Nitronates five-membered

Reactions offluorinated dipoles. In recent years, much effort has been devoted to the preparation of tnfluoromethyl-substituted 1,3-dipoles with the goal of using them to introduce trifluoromethyl groups into five-membered nng heterocycles Fluorinated diazoalkanes were the first such 1,3-dipoles to be prepared and used in synthesis A number of reports of cycloadditions of mono- and bis(tnfluo-romethyl)diazomethane appeared prior to 1972 [9] Other types of fluonne-substi-tuted 1,3-dipoles were virtually unknown until only recently However, largely because of the efforts of Tanaka s group, a broad knowledge of the chemistry of tnfluoromethyl-substituted nitrile oxides, nitnle imines, nitnle ylides, and nitrones has been accumulated recently... 

Nitrones are among the most highly studied and useful reagents for the synthesis of five-membered-nng heterocycles. The first fluonnated nitrone, N-methyl-C-(trifluoromethyl)nitrone, was prepared recently and used to introduce Irifluoromethyl groups into such heterocycles... 

Dipolar cycloadditions of five-member cyclic nitrones to a,(3-unsaturated acid derivatives 99H(50)1213. 

Dipolar cydoadditions are one of the most useful synthetic methods to make stereochemically defined five-membered heterocydes. Although a variety of dia-stereoselective 1,3-dipolar cydoadditions have been well developed, enantioselec-tive versions are still limited [29]. Nitrones are important 1,3-dipoles that have been the target of catalyzed enantioselective reactions [66]. Three different approaches to catalyzed enantioselective reactions have been taken (1) activation of electron-defident alkenes by a chiral Lewis acid [23-26, 32-34, 67], (2) activation of nitrones in the reaction with ketene acetals [30, 31], and (3) coordination of both nitrones and allylic alcohols on a chiral catalyst [20]. Among these approaches, the dipole/HOMO-controlled reactions of electron-deficient alkenes are especially promising because a variety of combinations between chiral Lewis acids and electron-deficient alkenes have been well investigated in the study of catalyzed enantioselective Diels-Alder reactions. Enantioselectivities in catalyzed nitrone cydoadditions sometimes exceed 90% ee, but the efficiency of catalytic loading remains insufficient. 

The parent five-membered nitronate having no substituent at the 3-position was too unstable to be isolated. However, 3-substituted derivatives were highly stabilized. Especially, the 3-ethyl derivatives having a terminal electron-withdrawing substituent are readily available by the dehydrochlorination of 3-chloro-l-nitropropane in the presence of electron-deficient alkenes. It was our delight that the reaction of 3-al-kyl-substituted five-membered nitronates was also successfully catalyzed by R,R-DBFOX/Ph-Ni(SbFg)2 complex to at room temperature. This reaction was highly endo-selective (cisjtrans= 91 9) and enantioselective for the endo cycloadduct (92% ee). 

The class of 1,3-dipolar cycloadditions embraces a variety of reactions that can accomplish the synthesis of a diverse array of polyfunctional and stereochemically complex five-membered rings.3 The first report of a 1,3-dipolar cycloaddition of a nitrone (a 1,3-dipole) to phenyl isocyanate (a dipolarophile) came from Beckmann s laboratory in 1890,4 and a full 70 years elapsed before several investigators simultaneously reported examples of nitrone-olefin [3+2] cycloadditions.5 The pioneering and brilliant investigations of Huisgen and his coworkers6 have deepened our under-... 

In addition to nitrones, azomethine ylides are also valuable 1,3-dipoles for five-membered heterocycles [415], which have found useful applications in the synthesis of for example, alkaloids [416]. Again, the groups of both Grigg [417] and Risch [418] have contributed to this field. As reported by the latter group, the treatment of secondary amines 2-824 with benzaldehyde and an appropriate dipolarophile leads to the formation of either substituted pyrrolidines 2-823, 2-825 and 2-826 or oxa-zolidines 2-828 with the 1,3-dipole 2-827 as intermediate (Scheme 2.184). However, the yields and the diastereoselectivities are not always satisfactory. 

Since Huisgen s definition of the general concepts of 1,3-dipolar cycloaddition, this class of reaction has been used extensively in organic synthesis. Nitro compounds can participate in 1,3-dipolar cycloaddition as sources of 1,3-dipoles such as nitronates or nitroxides. Because the reaction of nitrones can be compared with that of nitronates, recent development of nitrones in organic synthesis is briefly summarized. 1,3-Dipolar cycloadditions to a double bond or a triple bond lead to five-membered heterocyclic compounds (Scheme 8.12). There are many excellent reviews on 1,3-dipolar cycloaddition, in particular, the monograph by Torssell covers this topic comprehensively. This chapter describes only recent progress in this field. Many papers have appeared after the comprehensive monograph by Torssell. Here, the natural product synthesis and asymmetric 1,3-dipolar cycloaddition are emphasized.630 Synthesis of pyrrolidine and -izidine alkaloids based on cycloaddition reactions are also discussed in this chapter. 

