Macrocycles by Intramolecular Diels‐Alder Reaction—Regioselective Synthesis of Anthracycline Precursors

The key step in the synthesis of the anthracyclin precursor 2, the simultaneous formation of the ring A and the 18‐membered lactone ring, proceeds astonishingly in 58% yield by intramolecular Diels‐Alder reaction of 1, R(CH₂)₆OCOCHCH₂.The analogue of 1 with three instead of six methylene groups does not cyclize. (Figure Presented.)