3-Nitrocoumarins as Dienophiles in the Diels−Alder Reaction in Water. An Approach to the Synthesis of Nitrotetrahydrobenzo[c]chromenones and Dihydrodibenzo[b,d]furans

• Published in: Journal of organic chemistry 2003-11, Vol.68 (24), p.9263-9268
• Main Authors: Amantini, David, Fringuelli, Francesco, Piermatti, Oriana, Pizzo, Ferdinando, Vaccaro, Luigi
• Format: Article
• Language: English
• Subjects: Chemistry; Exact sciences and technology; Heterocyclic compounds; Heterocyclic compounds with o, s, se, te hetero atom and condensed derivatives; Organic chemistry; Preparations and properties
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo034956e

The [4 + 2] cycloadditions of 3-nitrocoumarin (1a), 6-chloro-3-nitrocoumarin (1b), and 6-, 7-, and 8-hydroxy-3-nitrocoumarins (1c, 5, and 6) with (E)-piperylene (7), isoprene (8), 2,3-dimethyl-1,3-butadiene (9), 2-methoxy-1,3-butadiene (10), 2,3-dimethoxy-1,3-butadiene (11), and cyclopentadiene (12) were investigated in aqueous medium, in organic solvent and under solventless conditions. The reactions performed in water occurred in heterogeneous phase but were faster than those executed in toluene or dichloroethane (DCE). 1a−c, 5, and 6 behaved as 2π components in the Diels−Alder cycloadditions with 7−10 and 12, and exo adducts were preferentially or exclusively produced. Surprisingly 1a, behaved as a 4π component in the cycloaddition in water with 11 and 4-substituted 3-nitrochromanones 20 and 21 were isolated. The cycloadditions of hydroxy-3-nitrocoumarins 1c, 5, and 6 with 1,3-diene 9 did not work in water or in organic solvent, but did work under solventless conditions. Nitrotetrahydrobenzo[c]chromenones 13−16, 24, and 25, originating from the normal electron-demand Diels−Alder reactions, were converted into dihydrodibenzo[b,d]furans 27−31 in water, via one-pot Nef-cyclodehydration reactions.