NBS‐Promoted Rearrangement of 1,1‐Diarylmethylenecyclopentane

The 1‐aroyl‐1‐aryl‐2‐bromocyclopentanes 3a , 3b , 3c and 3d (Ar = C₆H₅, 2‐FC₆H₄, 3‐FC₆H₄, 4‐FC₆H₄) were prepared from N‐bromosuccinimide (NBS)‐promoted rearrangement of 1,1‐diarylmethylenecyclopentane 2 . The possible mechanism was proposed. Two 1‐phenyl‐cyclopentane carbamides 5a and 5b with the anti‐influenza effect were also accomplished from compound 3a .     

Properties and Reactions of Substituted 1,2‐Thiazetidine 1,1‐Dioxides: Chiral Mono‐ and Bicyclic 1,2‐Thiazetidine 1,1‐Dioxides from α‐Amino Acids

New chiral mono‐ and bicyclic β‐sultams, valuable building blocks for drug synthesis, have been prepared from L ‐Ala, L ‐Val, L ‐Leu, L ‐Ile, L ‐Phe, L ‐Cys, L ‐Ser, L ‐Thr, and D ‐penicillamine by transformation of the COOH group into a methylsulfonyl chloride function, followed by cyclization under basic conditions. Selected properties, derivatives, and reactions of the β‐sultams are described.     

Stereoselective SE2′ Protonation of α‐Hydroxyallylsilanes Mediated by a Brook Rearrangement

Brooked up! Treatment of (R,Z)‐3‐(tert‐butyldimethylsilyl)‐1‐cyano‐3‐hydroxyprop‐1‐enyl carbamate with a catalytic amount of a base afforded (S,E)‐3‐(tert‐butyldimethylsilyloxy)‐1‐cyanoallyl diisopropylcarbamate, showing that S_E2′‐type reaction of allylsilicates proceeds in an anti fashion. The overall process is equivalent to trapping of an enantioenriched C‐chiral carbanion at the α‐position of nitrile group in up to 77 % ee (see scheme).

     

A novel photoisomerzation of 1,2‐benzothiazine 1,1‐dioxides to 1,3‐benzothiazine 1,1‐dioxides

A novel facile photoconversion of 4‐hydroxy‐1,2‐bezothiazine 1,1‐dioxides ( 3a‐e ) into 4‐oxo‐1,3‐2H‐benzothiazine 1,1‐dioxides ( 4a‐e ) and 4‐hydroxy‐2‐methyl‐N‐(pyridin‐2‐yl)‐2H‐1,2‐benzothiazine‐3‐carboxamide 1,1‐dioxide (PRX) into N‐methyl saccharin ( 2 ) upon 254 nm irradiation in methanol or acetonitrile is reported. The structures of the products have been elucidated by spectroscopic methods and single crystal X‐ray structure determination for 4a and 4d .     

A Facile Synthesis of 2‐Methyl‐3‐oxoindoline‐2‐carboxylates Utilizing Aza‐Brook Rearrangement as a Crucial Step

Synthesis of 2‐methyl‐3‐oxoindoline‐2‐carboxylates is developed using a bis(trimethylsilyl)aluminum chloride‐induced aza‐Brook rearrangement as a crucial step. Regarding the organoaluminum species, use of readily available tris(trimethylsilyl)aluminum·ether complex was not suitable for the present cyclization. After a series of examination of the reaction conditions, the aza‐Brook rearrangement and the subsequent cyclization with 2 equivalens of bis(trimethylsilyl)aluminum chloride were found to be the most effective. An application to the synthesis of a potential intermediate of duocarmycin A is also described, in which ozonolysis of the styrenyl moiety and the Barton–McCombie deoxygenation of the hydroxymethyl group were successfully carried out.     

