Asymmetric Reduction of Cyclic Enones to Allylic Alcohols

Abstract

Asymmetric transfer hydrogenation of cyclic enones results in highly enantioselective reduction to cyclic allylic alcohols.

Key words

asymmetric catalysis - enones - enantioselective hydrogen transfer - regioselectivity - ruthenium

References

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13

The unsaturated ketone 3a was prepared by allylic oxidation of 1-phenylcyclohex-2-ene using a 1:1 molar combination of tert-butyl hydroperoxide and pyridinium dichromate. [6]

14

1,2-cyclohexanedione was treated with benzyloxytri-methysilane to afford diosphenol ether 3b [7] ; the enone 3c
was synthesised in 3 steps starting from cyclohexanone
via 2-azido-cyclohexan-1-one. [8] The enone derivated 3 with R = t-Bu was obtained by addition of t-BuLi to 7-oxabi-cyclo[4.1.0]heptan-2-one, followed by water elimination. [11]

15

Ketone 5 was prepared by radical addition of N-chloro-carbamate to 1-methoxycyclohexene; [10] racemic 4c was obtained by reduction of 3c with sodium borohydride and cerium chloride. The acyclic enone 8 was synthesised in 2 steps starting from the commercially available 3-chloro-butan-2-one via 3-azidobutan-2-one. [12]

16

Acyclic enones 7 and 8 were reduced under the same conditions as displayed in Schemes [1] and [2] for cyclic enones 3a-c.