Lithiated epoxides have been found to react with a number of different activated electrophiles. A new study examines the reactivity of lithiated epoxides with nitrones to prepare 3,y-epoxyhydroxylamines, 46, and oxazetidine, 47 <06OL3923>. Upon deprotonation of styrene oxide, the lithiated reactant was then added to nitrone 45 to form the P,y-epoxyhydroxylamine 46 in good yield as a single diastereomer. A number of additional nitrones were examined as well and all provided similar yields of the 3,y-epoxyhydroxylamines. Treatment of 46 with additional base provided the 1,2-oxazetidine ring system 47 in excellent yield. Interestingly, none of the five-membered isoxazolidines from the 5-endo-tet cyclization were formed in this cyclization. 

Enantio-pure five-membered cyclic nitrones (154) and (155) were formed in a one-pot synthesis from the corresponding lactols (152) and (153) as the result of their reactions with unsubstituted hydroxylamine and by (a) subsequent treatment with MsCl and NaOH (Scheme 2.55) (310a) or by (b) subsequent treatment with TBDMSC1,12, TPP, imidazole, and tetrabutylammonium fluoride (TBAF) (310b). 

Cycloaddition of a-aryl-A-phenylnitrones to the C16-C17 n-bond in 16-dehydropregnenolone-3P-acetate (545) involves only the minor rotamer (A-form) of the nitrones. It proceeds regio-, stereo- and Jt-facial-selectively to give steroido[16,17-d]isoxazolidines (546) in high yield (Scheme 2.257), (Table 2.24) (760). Similarly the cycloaddition of a,N -diphenylnitrones proceeds with five-membered heterocyclic enones (761). 

The second method (path b) involves the initial transformation of AN into nitroso acetals A containing the good leaving group Y followed by 1,3-elimination of the Si Y fragment from the above mentioned intermediate to form the target nitronates. The use of this method in the synthesis of only five-membered cyclic nitronates has been documented (see Section 3.2.1.2.1.2). However, there are no obvious obstacles to the extension of the scope of this method. 

Another approach to the synthesis of cyclic nitronates is based on cycloaddition reactions (Scheme 3.11, Eq. 2), where two bonds (C-C and C-O) are simultaneously formed. This strategy allows one to perform stereoselective processes with the use of very simple precursors. However, this approach to the synthesis of five-membered cyclic nitronates implies that reactive and very unstable nitrocarbenes are involved in the process. 

Intramolecular Cyclization of y-Functionalized AN The most commonly used procedure for the synthesis of five-membered cyclic nitronates (5) is... 

In some cases, five-membered cyclic nitronates can be prepared by the chemos-elective replacement of one of two different halogen atoms in 1,3-dihalopropanes... 

Yet another approach to the synthesis of five-membered cyclic nitronates (5) is based on the Henry condensation of a-halo-substituted aldehydes (9) with primary AN followed by cyclization of nitroaldols (Scheme 3.14, Eq. 4) to give five-membered nitronates containing the hydroxy group at the C-4 atom. 

Finally, Scheme 3.14 presents the Michael addition of bromomalonic ester to conjugated nitro olefins 10. This approach allows one to synthesize five-membered cyclic nitronates (5) doubly functionalized at the C-5 atom (Scheme 3.14, Eq. 5). 

In all the above examples, the synthesis of nitronates (5) is rather chemose-lective. In any case, data on the formation of their structural isomers, viz, the corresponding nitrocyclopropanes (13), are lacking. However, the synthesis of five-membered nitronates (5) with the use of sulfur or selenium ylides is not chemoselective (see Scheme 3.16). 

The process shown in Scheme 3.16 is rather interesting. It should be noted that in most cases this reaction is very stereoselective with respect to the arrangement of the substituents at C-4 and C-5 atoms. In light of recent data on the possible isomerization of nitrocyclopropanes (13) to form five-membered cyclic nitronates (5) (for more details, see Section 3.2.2.1.2), low chemoselectivity of many reactions involving sulfur ylides does not seem to be so fatal. 

Unfortunately, only two attempts were made to use this approach in the synthesis of five-membered cyclic nitronates (5), and only one of them could be considered as successful. In the latter case, isomeric nitrocyclopropane was obtained as the major product. Only a-functionalized nitro alcohols are readily involved in the Mitsunobu cyclization. However, the possibility of isomerization of by-products, nitrocyclopropanes, which was mentioned in the discussion of Scheme 3.16, caused the revision of this process as a procedure for the synthesis of five-membered cyclic nitronates. (A new approach to the synthesis of initial y-nitro alcohols from readily available AN was documented in Reference 64)... 

