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. 2018 Mar 7;140(9):3228-3231.
doi: 10.1021/jacs.8b01011. Epub 2018 Feb 22.

Enantioselective Intermolecular [2+2] Photocycloaddition Reaction of Cyclic Enones and Its Application in a Synthesis of (-)-Grandisol

Saner Poplata  1 , Thorsten Bach  1
Affiliations

Enantioselective Intermolecular [2+2] Photocycloaddition Reaction of Cyclic Enones and Its Application in a Synthesis of (-)-Grandisol

Saner Poplata et al. J Am Chem Soc. .

Abstract

The intermolecular [2+2] photocycloaddition of typical cyclic α,β-unsaturated enones, such as 2-cyclohexenone, with olefins was performed in moderate to good yields (42-82%) and with high enantioselectivity (82%-96% ee). An unusual substitution pattern at the chiral oxazaborolidine-AlBr3 Lewis acid complex that promotes the reaction was found to be crucial for the success of the reaction. The method was applied to the enantioselective synthesis of the monoterpene (-)-grandisol, which could be accomplished in six steps and with an overall yield of 13% starting from 3-methyl-2-cyclohexenone.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Scheme 1
Scheme 1. Corey’s Landmark Synthesis of Racemic Caryophyllene with an Intermolecular [2+2] Photocycloaddition Reaction of 2-Cyclohexenone (1a) as the Key Step
Figure 1
Figure 1
UV/vis spectra of 2-cyclohexenone (1a) in the absence (—) and in the presence of 20 equiv of either EtAlCl2 (− – −) or BCl3 (- - -) (c = 0.5 mM in CH2Cl2).
Scheme 2
Scheme 2. Enantioselective Total Synthesis of (−)-Grandisol (8)
Figure 2
Figure 2
Suggested structure of 1:1 enone/Lewis acid complexes 1a·3a and 1a·3d and of putative 1,4-diradical intermediate 9.
See this image and copyright information in PMC

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