Ring-opening and ring-expansion reactions of carborane-fused borirane

Though the reaction chemistry of three-membered ring molecules such as cyclopropanes and their heteroatom-containing analogues has been extensively studied, the chemical properties of their boron analogues, boriranes, are little known thus far. This work describes the diverse reactivity patterns of carborane-fused borirane 2. This borirane engages in ring-opening reactions with different types of Lewis acids, such as BBr₃, GeCl₂, GaCl₃, BH₃(SMe₂) and HBpin, affording a series of ring-opening products, in which M–X or B–H bonds add across the B–C(cage) bond of the three-membered ring in 2. On the other hand, borirane 2 can undergo ring-expansion reactions with unsaturated molecules such as PhCHO, CO₂ and PhCN to give ring-expansion products, five-membered boracycles, via a concerted reaction mechanism as supported by DFT calculations. The results of this work not only enrich the reaction chemistry of boriranes, but also offer new routes to boron-containing compounds and heterocycles.

Carborane-fused borirane can not only engage in ring-opening reactions with different types of Lewis acids, but also undergo ring-expansion reactions with unsaturated molecules such as PhCHO, CO₂ and PhCN to give five-membered boracycles.     

Enantiodivergent synthesis of both enantiomers of marine alkaloids haliclorensin and isohaliclorensin, a constituent of halitulin

Starting from (3R)-5-benzotriazolyl-3-phenylperhydropyrido[2,1-b][1,3]oxazole 9, the enantiodivergent syntheses of both enantiomers of the marine alkaloids haliclorensin 1 and isohaliclorensin 3 have been achieved. Our syntheses feature ring-expansion reactions for the formation of the aza-macrocycle ring system of 3 and sequential ring-expansion reactions (aza-Claisen rearrangement and Zip reaction) for the formation of the aza-macrocycle ring system of 1.     

Stereochemistry of sulphilimine and sulphoxide formations in reactions of sulphides with chlorinating agents and nucleophiles

The stereochemistry of sulphilimine and sulphoxide formations was studied in reactions of chiral alkyl aryl sulphides either with N-chloro toluenesulphonamides or with t-BuOCl followed by TsNH^?Na⁺ both leading to unequal amounts of diastereomers of products. Configurations were determined by spectroscopic methods and stereospecific reactions, and diastereomeric product distributions were measured by hplc. Results are discussed and reaction pathways are suggested for the product-controlling steps. From sulphonium type intermediates sulphoxides are formed by hydrolysis with inversion, while sulphilimines with retention or inversion of configuration at sulphur depending on manner of attack by N-nucleophile. In reactions of ortho-carboxy-substituted sulphides both sulphilimines and sulphoxides are formed from cyclic acyloxysulphonium intermediates with inversion of configuration at sulphur.     

A photo-reagent system of benzimidazoline and Ru(bpy)3Cl2 to promote hexenyl radical cyclization and Dowd–Beckwith ring-expansion of α-halomethyl-substituted benzocyclic 1-alkanones

A combination of 2-aryl substituted 1,3-dimethyl-benzimidazolines (DMBIHs) and tris(2,2′-bipyridine)ruthenium(II) chloride, Ru(bpy)₃Cl₂ was used to promote photoinduced electron-transfer (PET) reactions of α-halomethyl-substituted benzocyclic 1-alkanones. This photo-reagent system stimulates free radical forming, cleavage of both carbon–bromine and carbon–chlorine bonds that are not activated by carbonyl groups. The resulting free radicals undergo 5-exo hexenyl cyclization as well as sequential cyclization and ring-expansion (Dowd–Beckwith) reactions to form radicals that abstract hydrogen atoms from the radical cation of DMBIH to yield the observed products. The results of a study of the effects of substituents located on the 2-aryl ring of DMBIH suggest that steric and hydrogen-bonding interactions influence the nature of the reaction pathways followed by the radical intermediates. PET reactions using an iridium complex and DMBIH were also investigated.     

