Ring opening versus ring expansion in rearrangement of bicyclic cyclobutylcarbinyl radicals

Ring opening and expansion of multicyclic cyclobutylcarbinyl radicals provides an appealing method for the construction of heavily substituted ring systems in a stereocontrollable fashion. Here we conducted the first, systematic study on the regioselectivity in the rearrangement of various synthetically relevant cyclobutylcarbinyl radicals. It was found that a two-layer ONIOM method, namely ONIOM(QCISD(T)/6-311+G(2d,2p):B3LYP/6-311+G(2df,2p)), could accurately predict the free energy barriers of the ring openings of cyclobutylcarbinyl radicals with a precision of 0.3 kcal/mol. By using this powerful tool we found that the regiochemistry for the ring opening of monocyclic cyclobutylcarbinyl radicals could be easily predicted by the relative stability of the two possible carbon radical products. A linear correlation was found between the activation and reaction free energies. This observation indicated that the ring opening of cyclobutylcarbinyl radicals was strongly affected by the thermodynamic factors. On the basis of the above results we extended our study to the rearrangement of bicyclic cyclobutylcarbinyl radicals that could undergo both ring opening and expansion. It was found that for bicyclic cyclobutylcarbinyl radicals whose radical center was located at the bridge methyl group, ring expansion was the favored rearrangement pathway unless a strongly radical-stabilizing substituent was placed in the cyclobutyl ring adjacent to the bridge methyl group. On the other hand, for bicyclic cyclobutylcarbinyl radicals whose radical center was located at the 2-position, ring opening was the favored rearrangement pathway unless a strongly radical-stabilizing substituent was placed in the cyclobutyl ring at the bridge position.     

2-Quinolyl- and 1-isoquinolylnitrenes: ring expansion and ring opening in heteroarylnitrenes

Argon matrix photolysis of tetrazolo[1,5-a]quinoline 8 and tetrazolo[5,1-a]isoquinoline 7 causes nitrogen elimination and ring expansion to 1,3-diazabenzo[d]cyclohepta-1,2,4,6-tetraene 13. The photolysis of tetrazolo[5,1-a]isoquinoline 7 also causes ring opening to o-cyanophenylketenimine 22. Mechanisms of ring opening of heteroarylnitrenes are discussed.     

2-pyrazinylnitrene and 4-pyrimidylnitrene. Ring expansion to 1,3,5-triazacyclohepta-1,2,4,6-tetraene and ring opening to (2-Isocyanovinyl)carbodiimide

Tetrazolo[1,5-a]pyrazine/2-azidopyrazine 9T/9A undergo photolysis in Ar matrix at cryogenic temperatures to yield 1,3,5-triazacyclohepta-1,2,4,6-tetraene 21 as the first observable intermediate, and 1-cyanoimidazole 11 and (2-isocyanovinyl)carbodiimide 22 as the final products. The latter tautomerizes to 2-(isocyanovinyl)cyanamide 23 on warming to 40 K. The same intermediate 21 and the same final products are obtained on matrix photolysis of the isomeric tetrazolo[1,5-c]pyrimidine/4-azidopyrimidine 24T/24A. These photolysis results as well as those of the previously reported thermal ring contraction of (15)N-labeled 2-pyrazinyl- and 4-pyrimidylnitrenes to 1-cyanoimidazoles can all be rationalized in terms of selective ring opening of 21 or nitrine 10 to a nitrile ylide zwitterion 28 prior to formation of the final products, 11 and 22. The results are supported by high-level ab initio and DFT calculations (CASPT2-CASSCF(6,6), G3(MP2), and B3LYP/6-31+G) of the energies and IR spectra of the intermediates and products.     

3-pyridylcarbene and 3-pyridylnitrene: ring opening to nitrile ylides

Photolysis of 3-pyridyldiazomethane in an Ar matrix at 7-10 K gives 3-pyridylcarbene. Further photolysis causes ring opening to nitrile ylide 26 (formonitrile pent-2-en-4-ynylide) as the major reaction together with a minor amount of ring expansion to 1-azacyclohepta-1,3,4,6-tetraene, 27. Matrix photolysis of 3-azidopyridine leads to ring opening to formonitrile N-cyanovinylmethylide, 33.     

Ring opening and expansion of aziridines in a silica-water reaction medium

Ring-opening reactions of N-tosylaziridines with water-soluble nucleophiles proceeded in a silica-water reaction medium. The system is applicable to a ring expansion of an aziridine with potassium thiocyanate, leading to a thiazolidine derivative.     

Ring opening and ring enlargement of a cyclopropene carboxylic acid

3,3-Dimethyl-2-phenyl-cyclopropenecarboxylic acid 4 is prepared, starting with chlorophenyldiazirine 1. With acids 4 yields the open chain carboxylic acids 5, 6 and 7. Thionychloride leads to the cyclobutenone 10, DCC to the acid anhydrides 11 and 13.     

