Phosphine-Catalyzed [2 + 4] Annulation of Allenoates with Thiazolone-Derived Alkenes: Synthesis of Functionalized 6,7-Dihydro-5H-pyrano[2,3-d]thiazoles

Abstract

Phosphine-catalyzed [2 + 4] annulation of allenoates with thiazolone-derived alkenes has been achieved under mild conditions, giving biologically important 6,7-dihydro-5H-pyrano[2,3-d]thiazole derivatives in high to excellent yields. With the use of Kwon's phosphine as the chiral catalyst, optically active products were obtained in good yields with excellent enantioselectivities.

Enantioselective Construction of Spiro[chroman‐thiazolones]: Bifunctional Phosphonium Salt‐Catalyzed [2+4] Annulation between 5‐Alkenyl Thiazolones and ortho‐Hydroxyphenyl‐Substituted para‐Quinone Methides

“…Generally, 5‐alkenyl thiazolones can act as either C2‐synthons or C4‐synthons for annulation reactions. Recently, many impressive examples on [4+n] annulation of 5‐alkenyl thiazolones have been achieved by the research groups of Wang, Ye, Guo, Li and others– to construct diverse chiral thiazo‐fused cyclic compounds (Scheme a, path A). Of note, almost all of reported examples focused on utilizing 5‐alkenyl thiazolones as C4‐syntons, and thus provided chiral thiazo‐fused cycles.…”

Catalytic Regio‐ and Enantioselective [4+2] Annulation Reactions of Non‐activated Allenes by a Chiral Cationic Indium Complex

“…[1][2][3] In this regard, [4+ +2] cycloaddition of allenes and a,b-unsaturated conjugated compounds provides ap owerful tool for the rapid construction of six-membered heterocycles. [5,6] Thecurrent successes along these lines have been mainly achieved with chiral Lewis base catalysis.S hi [5b] and Lu [6a] have developed chiral tertiary amine and chiral phosphine catalysis for the asymmetric [4+ +2] cycloaddition of b,g-unsaturated a-keto-compounds with activated allenes such as allenic esters and allenones,r espectively (Scheme 1A). [5,6] Thecurrent successes along these lines have been mainly achieved with chiral Lewis base catalysis.S hi [5b] and Lu [6a] have developed chiral tertiary amine and chiral phosphine catalysis for the asymmetric [4+ +2] cycloaddition of b,g-unsaturated a-keto-compounds with activated allenes such as allenic esters and allenones,r espectively (Scheme 1A).…”

“…We found that indium salts were the sole active Lewis acid catalyst and that enhanced cationic nature of the indium center (by using the BArF À salt instead of the halide salt;B ArF = [3,5-(CF 3 ) 2 C 6 H 3 ] 4 B) was essential for reactivity,s ince no reaction was observed with InBr 3 or indium phosphate (Table 1, entry 7). [6] To further improve the enantioselectivity,d ifferent free phosphoric acids and their metal salts were examined in the presence of In(BArF) 3 (Table S1). Accordingly,t he binary acid complex InBr 3 /1 was first treated with three equivalents of AgBArF before the substrates were added (Table S1).…”

“…[4] Catalytic asymmetric versions have been actively pursued. [5,6] Thecurrent successes along these lines have been mainly achieved with chiral Lewis base catalysis.S hi [5b] and Lu [6a] have developed chiral tertiary amine and chiral phosphine catalysis for the asymmetric [4+ +2] cycloaddition of b,g-unsaturated a-keto-compounds with activated allenes such as allenic esters and allenones,r espectively (Scheme 1A). On the other hand, transition-metal catalysts such as aN i 0 catalyst have been reported to promote the [4+ +2] cycloaddition of non-activated allenes under high temperature.…”

“…Ther esulted Lewis acid catalysis also demonstrates unprecedented regioselectivity that has not been observed in previous studies. [5,6,8] Different Lewis acids,c hiral phosphoric acids, [10] and combinations [11] thereof were examined in the model reaction of allene 2a and keto ester 3a (Table 1a nd Tables S1, S2 in the Supporting Information). We found that indium salts were the sole active Lewis acid catalyst and that enhanced cationic nature of the indium center (by using the BArF À salt instead of the halide salt;B ArF = [3,5-(CF 3 ) 2 C 6 H 3 ] 4 B) was essential for reactivity,s ince no reaction was observed with InBr 3 or indium phosphate (Table 1, entry 7).…”