Camphor Derivatives in Asymmetric Cycloadditions and Rearrangements

Abstract

Camphor-derived α,β-unsaturated oxazolines and oxazoline N-oxides are, respectively, useful dienophiles and dipolarophiles in [2+4] and [2+3] diastereoselective cycloadditions. The scope and limitations of these two reactions as well as their synthetic applications in the synthesis of various natural products are discussed. The Account also covers σ-[2,3] rearrangement of oxazoline N-oxide, which gives a new insight into the mechanism of this type of reaction. Finally, recent developments in Diels-Alder organocatalyzed cycloaddition with camphor-derived sulfonylhydrazines are also discussed.

1 Introduction

2 Diels-Alder Cycloadditions

2.1 Camphor-Derived α,β-Unsaturated Oxazolines in Diels-­Alder Cycloadditions

2.2 Diels-Alder Cycloaddition with 2-Azadienes

2.3 Diels-Alder Cycloaddition with a Captodative α,β-Unsaturated Oxazoline

2.4 Approaches to the Oxahydrindane Subunit of Avermectins

3 [3+2] Cycloadditions

3.1 Camphor-Derived Oxazoline N-Oxides in [3+2] Cycloadditions

3.2 Synthetic Applications of Asymmetric [3+2] Cycloadditions with Oxazoline N-Oxides

4 Stereoselective Hetero-Claisen Rearrangement of Camphor-Derived Oxazoline N-Oxides

5 Camphor-Derived Sulfonylhydrazines as Catalysts for ­Diels-Alder Cycloadditions

6 Conclusion

Key words

cycloadditions - Diels-Alder reactions - chiral auxiliaries - rearrangements - asymmetric catalysis

References

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7

Trifluoromethanesulfonic anhydride, even freshly distilled, promoted at -78 ˚C the polymerization of cyclopentadiene; however, the use of methyloxirane as proton scavenger precluded this polymerization. See ref. 14.

9

Danishefsky’s diene was not stable in the presence of trifluoroacetic anhydride.

11

All product diastereomer analyses were carried out by capillary chromatography coupled with mass spectrometric analysis or by HPLC. The four diastereomers corresponding to the adduct 21 were prepared as a sample for comparison.

16

Cycloadduct 54 was prepared independently by thiophenyl-ation of adduct 26 of known configuration. The configura-tions at C2 in adducts 55, 56, and 57 were deduced after NMR experiments.