Nitro-2/7-chromenes, reaction with

The reaction with nitroethanol in the presence of di-)i-bntylammmonium chloride in refluxing iropentyl acetate gives 2-unsnbsdtnted 3-nitro-2H-chromene in 50% yield. Some 3-nitro-2H-chromenes display efficient optical second harmonic generadon for nonhnear opdcal apphcations. 

The high diastereoselective synthesis of multifunctionalized 3,4-dihydro-coumarins bearing a quaternary stereocenter was developed through tandem Michael additions of indole and its derivatives (1-methyl, 2-methyl, 4-methoxy, 5-methoxy, 5-bromo, 6-benzyloxy) to 3-nitrocoumarines (3-nitro-chromen-2-one, 6- and 7-methyl-3-nitro-chromen-2-one) followed by methyl vinyl ketone in a one-pot step. For the tandem Michael additions, after the first Michael reaction of indole (2) with 3-nitrocoumarine (51) catalyzed... 

Reaction of nitro-2f/-chromene derivatives 134 with 135 in methanol at room temperature afforded a mixture of the Z-isomer 136 and tricyclic compound 137, which could be formed by denitrocyclization reaction of the corresponding primarily formed E-isomer and the following dehydrogenation (Eq. 15). The structural identification was based on the MS and H-NMR, however, it is not sufficiently documented and similar examples are not known (91IJC(B)297). 

The Michael aclcliQon of oxygen-nucleophiles followed by subsequent cyclizadon or cycloaddidon provides an important method for the preparadon of oxygen-heterocycles such as tetrahydrofurans For example, 3-nitro-2//-chromenes bearing various snbsdtnents are prepared by the reaction of snbsdtnted sahcylaldehydes with nitro ilkenes fEq 4 14 ... 

Sequential inter- and intramolecular Michael additions are proposed to account for the formation of 3-nitro-4-nitromethylchromans from the reaction of nitroalkenes with 2-(2-hydroxyaryl)-l-nitroethene derivatives 16 that proceeds with high stereoselectivity. Treatment of the products with base yields chromenes <03TL3813>. 

In a quantitative study of the photostability of spiropyrans, Malkin et al.11 concluded that the photodegradation quantum yield d is independent of the heterocyclic part and is determined instead by the structure of the chromene part of SPs. According to these authors, photodegradation of nitro-substituted SPs originates from the reaction of the cis-cisoid X isomer with solvent or impurities (Scheme 6) ... 

Recently, Chen and coworkers found that readily available (2S,4R)-4-hydro>qqDrolinamide 44 in combination with 4-nitrophenol as a cocatalyst effectively catalysed an asymmetric domino oxa-Michael/Henty reaction of salicylaldehyde derivatives 50 with trans-p-nitro olefins 51 to afford 3-nitro-2//-chromenes 52 in moderate to excellent yields and with up to 90% enantiomeric excess under mild conditions (Scheme 10.10). With the corresponding hydrojgr group-free prolinamide a significant reduction in... 

An enantioselective domino oxa-Michael Hemy reaction of substituted salicylaldehydes with nitroalkenes, proceeding through aromatic iminium activation has been developed by Xu et al. by using a chiral secondary amine organocatalyst and salicylic acid as a co-catalyst. This novel domino reaction served as an efficient method for the preparation of chiral 3-nitro-2/f-chromenes with moderate to good enantioseleetivities of up to 92% ee (Scheme 1.78). 

The 1,3-dipolar cycloaddition reaction of non-stabilized azomethine ylides, derived from A-alkyl-cf-amino acids, with 3-nitro-2-trifluoro(trichloro)methyl-2//-chromenes produced l-benzopyrano[3,4-c]pyrrolidines in good yields." AgOAc-catalysed asymmetric 3 + 2-cycloaddition reactions of azomethine ylides with e-deficient alkenes yielded enr/o-adducts with up to 99% ee. New chiral ferrocenyl P,N-ligands possessing a benzoxazole ring as the Af-donor (35) are effective asymmetric catalysts... 

Coumarines as dienophiles were chosen on the basis of earlier study in which the 9kbar pressure and moderate heating (30-70 °C) were used for Diels-Alder reactions of dienes with coumarins substitute at C-3 position with phenylsulfonyl, carboxy, ethoxycarbonyl, nitro, thiophenyl, and pyridyl groups [73]. The corresponding tetrahydro-6H-benzo[c]chromen-6-ones were prepared in excellent yields. When carried out at atmospheric pressure, the reaction yields and selectivities were greatly reduced. 

---