Recently, Italian researchers have developed a new procedure for the synthesis of five-membered cyclic nitronates with the use of enantiomerically pure epoxides (65-67) and aziridines (68) as the starting substrates (15) (Scheme 3.18, see also substrate B in Scheme 3.11, Eq. 1). 

In the synthesis of five-membered cyclic nitronates (5), the problem of stereoselectivity is in preparing these products with desired relative configurations of the stereocenters at the C-4 and C-5 atoms (see Scheme 3.12). Generally, the trans configuration of these substituents is most preferable. Several procedures giving exclusively this configuration were documented (see,e.g. (50, 55, 58, 63, 68)). 

In spite of the obvious simplicity, this approach has not been examined in practice until recent years. In the very recent past, French researchers demonstrated that this approach can be used for the synthesis of five-membered nitronates 5c,d starting at least from nitro ketones (17a) containing the PhS substituent at C-2. These ketones have been generated from the corresponding 3-nitroenones (Scheme 3.20) (69). 

In 1981, it was demonstrated (70) that anions of nitro compounds can be involved in C,C-coupling with allyl acetates at the allylic carbon atom with the use of metal complex catalysis. For many years, this observation did not come to the attention of chemists interested in the synthesis of cyclic nitronates. However, Trost demonstrated (71) that this process can be used in the synthesis of five-membered cyclic nitronates from olefins (18) containing two acyl groups in the different allylic positions (Scheme 3.21). 

Lastly, the radical inter- and intramolecular cyclizations in the presence of one-electron oxidizing agents as a procedure for the synthesis of five-membered cyclic nitronates can be considered. Radical oxidation of a-nitro ketones (19) in the presence of disubstituted olefins under the action of Mn(OAc)3 was documented (72a) (Scheme 3.22, Eq. 1). 

Synthesis of Five-membered Cyclic Nitronates by Cycloaddition Reactions... 

Here, we have already noted that cyclopropanes (23) are structural isomers of five-membered cyclic nitronates (24). There was evidence that functionalized cyclopropane (23b) can be isomerized to give the corresponding five-membered cyclic nitronate (24b) under the action of halide anions (82) (Scheme 3.28, Eq. 1). Moreover, the in-depth study (79) demonstrated that the above mentioned... 

Synthesis of Five-membered Cyclic Nitronates from a-Halogen-substituted AN. The key step of this approach is presented in Scheme 3.1, path (b). Until recently, this synthetic route to nitronates (24) has been of no preparative interest, because only two examples, such as elimination of trimethylsilyl nitrite (75) and 1,2-dinitrophenylethane (85) from the corresponding nitroso acetals were documented. 

Since inactivated alkenes do not contain the substituent X stabilizing the positive charge in intermediate B (see Scheme 3.40), the latter can undergo a hydride shift to form a more stabilized cationic center (intermediate B in Scheme 3.42), which finally gives rise to an impurity of five-membered cyclic nitronate (24). 

The reactions of ammonia or primary amines with five-membered cyclic nitronates containing the EWG -group at the C-5 atom involve deoxygenation of the nitronate fragment, aromatization of the ring, and amidation of the ester... 

Five-membered cyclic nitronates can be subjected to aromatization by alkali (21) or dilute acids (37, 319) (Scheme 3.110). 

Trost and coworkers (71) used tin dichloride for deoxygenation of annelated five-membered cyclic nitronates (140a,b) with retention of stereocenters (Scheme 3.115). 

An analogous result was obtained by the hydrogenation of five-membered cyclic nitronate. The latter reaction afforded a-oximino-Y-hydroxypentane-1,5-dicarboxylic acid derivative (Scheme 3.116, Eq. 2) in high yield (83%) (319). 

Ozonolysis of Cyclic Nitronates This very interesting approach to cleavage of the nitronate fragment has been recently demonstrated by Linker and coworkers using two optically pure five-membered cyclic nitronates as an example (263a) (Scheme 3.118). 

The ester groups in five-membered cyclic nitronates were successfully subjected to different transformations. Treatment of nitronates containing the methoxycarbonyl groups at positions 3 and 5 with butylamine at 20°C afforded the corresponding diamides (237b) (Scheme 3.119, Eq. 2). Their yields depend dramatically on the nature of the substituent R (further transformations of these products are shown in Scheme 3.109). 

Five-membered Cyclic Nitronates In spite of the fact that five-membered cyclic nitronates are known for more than a hundred years and are classified as the most stable representative of this class of compounds, their... 

Chart 3.13 [3 + 2]Reactivity of five-membered cyclic nitronates. 

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