Synthesis of 2‐phosphoro‐1,2‐thiazetidine 1,1‐dioxide

The reactions of phosphorochloridites 5a–c with an equimolar amount of 1,2‐thiazetidine 1,1‐dioxide (2) or L(−)‐3‐carboethoxy‐1,2‐thiazetidine 1,1‐dioxide (7) in the presence of triethylamine, affords the N‐phosphitylated β‐sultams 6a–b and L(−)‐8a,c. Their oxidation by addition of oxygen, sulfur, or selenium results in formation of stable organophosphorus β‐sultams 10a–b, L(−)‐11a,c, 12a, 13a, L(−)‐14c, and L(−)‐15c. © 1999 John Wiley & Sons, Inc. Heteroatom Chem 10: 61–67, 1999     

Catalyst‐Controlled 1,2‐ and 1,1‐Arylboration of α‐Alkyl Alkenyl Arenes

Two methods are reported for the 1,2‐ and 1,1‐arylboration of α‐methyl vinyl arenes. In the case of 1,2‐arylboration, the formation of a quaternary center occurred through a rare cross‐coupling reaction of a tertiary organometallic complex. 1,1‐Arylboration was enabled by catalyst optimization and occurred through a β‐hydride elimination/reinsertion cascade. Enantioselective variants of both processes are presented as well as mechanistic investigations.     

Retro‐Brook Rearrangement of Ferrocene‐Derived Silyl Ethers

An intramolecular Li–Si exchange was observed on various lithiated ferrocenylbenzyl silyl ethers. The thermodynamically more stable C‐silylated isomers were isolated in good yields and fully characterized. The reaction mechanism of the [1,4] retro‐Brook rearrangement was investigated by DFT calculations. Two distinct reaction routes were proposed and a possible stabilization effect of the ferrocenyl fragment on the C‐silylated isomers was described. The diastereoselective rearrangement of the trimethylsilyl group to the ortho position of the ferrocenyl cyclopentadienyl ring was also accomplished and the absolute configuration of the product was determined.     

Synthesis of Phenanthrene Derivatives by Intramolecular Cyclization Utilizing the [1,2]‐Phospha‐Brook Rearrangement Catalyzed by a Brønsted Base

The synthesis of functionalized phenanthrene derivatives was achieved by intramolecular cyclization utilizing the [1,2]‐phospha‐Brook rearrangement under Brønsted base catalysis. Treatment of biaryl compounds having an α‐ketoester moiety and an alkyne moiety at the 2 and 2′ positions, respectively, with diisopropyl phosphite in the presence of a catalytic amount of phosphazene base P2‐tBu provides 9,10‐disubstituted phenanthrene derivatives in high yields. This reaction involves the generation of an ester enolate through an umpolung process, that is, addition of diisopropyl phosphite to a keto moiety followed by the [1,2]‐phospha‐Brook rearrangement, the intramolecular addition to an alkyne, and the [3,3] rearrangement of the allylic phosphate moiety in a consecutive fashion.     

Synthesis of 1‐silacyclopent‐2‐ene derivatives using 1,2‐hydroboration, 1,1‐organoboration and protodeborylation

