Reaction of (2-Nitro- and 2-Bromo-2-nitroethenyl)phosphonates with 1,3-Cyclohexadiene

Abstract

Specific features of the reactions of bis(chloroethyl) 2-nitro-and 2-bromo-2-nitroethenylphosphonates with 1,3-cyclohexadiene were studied. It was found that the reaction with 2-nitroethenylphosphonate occurs stereoselectively and provides bis(2-chloroethyl) endo- (3-nitrobicyclo[2.2.2]oct-5-en-2-yl)phosphonate. 2-Bromo-2-nitroethenylphosphonate under the same conditions gives a mixture of the endo and exo isomers of the corresponding nitrobicyclooctenes. Enhanced tendency of adducts derived from gem-bromonitroethenylphosphonates for intramolecular transformations, such as dehydrohalogenation and aromatization, under the cycloaddition conditions was revealed.We have previously shown that nitro-and halonitroethenylphosphonates act as highly active dienophiles in the Diels3Alder reaction with aliphatic dienes, cyclopentadiene, and furan [133]. In the present work we have studied reactions of nitro-and gem-bromonitroethenylphosphonates I and II with cyclohexadiene, aiming at revealing specific features of these reactions compared with similar reactions with cyclopentadiene. Such reactions are interesting to study in terms of development of convenient synthetic approaches to polyfunctional bicyclooctenes, important intermediates in the synthesis of fragments of natural compounds, such as hormones, vitamin D 3 , etc. [4,5].It is known that cyclopentadiene reacts with nitroethenylphosphonate I in benzene under reflux for 1 h to give a mixture of the endo and exo diastereomers of the corresponding phosphorylated norbornene in 80% yield [1]. Unlike cyclopentadiene, 1,3-cyclohexadiene reacts under more rigid conditions, viz. in benzene under reflux for 36 h. The reaction results in exclusive formation of the endo isomer of bis(2-chloroethyl)(3-nitrobicyclo[2.2.2]oct-5-en-2-yl)phosphonate (IIIa) in 69% yield (see table). Yields, R f values, and IR and 1 H and 31 P NMR spectral parameters of phoshorylated nitrobicyclooctenes IIIa, IVa, and IVb ÄÄÄÂÄÄÂÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄ ³ ³ R f ³ IR spectrum ³ 1 H NMR spectrum (CDCl 3 ), d, ppm (J, Hz)