Stereochemistry of protonation at carbon of nitronate adducts from 1,6-conjugate addition of Grignard reagents to 9-nitroanthracenes. Crystal structure of cis-9-benzyl-10-nitro-9,10-dihydroanthracene

Abstract

Addition of RMgX (a, R = CH₃; b, R = PhCH₂) to 9-nitroanthracene (4) leads to formation of 1,6-adduct (5) in high yield. On treatment with dilute acetic acid, (5) is converted into the corresponding 10-alkyl-9,10-dihydroanthracene-9-nitronate anion (7), which quickly undergoes a stereoselective protonation at C-9 to give almost exclusively cis-10-alkyl-9-nitro-9,10-dihydroanthracene (8). Both (8a and b) remain unaltered under isomerization conditions. The formation of the more stable isomer, under kinetic control conditions, in a true boat system as (6), is interpreted in terms of a steric control on the proton approach, exerted by the axial alkyl group in position 10. The structure assignment of (8a, b) has been made by X-ray analysis of the benzyl derivative (8b). Structural parameters are discussed in comparison with those reported in the literature for analogous systems.