Highly Regioselective Palladium-Catalyzed Annulation Reactions of Heteroatom-Substituted Allenes for Synthesis of Condensed Heterocycles

Abstract

Palladium-catalyzed annulation processes have proved to be extremely useful and powerful methods for the construction of diverse carbocycles and heterocycles.1-4) Among them, reactions that involve carbopalladation of allenes, which generate p-allylpalladium intermediates, followed by internal nucleophilic attack by a heteroatom or carbanion, are particularly valuable (Fig. 1). [5][6][7][8][9][10][11][12][13][14][15][16] Larock et al. have extensively investigated the palladium-catalyzed annulation reactions of carbon-atom-substituted allenes using functionally substituted aryl iodides or vinyl halides. [13][14][15][16] Yields were generally high and most of the reactions are both stereo-and regioselective; however, the reactions sometimes afforded mixtures of regioisomers, especially in six-membered-ring formation. 13,14) Meanwhile, only one example of an annulation reaction of heteroatom-substituted allenes has been reported, and in that case the presence of the heteroatom on allenes and its electronic effect dominated the regioselectivity (Fig. 2).17) With the aim of regioselectively synthesizing highly functionalized heterocycles, we employed heteroatomsubstituted allenes for palladium-catalyzed annulation. We found that various aryl iodides and bromides reacted smoothly and that one regioisomer was produced exclusively both in five-and six-membered-ring formations. Furthermore, in the course of our extensive studies on the synthetic application of annulated products, we also found that alkyl metal reagents reacted unexpectedly in SN2Ј fashion at the exomethylene carbon of the annulated products obtained by the palladium-catalyzed annulation reaction. Several metal reagents were reactive and an alkyl moiety could be introduced by this process.Synthesis of Heteroatom-Substituted Allenes Heteroatom-substituted allenes (1, 2) used for the palladium-catalyzed annulation reaction were prepared according to the protocol as follows (Fig. 3). [18][19][20] O-Substituted allene 1 was obtained via O-benzylation of propargyl alcohol followed by treatment withN-Substituted allene 2 was synthesized using commercially available N-Boc-N-tosylamide as a starting material. Mitsunobu reaction of N-Boc-N-tosylamide with propargyl alcohol, followed by removal of Boc using 3 N HCl in MeOH under reflux, afforded N-propargyl-N-tosylamide 3 in 91% yield. Benzylation of amide 3 and subsequent isomerization produced the desired allene 2.Palladium-Catalyzed Annulation Reactions of Allenes Using Various Aryl Iodides/Bromides (Table 1) With heteroatom-substituted allenes 1 and 2 in hand, we first examined the annulation reaction of them using various aryl iodides.A variety of reaction conditions have been examined, and the use of an inorganic base, K 2 CO 3 , and a polar solvent, DMF, has proved to be effective for this process. All reactions were highly regioselective and only one regioisomer was observed.In annulation of allene 1 using N-tosyl-2-iodoaniline as an aryl halide, the reaction proceeded smoothly at 80°C in the * To whom cor...

Allenamides: A Powerful and Versatile Building Block in Organic Synthesis

“…Hiroya 99 published a Pd-catalyzed annulation reaction of allenamide 349 with aryl iodides 348 (Scheme 95). In this work, the Pd-π-allyl species 350 readily underwent an intramolecular nucleophilic attack by an oxygen- or nitrogen-based nucleophile to give the annulated product 354 .…”

The Heck Reaction Applied to 1,3- and 1,2-Unsaturated Derivatives, a Way towards Molecular Complexity

“…Recently a synthesis of condensed heterocycles 42 was proposed . It exploited the intramolecular Heck reaction applied to heteroatom substituted allenes and aryl halides ( Scheme 20 ) [ 28 ]. The initially formed arylpalladium complex added to the allene and produced the corresponding π-allylpalladium intermediate, which readily underwent intramolecular nucleophilic attack by an oxygen or a nitrogen affording the annulated product 42 .…”

Metalated Heterocycles in Organic Synthesis: Recent Applications

“…Thus, these reagents have been employed recently, for instance, in Michael addition reactions, 442 an example being the transfer of a butadienyl moiety to the unsaturated aldehyde 244 by means of the organocuprate 243 to give mainly the cis-product 245 (Scheme 72). 442e The ring-opening of aziridines 443 and epoxides, 444 as well as S N 2' reactions, 445 have also been performed using this copper-derived reagents. …”