A new variant of the intramolecular Diels−Alder oxazole (IMDAO) cycloaddition that provides direct access to 6azaindoles was developed. The IMDAO reaction was applied in a total synthesis of the aminophenylpyrrole-derived alkaloid marinoquinoline A, also featuring the use of a Curtius reaction for preparation of a 5-aminooxazole, a propargylic C,H-bond insertion, an in situ alkyne−allene isomerization, and a rutheniumcatalyzed cycloisomerization for benzene ring annulation to the 6-azaindole.T he intramolecular hetero-Diels−Alder reaction is a versatile method for generating monocyclic and polycyclic heteroarenes, 1 including pyridines, 2−4 pyrimidines, 5,6 quinolines, 7,8 and isoquinolines, 9,10 and has frequently been applied to complex target molecule synthesis. 11 As an extension of our intramolecular Diels−Alder furan (IMDAF) reaction for indole synthesis, 12,13 we now report a new variant of the intramolecular Diels−Alder oxazole (IMDAO) cycloaddition, 14,15 leading to 6-azaindoles. Furthermore, we apply this new reaction to the preparation of the pyrroloquinoline marinoquinoline A (Scheme 1).IMDAO reactions are well-established in the heterocyclic literature 16 and have previously been used in natural product synthesis. 17−19 However, to the best of our knowledge, this is the first application of this Diels−Alder variant for 6-azaindole synthesis, with the exception of some precedent for the generation of partially saturated heterocycles containing this core structure (Scheme 2). 20−23 We had previously identified conditions for a Diels−Alder cycloaddition route to indoles 4 from allenyl furans 3, but when we applied analogous conditions to oxazoles 5, we encountered experimental challenges with the preparation and
Alkenes with three boryl groups of differing reactivities were synthesized and subsequently cross‐coupled regioselectively with aryl halides in a stepwise manner to afford tetrasubstituted alkenes. The key triborylalkene is derived from the platinum‐catalyzed diboration of alkynyl−B(dan)s with B2(pin)2. Due to excellent regioselectivity and reaction efficiency of each step starting with alkynyl−B(dan)s, tetrasubstituted alkenes with desired carbon frameworks at desired positions can be prepared in high yields. For example, an alkene with four distinct aryl groups, p‐MeOC6H4, p‐CF3C6H4, p‐MeC6H4, and p‐NCC6H4, was obtained in 71% overall yield via six steps, starting with the dehydrogenative borylation of p‐MeC6H4C≡CH with HB(dan). Moreover, a variety of tetrasubstituted alkenes, including regio‐ and stereoisomers of the above tetraarylalkene, AIE‐active TPTPE and derivatives thereof, and (Z)‐tamoxifen, a well‐known breast cancer drug, were accessed via the developed strategy.
A seven-membered carbon-ring-forming annulation occurred, upon treatment of heteroarylindoles with α,β-unsaturated carbonyl compounds in the presence of an indium catalyst. The reductive annulation was observed when octylsilane was used. With...
With a zinc Lewis acid catalyst and proton sponge in toluene, terminal alkynes were found to undergo the alkylation by alkyl triflates to provide unsymmetrical internal alkynes. This is the first example that a simple alkyl chain other than benzylic and norbornyl units can be introduced onto the alkynyl carbon atom under Lewis acid catalysis. Mechanistic studies revealed that the activation of the alkyne by the zinc Lewis acid and proton sponge is the trigger of the reaction to give a monoalkynylzinc species, which successively reacts with the alkyl triflate to afford the internal alkyne. A radical pathway is unlikely in this system.magnified image
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