Nitroso compounds Diels-Alder reactions

Fluorinated heterodienophiles and heterodienes Diels-Alder reactions in which the dienophiles have perfluoroalkyl-substituted multiple bonds between carbon and a heteroatom are quite common Reported earlier were reactions of perfluoroketones, thiones, ketimines, thioesters, nitroso compounds, and nitriles [9] Examples of a-fluoroimines [107], co-hydroperfluorothioaldehydes [108], perfluorosulfines [109, IIO], and selenocarbonyidifluoride [III] (equations 89-92) have been reported recently... 

The intramolecular /zetero-Diels-Alder reactions of 4-O-protected acyl-nitroso compounds 81, generated in situ from hydroxamic acids 80 by periodate oxidation, were investigated under various conditions in order to obtain the best endo/exo ratio of adducts 82 and 83 [65h] (Table 4.15). The endo adducts are key intermediates for the synthesis of optically active swainsonine [66a] and pumiliotoxin [66b]. The use of CDs in aqueous medium improves the reaction yield and selectivity with respect to organic solvents. 

The nitroso moiety of the N-acylnitroso function is a powerful dienophile and therefore N-acylnitroso compounds are trapped rapidly, especially in an intramolecular reaction, with a diene allowing the Diels Alder reaction to occur also in water, although N-acylnitroso compounds are short-lived in aqueous medium. 

On transforming the cyclopentenone 1431 into the trimethylsilyloxy diene 1432 the ensuing Diels-Alder cyclization gives rise to 69% tricychc compound 1433 [10] (Scheme 9.8). For Diels-Alder-reactions of thioaldehydes, selenoaldehydes, or unsaturated nitroso compounds with cyclopentadiene, see the reactions of 602 to 603 and of 605 to 606 in Scheme 5.48 and of 1092 to 1093 in Scheme 7.43. For Diels-Alder-reactions of silyloxyazadienes such as 510 with maleic anhydride to give 511, see Scheme 5.29. 

Acyl nitroso compounds react with 1, 3-dienes as N-O heterodienophiles to produce cycloadducts, which have found use in the total synthesis of a number of nitrogen-containing natural products [21]. The cycloadducts of acyl nitroso compounds and 9,10-dimethylanthracene (4, Scheme 7.3) undergo thermal decomposition through retro-Diels-Alder reactions to produce acyl nitroso compounds under non-oxidative conditions and at relatively mild temperatures (40-100°C) [11-14]. Decomposition of these compounds provides a particularly clean method for the formation of acyl nitroso compounds. Photolysis or thermolysis of 3, 5-diphenyl-l, 2, 4-oxadiazole-4-oxide (5) generates the aromatic acyl nitroso compound (6) and ben-zonitrile (Scheme 7.3) [22, 23]. Other reactions that generate acyl nitroso compounds include the treatment of 5 with a nitrile oxide [24], the addition of N-methyl morpholine N-oxide to nitrile oxides and the decomposition of N, O-diacylated or alkylated N-hydroxyarylsulfonamides [25-29]. 

Extensive structure-activity relationships for the oxidative formation of C-acyl nitroso compounds or the release of NO or HNO from C-acyl nitroso compounds do not exist. However, the -R group of the cycloadducts of acyl nitroso compounds and 9, 10-dimethylanthracene (4, Scheme 7.3) appears to strongly influence the rate that these compounds undergo retro-Diels-Alder reactions to produce acyl nitroso compounds. 

In general, C-acyl nitroso compounds-9,10-dimethylanthracene cycloadducts derived from hydroxamic acids (-R = alkyl, aryl, ti/2 = 4.1 h for -R = -Ph at 60°C) decompose more slowly than those derived from N-hydroxycarbamates or N-hydroxyureas [11, 13, 14]. Further addition of alkyl groups to the N atom of N-hydroxyurea-derived cycloadducts produces a further increase in the rate of the retro-Diels-Alder reaction of these cycloadducts [36]. These general trends suggest the possibility of the development of acyl nitroso compound-9, 10-dimethylanthracene cycloadducts as a general class of HNO or NO donors with varied release profiles. 

The most important synthetic methods involve condensation of hydrazine, hydroxylamine or hydrogen peroxide with a 1,4-oxygenated carbon chain, and these procedures are particularly useful for the preparation of pyridazines and 1,2-oxazines. Other methods include Diels-Alder reactions of a diene with an azo or nitroso compound. 

