Europium catalyzed intramolecular oxazole Diels-Alder reactions for the synthesis of benzopyrano[4,3-b]pyridines and benzo[h]-1,6-naphthyridines

Intramolecular Diels–Alder Reactions of Oxazoles, Imidazoles, and Thiazoles

The development of the intramolecular Diels–Alder cycloaddition­ of azole heterocycles, i.e. oxazoles (IMDAO), imidazoles (IMDAI), and thiazoles (IMDAT), has had a significant impact on the efficient preparation of heterocyclic intermediates and natural products. In particular, highly efficient and versatile IMDAO reactions have been utilized as a key step in several synthetic schemes to provide alkaloids and terpenoid target molecules. More limited studies have been performed on IMDAI and IMDAT cycloadditions. Some drawbacks, such as the occasionally­ challenging preparation of IMDA precursors, are also highlighted in this review. Perspectives are provided on how IMDAI and IMDAT­ transformations can be further expanded for target-directed syntheses.

1 Introduction

2 Oxazoles

2.1 IMDAO Approaches to Furanosesquiterpenes and Furanosteroids

2.1.1 Syntheses of Highly Oxygenated Sesquiterpenes

2.1.2 Syntheses of (±)-Gnididione and (±)-Isognididione

2.1.3 Synthesis of (±)-Stemoamide

2.1.4 Synthesis of (±)-Paniculide A

2.1.5 Syntheses of (+)- and (–)-Norsecurinine

2.1.6 Synthesis of Evodone

2.1.7 Syntheses of (±)-Ligularone and (±)-Petasalbine

2.1.8 Syntheses of Imerubrine, Isoimerubrine, and Grandirubrine

2.1.9 Syntheses of Furanosteroids

2.1.10 Syntheses of Substituted Indolines and Tetrahydroquinolines

2.2 IMDAO Approaches to Pyridines: the Kondrat’eva Reaction

2.2.1 Syntheses of Suaveoline and Norsuaveoline

2.2.2 Synthesis of Eupolauramine

2.2.3 Syntheses of (–)-Plectrodorine and (+)-Oxerine

2.2.4 Synthesis of Amphimedine

2.2.5 Synthetic Approach to the Western Segment of Haplophytine

2.2.6 Synthesis of Marinoquinoline A

2.2.6.1 IMDAO Approach to Marinoquinoline A

2.2.6.2 Scope of Allenyl IMDAO Cycloaddition

2.3 Lewis Acid Catalysis in IMDAO Reactions

2.3.1 Effects of Europium Catalysts on IMDAO Reactions

2.3.2 Effects of Copper Catalysts on IMDAO Reactions

3 Imidazoles

4 Thiazoles

4.1 Syntheses of Menthane and Eremophilane

4.2 Further Comments on the Intramolecular Cycloadditions of Thiocarbonyl Ylides

5 Conclusions and Outlook

Use of the Oxazole-Olefin Diels-Alder Reaction in the Total Synthesis of the Monoterpene Alkaloids (-)-Plectrodorine and (+)-Oxerine

A full account of the total synthesis of two monoterpene alkaloids, (-)-plectrodorine [(-)-1] and (+)-oxerine [(+)-3], is presented. The key steps involved are the formation of the oxazole alcohol 10 from the gamma-butyrolactone 9 and the intramolecular Diels-Alder reaction of the oxazole-olefins 13a, b. Since the sign of specific rotation for the synthetic (+)-3 was different from that reported for natural oxerine, the absolute configuration of this alkaloid is not yet fully understood.