Bromoallenes as Synthetic Equivalents of Allyl Dications: Synthesis of Medium‐Sized Nitrogen Heterocycles through the Cyclization of Bromoallenes in the Presence of a Palladium(0) Catalyst and an Alcohol

Palladium(0)-Catalyzed Synthesis of Medium-Sized Heterocycles by Using Bromoallenes as an Allyl Dication Equivalent

“…For example, while the bromoallene 15d yielded 1,4-oxazepine 24d in 73% yield (Table ), the corresponding propargylic carbonate was converted to the diol by solvolysis under identical reaction conditions. Similarly, the corresponding propargyl bromide to 17b was found to be relatively unstable under the cyclization conditions, and only a small amount of the desired cyclized product 26b was obtained (12% yield) by treatment with NaOMe in MeOH in the presence of Pd(PPh 3 ) 4 12a…”

“…On the other hand, if the intramolecular nucleophilic attack takes place predominantly, cyclization at the central carbon atom of the allenic moiety would proceed to give 11 and/or 12 . In this contribution, we detail a highly regioselective synthetic method for medium-sized heterocycles 11 containing one or two heteroatoms by the palladium(0)-catalyzed cyclization of bromoallenes . In all cases examined, the cyclization takes place at the central carbon regioselectively via path B to give a variety of medium-sized heterocycles.…”

Palladium‐Catalyzed [6+2] Double Allene Annulation for Benzazocines Synthesis

“…[2] It is well-known that unfavorable enthalpic and entropic factors have been the major stumbling block for the synthesis of the eight-membered rings, thus much attention has been focused on this topic. [3,4] On the other hand, allenes bearing a leaving group at α-position could react with transition metal catalysts to generate the methylene-π-allylic metal intermediate, [5] which may readily undergo substitution reaction with stabilized nucleophiles or non-stabilized nucleophiles to give either allenes [6,7] and/or conjugated 1,3dienes, [8] respectively (Scheme 1b). With our longstanding interest in developing new catalytic reactions of 2,3-allenyl derivatives, [6c-f, 7e-l, 8m] we envisaged that an insertion reaction of such methylene-π-allylic metal intermediates with an unsaturated CÀ C bond would generate intermediates Int-A and/or Int-B (Scheme 1c).…”

Potassium Carbonate-Promoted Stereospecific 5-Endo-Trig Cyclization of Unactivated Allenes in the Absence of Any Transition Metals

“…As a part of our ongoing program directed toward economical and environmentally friendly cyclization of allenic compounds, − we investigated cycloisomerization of amino allenes without using any transition-metal catalysts. Herein, we describe K 2 CO 3 -mediated stereospecific cycloisomerization of α-amino allenes, which is the first example of base-induced 5- endo - trig mode cycloisomerization of simple unactivated allenes in the absence of any activating reagents toward the allenic π-bond.…”