![]() In the two calculated transition states (endo and exo), the ratio of the distance between C–O to that between C–N was more than one, whereas in the product, this was reversed ( Figure 2). Pioneering computational studies on the mechanism of the intermolecular nitroso hetero-Diels–Alder reaction by Houk demonstrated that the reaction proceeds in a concerted fashion through an asynchronous transition state. Mechanistic studies of the nitroso hetero-Diels–Alder reaction In the reaction of arylnitroso dienophiles containing a nitro group in close proximity to the reaction center, unsubstituted dienes show a higher reactivity than substituted dienes, due to steric effects. However, the electronic effect is not the only factor responsible for the ease of the nitroso hetero-Diels–Alder reaction. In general, cyclic dienes were more reactive than their acyclic counterparts, and dienes with electron-donating substituents, such as α-terpinene and 2,4-hexadien-1-ol, were more reactive than those with electron-withdrawing substituents such as sorbic acid or ethyl sorbate and their unsubstituted counterparts, 1,3-cyclohexadiene and 2,4-hexadiene. The reactivity of dienes has also been studied for the reaction with acyl- and arylnitroso dienophiles that are bound to a solid support. This compatibility with such a range of dienes reflects the remarkable inherent activity of the nitroso agents, especially the acylnitroso moieties. Acyclic dienes are still reactive even while bearing a number of substituents, including both electron-donating and withdrawing groups at the 1 and/or 2 positions. Cyclic dienes such as cyclopenta-, cyclohexa- and cycloheptadiene as well as a number of more complex substituted derivatives have been reported as being highly reactive substrates for the reaction. ĭienes: There is a wide range of acyclic and cyclic dienes available for the nitroso hetero-Diels–Alder reaction. Additionally, acylnitroso compounds can be generated by the rearrangement of diazonitroalkanes 26, the photochemical cleavage of 1,2,4-oxadiazole-4-oxides 25 and the cycloreversion of 9,10-dimethylantracene adducts 27 ( Scheme 7). In 2015, the Whiting group reported an extensive study of acylnitroso compounds prepared in situ by the catalytic aerobic oxidation of hydroxycarbamate using CuCl 2 and 2-ethyl-2-oxazoline in methanol. In a recent work by Tusun dirhodium caprolactamate, and the aerobic oxidation in the presence of catalytic amounts of Cu(II) and pyridine were used for the preparation of acylnitroso compounds. These include the oxidation of nitrile oxides or the corresponding hydroxamic acids using, for example, periodate, Dess–Martin periodinane, Swern oxidation conditions, lead and silver oxide, and transition-metal oxidation with peroxide as the oxidant. Compounds 1a–c will be studied in more detail in this review.Īcylnitroso compounds are generally prepared and used in situ due to their extremely reactive nature and the conditions have been summarized previously. Nitroso compounds: Nitroso compounds are highly reactive dienophiles often used for the hetero-Diels–Alder reaction and the most frequently used representatives are depicted in Figure 1. The main aim of this review is to provide insight into the fundamental relationships between the structures of the reactants and the regio- and stereoselectivity of the hetero-Diels–Alder reaction as well as the possibility of controlling the regio- and stereoselectivity of 3,6-dihydro-2 H-1,2-oxazine formation. Special emphasis is drawn to the influence and control of the reaction under solution and solid-state reaction conditions. This review will thus focus on the regio- and stereoselectivity of the nitroso hetero-Diels–Alder reaction. ![]() The possibility to control nitroso HAD reactions is the most relevant fact for using the nitroso hetero-Diels–Alder reaction for the syntheses of biologically important molecules. These aspects were partially covered in a review by Yamamoto in 2006, which focused on asymmetric nitroso hetero-Diels–Alder reactions, following the review by Miller that was published in 1998. However, none of these reviews focused on general aspects of the regio- and stereoselectivity of the nitroso hetero-Diels–Alder reaction and the possibility of its control. Several excellent reviews on nitroso hetero-Diels–Alder reactions have been published in the past, including general reviews on hetero-Diels–Alder reactions and their applications in organic synthesis, applications of nitroso hetero-Diels–Alder reactions for the synthesis of azasugars, and the utilization of nitroso hetero-Diels–Alder reactions in natural product synthesis and the synthesis of bioactive molecules.
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