Recent Advances in the Synthesis of Angucyclines

The different strategies recently reported to construct the tetracyclic skeleton of angucyclines are presented: Diels-Alder and Friedel-Crafts reactions, nucleophilic additions, free radical annulations, rearrangements of cyclobutenones and cobaltmediated [2+2+2] cycloadditions. Among the asymmetric approaches, the most efficient corresponds to Diels-Alder reactions including the use of chiral catalysts, enantiopure vinyl cyclohexenes as dienes, synthesized from quinic acid or (S,S)[(p-tolylsulfinyl)methyl]-p-quinol, and chiral dienophiles such as (S,S)-2-(p-tolylsulfinyl)-1,4-naphthoquinones. 1 Introduction 2 Diels-Alder Reactions 2.1 Formation of B Ring: DC+A Strategy 2.2 Formation of C Ring: D+BA Strategy 2.3 Formation of D Ring from a Dienophile Bearing the CBA Moiety 3 Friedel-Crafts Reactions 4 Nucleophilic Additions 4.1 Anionic Annulations with Phthalides 4.2 Other Anionic Annulations 5 Free Radical Annulations 6 Rearrangements of Cyclobutenones 7 Cobalt-Mediated [2+2+2] Cycloadditions 8 Summary