In this paper we describe the development and application of a sequential Fries-Claisen rearrangement strategy as a new route for annelation on an aromatic ring. We demonstrate the potential utility of this protocol through the synthesis of a pyranonaphthoquinone target.Whilst the application of double Claisen rearrangements has been investigated by others, 1 including in conjunction with ring-closing metathesis (RCM) methodology, as a method for annelating on a benzene ring, 2 one advantage of developing a process in which different classes of sigmatropic rearrangement are combined and applied consecutively to an aromatic substrate is that of potential differentiation of any resulting (possibly annelated) product. Scheme 1 shows a typical product of double Claisen rearrangement from the work of Kotha. 2 Given the symmetrical nature of the substrate it is clear that this process would not lead to ready differentiation of emerging functionality within the new ring, whereas one can envisage an alternative process in which a dual Fries-Claisen strategy (Scheme 1) could lead to a more flexible target: one in which selective chemistry might then be carried out on the 'upper' or 'lower' edge of the product (as drawn) as the molecular framework is further developed. We now report our progress in developing a unique synthetic strategy based on the coupling of the anionic ortho-Fries and thermal Claisen rearrangements. Individually, the Claisen and anionic ortho-Fries rearrangements 3 are well-known synthetic routes, however, their incorporation into a dual (consecutive) process on the same aromatic substrate has not, to the best of our knowledge, been previously reported, although there has been one report of an unsuccessful attempt to combine and control these useful rearrangements. 4 In this study, Green and co-workers were able to perform a thermally induced Claisen rearrangement on substrate 4, but unfortunately the only product to be isolated in low yield from a subsequent Fries rearrangement of 5 was the furan 6 as a byproduct in a complex mixture, rather than the desired allyl-substituted ketone 7 (Scheme 2). 4 We began our study with an investigation of the rearrangement potential of substrate 8, obtained through a simple two-step procedure from the readily available diol 6-benzyloxy-1,4-dihydroxynaphthalene. 5 We were Scheme 1 A comparison of dual sigmatropic rearrangements, highlighting the opportunity for edge differentiation in a mixed-type rearrangement process O O i) double Claisen ii) methylation OMe OMe RCM OMe OMe R R R R R R O O X Fries-Claisen sequence OMe OMe X annelate OMe OMe X R R R R R R O O O symmetrical "edges" differentiation SYNLETT
In this paper we describe the application of a recently developed dual Fries-Claisen protocol as a novel synthetic approach to the pyranonaphthoquinone natural products eleutherin and isoeleutherin.
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