Of nitroolefines

To a solution of nitroolefin 4 (200 mg) and isocyanide 16 (169 mg) in a 1 1 mixture of THF and isopropanol (5 mL) was added the guanidine base A -t-BuTMG (180 mg). The resulting solution was heated to 50°C for 3 h, poured into water, and extracted with dichloromethane. The organic layer was dried over sodium sulfate and filtered through a short column of silica gel (eluent dichloromethane). Evaporation under vacuum of the solvent gave the desired pyrrole as a pale crystalline solid (272 mg, 90%) mp... 

The regiochemical control of Pd-catalyzed hydnde transfer reacdon is much more effeedve than that of the radical denitradon, as Shown in Eq 7 98 The base-catalyzed reacdon of nitroolefins with aldehydes followed by denitradon provides a new syntbedc method of homoallyl aicohols fEq 7 99 Exomethylene compounds are obtined by denitradon of cyclic allylic nino compounds with PdfO, HGO-,H and Et- N fEq 7 100 ... 

Sakai, K, A Nakazawa, K Kondo, and H Ohta (1985) Microbial hydrogenation of nitroolefins. Agric Biol Chem 49 2231-2236. 

Meah Y, V Massey (2000) Old yellow enzyme stepwise reduction of nitroolefins and catalysis of acid-nitro tautomerization. Proc Natl Acad Sci USA 97 10733-10738. 

The regiochemical control of Pd-catalyzed hydride transfer reaction is much more effective than that of the radical denitration, as shown in Eq. 7.98. The base-catalyzed reaction of nitroolefins with aldehydes followed by denitration provides a new synthetic method of homoallyl alcohols (Eq. 7.99).140 Exomethylene compounds are obtained by denitration of cyclic allylic nitro compounds with Pd(0), HC02H and Et3N (Eq. 7.100).140b... 

As follows from above, the rate of [4+ 2]-cycloaddition of nitroolefin (42) is determined by the nature of partner (43) and decreases in the following series of olefines electron-rich > electroneutral > electron-deficient (100). 

Nitroolefins are good Michael acceptors and their reactivity toward diethylzinc is different depending on the presence or absence of Lewis acids. Since the nitro group can be further transformed to a variety of useful containing functionalities,76,7641 the asymmetric 1,4-additions of nitroolefins may provide an easily accessible pathway to highly versatile optically active synthons. 

In this context, Yadav and coworkers have recently developed a mild and efficient procednre for the selective rednction of / -nitrostyrenes 22 to oximes 23 using indium metal in aqueous methanol under neutral reaction conditions (Scheme 19). Similarly, a-alkoxy oximes 24 are formed in good yields by the treatment of nitroolefins with metalhc indium and trimethylsilyl chloride in anhydrous methanol (Scheme 19). 

Nitroolefins are attractive alternative acceptors to enones. In 1999, Barnes and Ji reported an efficient catalyst system for reaction of nitroolefins with 1,3-dicarbonyl compounds with high enantioselectivity (up to 97% ee Scheme 19) [22], Using this method, a highly functionalized nitro compound 38, an important intermediate in the synthesis of an endothelin-A antagonist, was prepared on a large scale in 88 % ee. In this case, the formation of a chiral Mg enolate as a reactive intermediate was proposed. 

Scheme 19. Catalytic reaction of nitroolefins with 1,3-dicarbonyl compounds.

Table 6.7 Cinchona alkaloid-catalyzed Michael reactions of nitroolefins.

An example for the use of the boron-zinc exchange reaction for copper-mediated SN2 -substitutions of allylic electrophiles is the hydroboration of nitroolefin 130 with diethylborane, followed by successive transmetallation of the borane 131 with diethylzinc and CuCN-2LiCl, and final trapping with allyl bromide to give the product 133 with 83% yield over four steps (Scheme 34).34,34a This transformation again demonstrates the tolerance of the method towards functional groups and acidic hydrogen atoms. 

A similar reaction of nitroolefins with kctcnc silyl acetals provides y-keto esters but in this case only TiCE is an effective catalyst. Yields arc low unless titanium(IV) iso-propoxidc is added to suppress hydrolysis of the ketene silyl acetals. 

The activation of alcohols with CuCl or CUCI2 Lewis acids in conjunction with di-cyclohexylcarbodiimide (DCC), developed by Corey [8], has been employed by Knochel and Seebach for the facile preparation of nitroolefins in high yield (Sch. 3) [9]. Sensitive dienoyl compounds can be prepared, as shown in the elimination of 11 to provide 12 in excellent yield [10]. 

Saikia, A. K., Barua, N. C., Sharma, R. P., Ghosh, A. C. The zinc-trifluoroacetic acid reaction in organic solvents afacile procedure for the conversion of nitroolefins into carbonyl compounds under mild conditions. J, Chem. Res., Synop. 1996,124-125. 

Bezbarua, M. S., Bez, G., Barua, N. C. A facile procedure for the conversion of nitroolefins to carbonyl compounds using the AI-NiCl2.6H20-THF system. Chem. Lett. 1999, 325-326. 

Reduction of nitroolefins. Hassner and Heathcock found that 6-nitrocholesteryl acetate can be reduced with sodium borohydride in ethanol to the saturated 6a-nitro-5a-steroid. 

Supported phosphine 54 also acts as an efficient catalyst for isomerization processes. The isomerization of nitroolefins to obtain the (Ej-isomer has been reported to occur in quantitative yields and without the activation of other side reactions [196]. Similarly, the isomerization of a,P-ynones to (/ , fP-y,8-dieriories (60) is catalyzed by 54 with high efficiency and stereoselectivity (Scheme 10.12) [197]. The recovery and reuse of these catalysts is generally reported, but this is strongly limited, as in many other applications involving phosphines, by their oxidative degradation. 

The reduction of nitroolefins is also possible using other microbial systems. In a microbial screening20 good reducing strains were found among actinomycetes belonging to Rhodococcus, Nocardia and Mycobacterium. The yield is comparable to the results obtained with yeast, however, the enantiomeric excess values remained low due to spontaneous racemization of the products. 

A convenient and novel one-pot organocatalytic methodology for the stereoselective synthesis of highly functionalized nitrocyclopropenes has been developed [90] (Table 3.9). DBU (1) catalyses the addition of dimethyl chloromalonate to a variety of nitroolefins to afford a Michael adduct, which cyclizes to form the cyclopropane in the presence of 1 under carefully controlled reaction conditions with outstanding diastereoselectivity. 

Table 9.13 Thiourea catalysed Michael reaction of nitroolefins with indole...

Seebach studied the conjugate addition reaction of nitroolefin 10 with butyllith-ium in the presence of the multidentate ligand 11 (accessible from tartaric acid), which yielded the adduct 12 in up to 58% ee (Scheme 4) [18]. 

There have been a number of investigations involving the study of nitroolefins as useful intermediates in the synthesis of energetic materials. A characteristic of many of them is a large twist about the double bond. Interest in the structural characteristics that accompany the rotation motivates a discussion of details. As would be expected, the rotation out of the normal planar conformation is associated with spatial crowding. Rgure 14 illustrates some of the largest out-of-plane rotations that have been measured and, in one case, calculated for crowded ethylenes [25j. 

In the course of the synthesis of potential D1 agonists, DoM of pyridine 2-carboxamide derivatives X = CR, Y = N, R = H, Me, F, OMe, n-Pr, and R = H, Me, w-Bu with 2 equiv of BuLi followed by Michael addition of nitroolefin and a triethylamine-mediated equilibration gives the trans-lslnlm products (eq 54)... 

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