Nickel-catalyzed reactions of silacyclobutanes with aldehydes: ring opening and ring expansion reaction

[reaction: see text]. The nickel-catalyzed ring opening reaction of silacyclobutanes with aldehydes affords the corresponding alkoxyallylsilanes. In contrast, the ring expansion reaction of benzosilacyclobutene with aldehydes occurs under nickel catalysis to give oxasilacyclohexenes.     

Ring cleavage and ring expansion of indoles by superoxide ion

The reaction of indoles ($\text{1}$) with superoxide ion resulted in ring cleavage to give $\text{o}$-formyl and $\text{o}$-acylaminoketones ($\text{6}$) or N-acylanthranilic acid ($\text{8}$) and ring expansion yielding 2-quinolones ($\text{7}$). All reactions are chemiluminescent except that of 2-methylindole ($\text{lh}$), which gave a coupled product ($\text{9}$).     

Ring expansion of azetidinium ylides: rapid access to the pyrrolizidine alkaloids turneforcidine and platynecine

[reaction: see text] Azetidinecarboxylate esters react readily with metallocarbenes in an inter- or intramolecular fashion to generate azetidinium ylides. Efficient [1,2]-shift by the ester-substituted carbon furnishes ring-expanded pyrrolidine products. In the case of substrate 1, this provides access to the pyrrolizidine alkaloids turneforcidine and platynecine via a high-yield, five-step sequence starting with readily available methyl 1-benzylazetidine-2-carboxylate.     

Azavinyl azomethine ylides from thermal ring opening of alpha-aziridinohydrazones: unprecedented 1,5-electrocyclization to imidazoles

Michael-type addition of aziridinecarboxylates to 1,2-diaza-1,3-butadienes under solvent-free conditions (SFC) resulted in the formation of alpha-aziridinohydrazone adducts. In toluene under reflux, alpha-aziridinohydrazones gave imidazoles in moderate to good yields. Such a reactivity pattern is explained by 1,5-electrocyclization of azavinyl azomethine ylide generated through thermal ring opening of alpha-aziridinohydrazones.     

Ring opening of 2-phenylazetidines with allylsilanes

N-Activated 2-phenylazetidines were opened regioselectively at the benzylic carbon with various allylsilanes or propargylsilane in the presence of BF₃·Et₂O, providing amino olefins, precursors of biomolecules such as phenyl-homo-kainoids.     

Ring opening in methylenecyclopropane

The indo molecular orbital calculations on methylenecyclopropane show that the ¹A₁ state of the (90,90) configuration is the most stable one at θ = 64°. The ground state of the ring-opened trimethylenemethane is predicted to be the ³A'₂ state (D_3h symmetry), however the³A″ state of non-planar (0,90) trimethylenemethane is about 10 kcal/mole higher in energy than the planar diradical species. This suggests intermediacy of the orthogonal diradical in the degenerate thermal rearrangement of substituted methylenecyclopropane. The calculated geometry, dipole moment of methylenecyclopropane, and spin densities of trimethylenemethane are in excellent agreement with the experimental values.     

Ring opening of 2-acylaziridines by acid chlorides

Good nucleophilicity of the ring nitrogen in chiral (2R,1′R)-2-acyl-(1′-phenylethyl)aziridines initiated the reaction with various acid chlorides to form the corresponding acylaziridinium ion intermediates whose rings were opened by the chloride anion to yield the β-amino-α-chlorocarbonyl compounds. The subsequent displacement of the chloride with the internal oxygen nucleophile originated from methylchloroformate, acetyl chloride, and methyl chlorooxoacetate yielded oxazolidin-2-ones, β-amino-α-acetyloxypropionates, and morpholin-2,3-diones, respectively.     

Attempted synthesis of 2-acetamido and 2-amino derivatives of salacinol. Ring opening reactions

The attempted synthesis of the 2-acetamido and 2-amino derivatives of salacinol, a naturally occurring glycosidase inhibitor, is described. Reaction of the protected acetamidothioarabinitol unit with the cyclic sulfate derived from L-erythritol gave the corresponding sulfonium sulfate, which underwent ring opening to give an acyclic amido sulfate. The corresponding reaction of the protected azidothioarabinitol unit with the cyclic sulfate proceeded to give the sulfonium sulfate. However, upon reduction of the azido function to an amine it formed an acyclic ammonium sulfate.     

Regioselective epoxide ring opening. Steroselective synthesis of a tetrahydropyran ring

The stereoselective synthesis of a 2-substituted tetrahydropyran with adjacent alkoxy-bearing stereogenic centre is described. The key steps of this synthesis were the stereoselective epoxidation of an allylic alcohol and the regioselective epoxide ring opening by lithium aluminum hydride. The regio and stereoselective synthesis of a trihydroxyselenide and a trihydroxysulfide is also described. The latter compounds are not suitable for cyclization to tetrahydrofuran ring.