. The putative, alkoxy-linked, non-heme metal-oxo species 45 undergoes a [2-1-2] cycloaddition to yield the metalloxetane intermediate 46. Reductive elimination then results in an overall syn-addition of the two oxygen atoms 47. Oxidation of the metal intermediate 47 to 48 and two further [2 + 2] cycloaddition-reductive elimination-metal oxidation cycles would result, after final hydroxylation, in monensin A 41. 

Recent advances in the cyclizations catalyzed by transition metals and their complexes are reviewed. The catalytic cyclizations discussed here include various carbocyclizations, for example, cycloisomerization, cycloaddition, reductive cyclization, and so on cascade carbocyclizations, for example, cyclotrimerization, silylcarbocyclization, and Heck reaction carbonylative carbocyclizations cyclohy-drocarbonylations intramolecular hydrosilylations intramolecular silylformyla-tions and aldol cyclizations. These reactions serve as efficient and useful methods for the syntheses of a variety of heterocycles and carbocycles that are important... 

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