Conjugate system nitroalkenes

The conjugate system of the C-2 nitroalkenes should posses some interesting chemical reactivity and it should be an excellent Michael reaction acceptor with reactive nucleophiles. Moreover, the steric effect of the bulky 1,6-anhydro ring should be similar to that of levoglucosenone. As a consequence, nitroalkenes are excellent precursors for the stereoselective introduction of an additional sugar moiety at C-2 with subsequent additional functional group such as nitromethylene or its reduced/acetylated analog. Moreover, this unsaturated C-2 functionality additionally fixes the conformation of the system and most importantly sterically hinders the P-D-face of both enone molecules. 

The following are data in ethanol [4] which illustrate the effect of lengthening the conjugated system in aliphatic unsaturated compounds Table (13). Kochany and Piotrowska [10] examined the ultraviolet-spectra of a number of nitroalkenes. Their w — tt were observed between 360 and 420 run. The ultraviolet absorption spectra of nitroethylene, nitropropenes and nitro methane have been taken and interpreted with the Pariser-Parr-Pople self-consistent field — MO calculation. The absorption bands it it are in good agreement with calculated values [11]. 

This chapter covers the recent literature reports (ca. 2003 onward) on asymmetric conjugate addition of triorganoaluminum reagents to enones, a,(3-unsaturated systems, nitroalkenes and also mechanistically closely related allylic alkylation of allylic phosphonates. It also includes cascade processes where the intermediate enolates (conjugate addition) and alkenes (allylic alkylation) are used for the synthesis of more complex molecules. This chapter is organized as follows. In Sect. 2 we present the results in the asymmetric conjugate addition. In this part, the... 

Clearly, the nitroalkene dipolarophile oxidoisoquinolinium betaine 123 is nonideal for the synthesis of the hetisine alkaloids, as mass throughput for the needed cycloadduct would be low, and conversion of the tertiary nitro group to carbon-based functionality, as would be required in the latter stages of the synthesis, could be problematic. On the other hand, an ene-nitrile dipolarophile has several potential advantages over nitroalkene dipolarophile. Most importantly, the ene-nitrile cycloadduct has carbon functionality installed at the C-10 position. Second, the conjugate addition byproduct pathway that occurs so readily for the nitroalkene oxidoisoquinolinium betaine 123 system (see Scheme 1.13) should be much slower... 

However, the introduction of sterically hindered substituents at the p-car bon atom of nitroalkene (42) completely changes the ring-chain tautomerism of conjugated nitroalkenes. Apparently, steric hindrance caused by two bulky Bu groups in product (42a) (Scheme 3.47) prevents effective conjugation of the jt systems of the C,C double bond and the nitro group, thus causing its deviation from the plane of the C=C bond as a result of which isomer (47a) becomes thermodynamically more favorable. 

Bordoloi, M. Cadmium chloride-magnesium-water a new system for regioselective transformation of conjugated nitroalkenes to ketocompounds conversion of 6-nitro-D5-steroids to 6-ketosteroids. J. Chem. Soc., Chem. Commun. 1993, 922-923. 

Das, N. B., Sarangi, C., Nanda, B., Nayak, A., Sharma, R. P. SnCl2.2H20-Mg-H20 a mild reagent system for the regioselective transformation of conjugated nitroalkenes to carbonyl compounds. J. Chem. Res., Synop. 1996, 28-29. 

In the review period, additional data became available (118) for nitroalkene systems (Table XXVI, note h). The structures with a double bond system separated from the N02 group by at least one saturated carbon atom give N02 shifts within the range characteristic of nitroalkanes. However, the presence of a double bond in a position conjugated to the N02 system results in an increased screening (Table XXVI) by 10-20 ppm [55], [56]. 

Michael addition of diethyl cyanomethylphosphonate to a-nitroalkenes in the presence of LDA (1 eq) followed by treatment with Mc,SiOI provides a one-pot synthesis of 2-isoxazoline derivatives. The reaction proceeds through a 1,3-dipolar cycloaddition of trimethylsilyl nitronate with an a-cyanovinylphosphonate. Similarly, the 1,4-addition of the sodium diethyl cyanomethylphosphonate to the azo-ene system of conjugated alkenes results in the formation of l,2-diamino-3-(dietlioxyphosphinyl)pyrroles in moderate to good yields via a hydrazonic intermediate. - In a route to phosphonylated pyrones, the sodium diethyl cyanomethylphosphonate reacts with 3-anilinometliylene derivatives of 4-hydroxycouinaiins in DMF with displacement of aniline. ... 

Aryl transfer from ArSi(OEt)3 to conjugated ketones, lactones and lactams is achieved with the aid of a palladium(ll) salt supported by 116. It is a variation of the reaction involving ArB(OH)2 with a similar system. The P,P -dioxide of the same ligand complements CuOTf to serve as catalyst for the addition of R2Zn to nitroalkenes. ... 

DMAP Catalysis to Form Key Substructures. DMAP has been used as a catalyst in the Morita-Baylis-HiUinan (MBH) reaction. Conjugated nitroalkenes can be reacted with an amine base to form a zwitterionic species that can add to many types of ketones and aldehydes. Remarkably, only DMAP and imidazole were able to effectively catalyze this reaction, likely due to their small size and ability to stabilize the intermediate. In another example, a unique catalyst system involving 1 1 1 DMAP TMEDA Mgl2 (each in 10 mol %) allowed the MBH reaction to proceed in fairly high yields between aldehydes and a , -unsaturated ketones, esters, or a thioester. ... 

Another important feature is that the nitro group is an electron-withdrawing substituent, increasing electrophilicity of an aromatic system. Due to this fact, nitroarenes sometimes exhibit the reactivity, which is similar to that of conjugated nitroalkenes, and are able to undergo pericychc reactions (Scheme 4). 

Among the most successful catal)rtic systems employed in enantioselective nickel-catalysed Michael additions of nucleophiles to nitroalkenes is that developed by Evans and Seidel in 2005, which allowed enantioselectivities of up to 95% ee to be achieved in the enantioselective conjugate additions of 1,3-dicarbonyl compounds to nitroalkenes (Scheme 2.3). Products resulting... 

Nitroalkenes can be converted into a variety of functionalities, and the transformation of conjugated nitroalkenes into carbonyl compounds is of particular interest. A simple, cheap and practicable procedure has been developed using the NaBH4/H202 system [72] ... 

Cyclopropanecarboxylic esters have been prepared, in 75—86 % yield, by intramolecular alkylation of 4-chloroalkyl esters, using phase-transfer catalysis. Monoalkylation of nitro-alkenes by acrylic esters occurs in a controlled manner if a two-phase system is used, to give products of Michael addition in 45—65 % yield for five examples. An interesting variant on this reaction involves the generation of the a-nitro-carbanion by conjugate reduction of a nitroalkene with sodium borohydride followed by its conjugate addition to methyl acrylate yields of 62—95% are reported for five cases (Scheme 39). ... 

Takizawa and Sasai developed a new class of acid-base chiral organocatalysts (122 and 123) bearing an imidazole unit for aza-MBH reaction of conjugated nitroalkenes (Scheme 31.32) [46]. The acidic phenolic hydroxy groups and basic imidazole unit cooperatively activate nitroalkenes to promote the aza-MBH reaction in good yields with moderate enantioselectivities. They also investigated the substrate scope of this catalytic system under the optimized reaction conditions. Regardless of whether the aromatic substituent of imine 125 is electron-... 

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