Manganese(III) Acetate Initiated Oxidative Free Radical Reactions between 2-Amino-1,4-naphthoquinones and β-Dicarbonyl Compounds

Abstract

Free radical reactions have become increasingly important in organic synthesis in the last two decades. 1 Compounds containing the quinone group represent an important class of biologically active molecules that are widespread in nature. 2,3 Electrophilic radicals produced from the manganese(III) acetate oxidation of β-dicarbonyl compounds undergo efficient addition to a C-C double bond. 1d,e,4,5 These reactions can be performed intermo-10.

Aminonaphthoquinones in heterocyclization

“…Jiang and Chuang indicated that upon treatment of 2‐(ethylamino)‐1,4‐naphthoquinone ( 66d ) with 2,4‐pentanedione and manganese (III)acetate under similar conditions, compound 76a was obtained in 65% yield and other by‐cyclized products 77 and 78 (Scheme ) [56]. Oxidation of the β‐keto ester by manganese (III) acetate gives radical 79 .…”

“…Oxidation of the β‐keto ester by manganese (III) acetate gives radical 79 . Addition of the radical intermediate to the quinone ring, followed by oxidation, gives 80 (Scheme ) [56]. Compound 80 underwent condensation to produce 76a (path a).…”

“…Radical 81 undergoes either cyclization with (R = H, Me, i ‐Pr) to give 82 , followed by alkyl group migration and oxidation to produce 77 (path b) or oxidation (with R = p ‐tolyl) by manganese (III) acetate to produce imine 84 . Imine 84 underwent further intramolecular nucleophilic addition followed by retro‐Claisen condensation and oxidation to produce 78 (Scheme ) [56]. The free radical mechanism might here explained the rather difficulty arises in Scheme , where an ethyl group on the nitrogen in 81 somehow becomes a propylidene group (in 84 ), thereafter becoming entangled with the incoming diacylmethyl group, the ethyl group eventually winding up on C(2) ( 85 → 78 ) (Scheme ) [56].…”

“…Imine 84 underwent further intramolecular nucleophilic addition followed by retro‐Claisen condensation and oxidation to produce 78 (Scheme ) [56]. The free radical mechanism might here explained the rather difficulty arises in Scheme , where an ethyl group on the nitrogen in 81 somehow becomes a propylidene group (in 84 ), thereafter becoming entangled with the incoming diacylmethyl group, the ethyl group eventually winding up on C(2) ( 85 → 78 ) (Scheme ) [56]. The proposed free radical mechanism can describe unexpected products formation such as those 77 and 78 (Scheme ).…”

“…Reaction of 2‐( p ‐toluidino)‐1,4‐naphthoquinone ( 66f ) with 2,4‐pentanedione in presence of manganese (III) acetate in acetic acid at room temperature afforded compounds 94 and 76 in 77% and 18% yields, respectively (Scheme ) [56].…”