A stereo-
and regioselective 1,3-dipolar cycloaddition of the stable ninhydrin-derived azomethine
ylide [2-(3,4-dihydro-2H-pyrrolium-1-yl)-1-oxo-1H-inden-3-olate, DHPO] to differently substituted cyclopropenes
has been established. As a result, an efficient synthetic protocol
was developed for the preparation of biologically relevant spiro[cyclopropa[a]pyrrolizine-2,2′-indene] derivatives. DHPO has
proved to be an effective trap for such highly reactive and unstable
substrates as parent cyclopropene, 1-methylcyclopropene, 1-phenylcyclopropene,
and 1-halo-2-phenylcyclopropenes. It has also been found that 3-nitro-1,2-diphenylcyclopropene
undergoes a nucleophilic substitution reaction in alcohols and thiols
to afford 3-alkoxy- and 3-arylthio-substituted 1,2-diphenylcyclopropenes,
which can be captured as corresponding 1,3-dipolar cycloadducts in
the presence of DHPO. These new approaches provide a straightforward
strategy for the synthesis of functionally substituted cyclopropa[a]pyrrolizine derivatives. The factors governing regio-
and stereoselectivity have been revealed by means of quantum mechanical
calculations (M11 density functional theory), including previously
unreported N
ylide–H
cyclopropene second-orbital interactions. The outcome
of this work contributes to the study of 1,3-dipolar cycloaddition,
as well as enriches chemistry of cyclopropenes and methods for the
construction of polycyclic compounds with cyclopropane fragments.
In triflic acid or sulfuric acid, diaryl‐substituted cross‐conjugated enynones undergo addition of the acid to the carbon–carbon triple bond to afford the corresponding vinyl triflates or sulfates. The vinyl triflates are stable under aqueous workup, whereas the vinyl sulfates are hydrolyzed to α,β‐unsaturated 1,3‐diketones (existing as conjugated enol forms). Extended reaction times lead to cyclization into dihydropyran‐4‐ones with yields of up to 95 %. The protonated forms of the vinyl triflates or sulfates in triflic and sulfuric acid, respectively, are studied as reactive intermediates by NMR spectroscopy. Plausible reaction mechanisms for the formation of dihydropyran‐4‐ones are discussed.
3-Methylbuta-1,2-dien-1-ylphosphonic acid derivatives (phosphorylallenes) [X2(O=)P–CR=C=CMe2, X = Cl, OMe, NR2, or SAr] undergo intramolecular cyclization into the corresponding 1,2-oxaphospholium ions in the Brønsted superacid TfOH. These cations have been thoroughly studied by means of NMR spectroscopy. The hydrolysis of superacidic solutions of these species afforded cyclic phosphonic acids and other phosphorus-containing compounds. Contrary to Brønsted acids, 3-methylbuta-1,2-dien-1-ylphosphonic dichloride [Cl2(O=)P–HC=C=CMe2] reacted with the Lewis acid AlCl3 in an intermolecular way forming noncyclic intermediates, which were investigated by NMR spectroscopy and DFT calculations. Hydrolysis of these species resulted in the formation of phosphoryl-substituted allyl alcohols and 1,3-butadienes. A strong coordination of the oxygen of the P=O group with AlCl3 prevented the formation of cyclic 1,2-oxaphospholium ions and played a crucial role in the different reactivity of such phosphorylallenes under the action of Brønsted or Lewis acids. Apart from that, the reaction of dichlorophosphorylallenes with arenes and AlCl3 led to products of hydroarylation of the allene system, phosphoryl-substituted alkenes and/or indanes. This is the first example of a Lewis acid-promoted intermolecular hydroarylation of allenes bearing electron-withdrawing substituents. Plausible reaction mechanisms have been proposed on the basis of the investigated reactions, and NMR analysis and DFT studies of the intermediate cationic species.
Diphenyl allenyl phosphonates are cyclized into 2,5‐dihydro‐1,2‐oxaphosphol‐2‐ium ions in Brønsted acids (TfOH, H2SO4, FSO3H). These species have been studied by means of 1H, 13C, 31P NMR. The cations react with various nucleophiles (H2O, MeOH, EtOH, Et2NH, PhH) on the phosphorus atom. The obtained products are very labile and undergo decomposition. Contrary to Brønsted acids, under the action of Lewis acids AlX3 (X=Cl, Br), these diphenyl allenyl phosphonates lead to another kind of heterocycles, 2,5‐dihydrobenzo[f][1,2]oxaphosphepine‐2‐oxides.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.