An efficient method for the one-pot construction of the 1H-naphtho[2,3-c]pyran-5,10-dione system

2-(1-Hydroxyalkyl)-1,4-naphthoquinones are found to react with pyrrolidino enamines in toluene to give 1H-naphtho[2,3-c]pyran-5,10-diones in good yields via a tandem conjugate addition–cyclization sequence, followed by an elimination of pyrrolidine. 2-Hydroxymethyl-1,4-naphthoquinone and morpholino enamines undergo a similar sequence, without loss of morpholine, to yield 3-morpholino-3,4-dihydro-1H-naphtho[2,3-c]pyran-5,10-diones. The 3-morpholino group of these products can be replaced with a hydro, a hydroxy, or a methoxy group. Imines also react with 2-(1-hydroxyalkyl)-1,4-naphthoquinones to give the corresponding 1H-naphtho[2,3-c]pyran-5,10-diones, including a natural product (pentalongin). The utility of these reactions is demonstrated in the synthesis of pyranonaphthoquinone antibiotics, viz. (±)-eleutherin and (±)-isoeleutherin.