Diastereoselective formation of seven-membered oxacycles by ring-expansion of cyclopropanated galactal

The cyclopropanation and ring-expansion of 4,6-O-di-(tert-butyl)silanediyl-d-galactal using catalytic amounts of TMSOTf and silated nucleophiles gave a range of substituted oxepanes. Good to excellent yields were obtained in addition to very high selectivities.     

Fluorinative ring-expansion of cyclic ethers using p-iodotoluene difluoride. Stereoselective synthesis of fluoro cyclic ethers

Fluorinated five- to seven-membered cyclic ethers were stereoselectively synthesized from four- to six-membered cyclic ethers having an iodoalkyl substituent by fluorinative ring-expansion reaction using p-iodotoluene difluoride.     

A formal synthesis of (+/-) parvifoline by an aromatic Cope rearrangement of a trans-1-aryl-2-ethenylcyclobutanecarbonitrile

A formal synthesis of the sesquiterpene parvifoline is described. The key step of the synthesis is an aromatic Cope rearrangement of a trans-1-aryl-ethenylcyclobutancarbonitrile. It is in fact a ring-expansion type reaction with the advantage of achieving the fusion, expansion and rearomatization in a single step.     

Subject index

1-Phenyl-2[²H¹]-propylcobaloxime(III) containing a large excess of one diastereoisomer reacts rapidly with sulphur dioxide to give an equimolar mixture of the two diastereoisomers of the insertion product. 6-Methylhept-5-en-2-ylcobaloxime(III) reacts with sulphur dioxide to give 1,1-dioxathiacyclohex-3-anesulphonylcobaloxime in which two molecules of sulphur dioxide have become incorporated. Each of these products is believed to result from an initial homolysis of the carbon—cobalt bond, followed by a series of non-chain free radical reactions.     

An improved catalyst system for the iron-catalyzed intermolecular ring-expansion reactions of epoxides

A considerable improvement is reported in the iron-catalyzed ring-expansion reactions of epoxides generating tetrahydrofuran derivatives by formal insertion of an alkene. Optimization of the catalyst system revealed that a preformed [Fe(salen)] complex minimizes the formation of polymerization side-products so that increased yields of intermolecular reactions were obtained. However, more importantly, the scope of the reaction could also be enlarged considerably. The iron-catalyzed ring-expansion reaction can now be applied to some styrene oxide derivatives, acting as radical donors, as well as to a wide variety of acceptor-substituted acyclic alkenes and cyclic dienes that act as radical acceptors. The use of unsymmetrical radical acceptors led to interesting questions concerning the regiochemistry of the reactions. The conservation of the stereochemistry of the starting materials in the products was investigated through a study of the reactions of E- and Z-configured acceptor-substituted double bonds. The reactions of fumaric and maleic esters were performed and the ratios of diastereomeric and regioisomeric products were determined.     

Synthesis of tryptophans by Lewis acid promoted ring-opening of aziridine-2-carboxylates: optimization of protecting group and Lewis acid

The preparation of tryptophan derivatives through the Lewis acid promoted substitution of aziridine carboxylates with indole was found to be accompanied by a ring-expansion reaction to generate an oxazolidinone byproduct. The ratio of tryptophan to oxazolidinone products can be optimized through a judicious choice of Lewis acid and N-protecting group.     

Unexpected ring-expansion of 1,2-benzisoxazol-3-ones

A novel and unexpected ring-expansion reaction of 1,2-benzisoxazol-3-ones is identified. The scope of this reaction is exemplified and the proposed mechanism is also implicated in another degradation process. This reaction also represents a new method for accessing the 4H-1,3-benzoxazin-4-one skeleton.     

Concise diastereospecific pyrrolo[1,2-a][1,4]benzodiazepinone synthesis

A concise synthesis of the novel pyrrolo[1,2-a]benzodiazepine system, by using the metallo carbenoid/spiro-[6,5]-ammonium ylide/Stevens[1,2]-shift with ring-expansion approach, was reported. The overall cascade process resulted stereospecific.     