Allyl amines can also be formed in an oxidative environment. Nicholas et al. have shown that instead of phenyl hydroxylamine as the nitrogen donor, it is possible using f-BuOOH as the terminal oxidant and the molybdenum catalyst described above [65]. The procedure is analogues to the in situ hetero-Diels-Alder reaction of nitroso compounds developed earlier by others [66]. 

As a current example of a stereoselective intramolecular Diels-Alder reaction using nitroso dienophiles, Kibayashi s studies aimed at the enantioselective total sysnthesis of (-)-pumiliotoxin C 4-31 shall be discussed here [356, 357]. The chiral nitroso compound 4-30 derived from L-malic acid was generated in situ... 

Aliphatic nitroso compounds can be prepared from IV-alkylhydroxylamines by oxidation with bromine, chlorine or sodium hypochlorite in weakly acidic solution, by reaction with potassium dichromate in acetic or sulfuric acid, and by oxidation with yellow mercury(II) oxide in suspension in an organic solvent. Silver carbonate on (Telite has also been used to prepare aliphatic nitroso compounds, such as ni-trosocyclohexane, in high yield from the corresponding hydroxylamines. Aqueous sodium periodate and tetraalkylammonium periodates, which are soluble in organic solvents, are the reagents most commonly used for the oxidation of hydroxamic acids and / -acylhydroxylamines to acylnitroso compounds (40). These compounds are rarely isolated, but are useful as highly reactive dienophiles in the Diels-Alder reaction. ... 

A useful trick is based on the reversible preparation of the adducts of nitroso compounds with 9,10-dimelhylanthracene, e.g.. 5 they undergo a retro-Diels Alder reaction on heating, releasing the nitroso moiety which undergoes a rapid inter- or intramolecular Diels-Alder reaction2,4. 

The dihydrooxazines which are produced from aryl nitroso compounds have not found wide use in synthesis. One application to the syntheses of an N-aiyl-3-pyrrolidone is shown in equation (31). The initial Diels-Alder reaction in this sequence was totally regioselective. 

Kresze and cowoikers have used nitroso Diels-Alder reactions in synthesis of a variety of inosamines and related compounds. One example of this work is the synthesis of conduramine-Fl (92) as shown in equation (34). In this case, diene (91) reacted with a-chloro nitroso cyclohexane to afford adduct (91) as the only product. Just why this stereoisomer was formed is not obvious. Compound (91) could be converted to the conduramine in a few steps. 

The use of acyl nitroso compounds as dienophiles was first described by Kirby. The primary method for generating these highly reactive, unstable species is by periodate oxidation of hydroxamic acids. The acyl nitroso compounds can be trapped in a Diels-Alder reaction with 9,10-dimethylanthracene to give adduct (95) (equation 36) which can act as an alternative source of the dienophile, since retro [4 + 2] cycloaddition occurs under mild thermal conditions. 

Intramolecular acyl nitroso Diels-Alder reactions have been utilized as key steps in some elegant alkaloid total syntheses. Keck has prepared compound (104) (equation 40) which upon thermolysis gave an intermediate acyl nitroso species leading to adduct (105) as a mixture of epimers. This mixture was... 

Nitrosyl cyanide (109) has been briefly examined as a dienophile by Kirby et This intermediate can be generated from nitrosyl chloride and silver cyanide (equation 42) and trapped with 9,10-diinethyl-anthracene to yield adduct (110). As with acyl nitroso compounds, this adduct can be used as a stable source of (109) via a retro Diels-Alder reaction. 

Methods for the reductive cleavage of the N—O bond in hydroxylamines have assumed increasing importance in synthesis because it is a key step in routes based on cycloaddition of nitrones and nitroso compounds. Inter- and intra-molecular cycloadditions of nitrones lead to the formation of tetrahydroisox-azoles (Scheme 25) these compounds are then converted into amino alcohols by reductive cleavage of the N—-O bonds. This type of reaction sequence has been exploited in the synthesis of a number of alkaloids.Nitroso compounds, particularly those activated by electron-withdrawing substituents, act as dienophiles in the Diels-Alder reaction (Scheme 26) and the cycloadducts can be used as synthetic intermediates by cleaving the N—O bonds. 

Efficient hetero Diels-Alder reactions have been carried out using a-chloronitroso saccharides as dienophiles and various symmetrical and unsymmetrical dienes. Thus, 2,3 5,6-di-0-isopropyli-dene-l-C-nitroso-a-D-mannofuranosyl chloride 122, easily prepared from hydroximolactone 121 and tert-butyIhypochlorite, was proven to be a stable and highly reactive dienophilic compound [92]. 

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