Regioselective Gold-Catalyzed Allylative Ring Opening of 1,4-Epoxy-1,4-dihydronaphthalenes

Abstract

In the presence of a gold catalyst, the ring opening of 1,4-epoxy-1,4-dihydronaphthalenes with allyltrimethylsilane affords allylnaphthalenes in high yield. For unsymmetrical substrates, high regioselectivity is observed in many cases. This reaction might proceed via tricyclic tetrahydrofuran intermediates which are formed stereoselectively.

Key words

allylation - ring opening - regioselectivity - naphthalene - gold catalyst

References and Notes

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13

For a two-step synthesis of 2-alkyl-1-naphthols by Pd-catalyzed ring opening of 1,4-epoxy-1,4-dihydro-naphthalenes and subsequent oxidation, see ref. 5j.

14

General Procedure for the Allylative Ring Opening to Synthesize 3d-m
Procedure A
To a solution of the substrate (0.2 mmol) in CH₂Cl₂ (2 mL) were added allylTMS (0.8 mmol) and AuCl₃ (0.01 mmol) at -40 ˚C under nitrogen. After being stirred until reaction was completed, the reaction mixture was concentrated under vacuum. The residue was purified by column chromatog-raphy using hexane-EtOAc (50:1) to give the pure allylation product.
Procedure B
To a solution of the substrate (0.2 mmol) in CH₂Cl₂ (2 mL) were added allylTMS (0.8 mmol), AgSbF₆ (0.03 mmol), and AuCl₃ (0.01 mmol) at -40 ˚C under nitrogen. After being stirred until reaction was completed, the reaction mixture was concentrated under vacuum. The residue was purified by column chromatography using hexane-EtOAc (50:1) to give the pure allylation product.

16

The determination of relative configuration of 5d,j was accomplished with NOE experiments.

18

Regioselective synthesis of 2-alkylated-1,2-dihydro-1-naphthols from unsymmetrical substrates could be only accomplished with Rh catalysts, see ref. 6d.

19

Regiochemistry of 3e-m was determined by NOE experiments.

20

In this paper, the same expressions as in ref. 4c are used. The position of R^¹ in substrates 1e-k is termed distal in relation to the dihydrofuran moiety. The position of R^² is termed proximal.

21

The relative configuration of 5l could not be assigned.

22

Reaction of 1d with crotyltrimethylsilane afforded 1,4-dimethyl-2-(1-methylallyl)naphthalene with 38% yield, which indicates that the allyl nucleophile directly attacks at C-2 of A. The formation of 3 by allylation at C-4 and subsequent Cope rearrangement is disfavored by the steric hindrance at C-4, and the high activation barrier for the Cope rearrangement of 1,5-dienes which usually requires strong heating.