Asymmetric synthesis of isohaliclorensin,a key intermediate of bisquinolinylpyrrole alkaloid halitulin

A new enantioselective total synthesis of N-(3′-aminopropyl)-3-methylazacyclodecane, a partial structure of halitulin, has been achieved in eight steps with 14% overall yield. The key steps are the photochemical ring-expansion reaction of spirooxaziridine to lactam for constructing the azacyclodecane moiety and 1,4-stereoinductive methylation of the resulting lactam.     

第5/6副族高价过渡金属氯化物的有机反应

从制备化学的角度, 对包括作者本人的研究工作在内的由第5/6副族高价过渡金属氯化物参加的有机反应进行了综述. 内容包括以下几个方面: (1)烷烃、烯烃和炔烃与第5/6副族高价过渡金属氯化物的卤化反应; (2)作为Lewis 酸催化的反应; (3)烯烃歧化反应及炔烃的聚合反应; (4) MoCl₅参与的芳烃氧化偶合反应; (5)硫醚化及扩环反应.     

Isocyanide based multicomponent reactions of oxazolidines and related systems

N-Alkyloxazolidines react in a multicomponent reaction with carboxylic acids and isocyanides to give N-acyloxyethylamino acid amides. The previously reported reaction conditions were improved using a design of experiments approach (DoE). Under the optimised conditions, good yields of the N-acyloxyethylamino acid amide products are obtained both via a three- or four-component approach from N-alkylethanolamines, aldehydes/ketones, isocyanides and carboxylic acids. The reaction of oxazolidines without a nitrogen substituent was found to give either the expected Ugi products or the N-acyloxyethylamino acid amides depending on the choice of reaction conditions. Optimised reaction conditions were also developed for the ring-expansion of oxazolidines to morpholin-2-ones via reaction with an isocyanide followed by hydrolysis. The mechanistic pathway of the multicomponent reaction was briefly investigated using an ¹⁸O labelling experiment. The carboxylic acid component can be replaced by a range of other acidic nucleophiles including thiobenzoic acid, thiophenol or 5-phenyltetrazole, which are incorporated via an alternative pathway. These latter reactions can also be applied to 2-aminotetrahydrofurans, 2-aminotetrahydropyrans or 4-hydroxybut-2-one, further extending the structural diversity of the multicomponent reaction products.     

First asymmetric total synthesis of (+)-sparteine

[reaction: see text] The total synthesis of (+)-sparteine was accomplished from 2,5-norbornadione in 15 steps and 15.7% overall yield. The key steps were two ring-expansion reactions, one involving an intramolecular Schmidt reaction and one using a novel variant of the photo-Beckmann rearrangement.     

A furan route to (−)-exo-isobrevicomin

A new enantiocontrolled synthesis of (−)-exo-isobrevicomin, isolated from male mountain pine beetles, Dendroctonus ponderosae, has been established starting from 5-ethyl-2-furfural by employing AD-mix-induced asymmetric induction and oxidative furan ring-expansion reaction as the key steps.     

Base-mediated hydrolytic cleavage with chain migration of 1-chloromethyl-tetrahydropyrano[3,4-b]indoles: an unusual pathway to 2-succinoyl tryptophols

An unusual hydrolytic cleavage with 1,2-alkyl chain migration of 1-chloromethyl-tetrahydropyrano[3,4-b]indoles by heating with a limited amount of water and base in DMF is reported. A mechanism for the formation of 1,2-alkyl chain migration products, 2-succinoyl tryptophols, and the ring-expansion products, dihydro-oxepine fused indoles, is reported. No comparable 1,2-chain migration from a structurally related 1-chloromethyl-isochroman is observed.     

Convenient syntheses of benzo-fluorinated dibenz[b,f]azepines: rearrangements of isatins, acridines, and indoles

Efficient procedures for the synthesis of benzo-fluorinated dibenz[b,f]azepines (iminostilbenes) from fluorinated isatins or indoles using a number of ring-expansion reactions are described. A range of mono- and difluorinated analogues is accessible, and the syntheses can deliver gram quantities of the final products, which are precursors of fluoro analogues of the important anticonvulsant carbamazepine.