The reaction of di(alkyn‐1‐yl)vinylsilanes R¹(H₂C═CH)Si(C≡C―R)₂ (R¹ = Me ( 1 ), Ph ( 2 ); R = Bu (a), Ph (b), Me₂HSi (c)) at 25°C with 1 equiv. of 9‐borabicyclo[3.3.1]nonane (9‐BBN) affords 1‐silacyclopent‐2‐ene derivatives ( 3a , 3b , 3c , 4a , 4b ), bearing one Si―C≡C―R function readily available for further transformations. These compounds are formed by consecutive 1,2‐hydroboration followed by intramolecular 1,1‐carboboration. Treated with a further equivalent of 9‐BBN in benzene they are converted at relatively high temperature (80–100°C) into 1‐alkenyl‐1‐silacyclopent‐2‐ene derivatives ( 5a , 5b 6a , 6b ) as a result of 1,2‐hydroboration of the Si―C≡C―R function. Protodeborylation of the 9‐BBN‐substituted 1‐silacyclopent‐2‐ene derivatives 3 , 4 , 5 , 6 , using acetic acid in excess, proceeds smoothly to give the novel 1‐silacyclopent‐2‐ene ( 7 , 8 , 9 , 10 ). The solution‐state structural assignment of all new compounds, i.e. di(alkyn‐1‐yl)vinylsilanes and 1‐silacyclopent‐2‐ene derivatives, was carried out using multinuclear magnetic resonance techniques (¹H, ¹³C, ¹¹B, ²⁹Si NMR). The gas phase structures of some examples were calculated and optimized by density functional theory methods (B3LYP/6‐311+G/(d,p) level of theory), and ²⁹Si NMR parameters were calculated (chemical shifts δ²⁹Si and coupling constants ⁿJ(²⁹Si,¹³C)). Copyright © 2014 John Wiley & Sons, Ltd.     

Enantio‐ and Diastereoselective 1,2‐Additions to α‐Ketoesters with Diborylmethane and Substituted 1,1‐Diborylalkanes

The catalytic enantioselective synthesis of boronate‐substituted tertiary alcohols through additions of diborylmethane and substituted 1,1‐diborylalkanes to α‐ketoesters is reported. The reactions are catalyzed by readily available chiral phosphine/copper(I) complexes and produce β‐hydroxyboronates containing up to two contiguous stereogenic centers in up to 99:1 e.r. and greater than 20:1 d.r. The utility of the organoboron products is demonstrated through several chemoselective functionalizations. Evidence indicates the reactions occur via an enantioenriched α‐boryl‐copper‐alkyl intermediate.     

Reaction‐path calculations and crystal structures of 1,1′‐(ethylene‐1,2‐diyl)dipyridinium dichloride dihydrate and 1,1′‐(ethylene‐1,2‐diyl)dipyridinium dibromide

The design of new organic–inorganic hybrid ionic materials is of interest for various applications, particularly in the areas of crystal engineering, supramolecular chemistry and materials science. The monohalogenated intermediates 1‐(2‐chloroethyl)pyridinium chloride, C₅H₅NCH₂CH₂Cl⁺·Cl⁻, (I′), and 1‐(2‐bromoethyl)pyridinium bromide, C₅H₅NCH₂CH₂Br⁺·Br⁻, (II′), and the ionic disubstituted products 1,1′‐(ethylene‐1,2‐diyl)dipyridinium dichloride dihydrate, C₁₂H₁₄N₂²⁺·2Cl⁻·2H₂O, (I), and 1,1′‐(ethylene‐1,2‐diyl)dipyridinium dibromide, C₁₂H₁₄N₂²⁺·2Br⁻, (II), have been isolated as powders from the reactions of pyridine with the appropriate 1,2‐dihaloethanes. The monohalogenated intermediates (I′) and (II′) were characterized by multinuclear NMR spectroscopy, while (I) and (II) were structurally characterized using powder X‐ray diffraction. Both (I) and (II) crystallize with half the empirical formula in the asymmetric unit in the triclinic space group P. The organic 1,1′‐(ethylene‐1,2‐diyl)dipyridinium dications, which display approximate C2h symmetry in both structures, are situated on inversion centres. The components in (I) are linked via intermolecular O—H…Cl, C—H…Cl and C—H…O hydrogen bonds into a three‐dimensional framework, while for (II), they are connected via weak intermolecular C—H…Br hydrogen bonds into one‐dimensional chains in the [110] direction. The nucleophilic substitution reactions of 1,2‐dichloroethane and 1,2‐dibromoethane with pyridine have been investigated by ab initio quantum chemical calculations using the 6–31G** basis. In both cases, the reactions occur in two exothermic stages involving consecutive S_N2 nucleophilic substitutions. The isolation of the monosubstituted intermediate in each case is strong evidence that the second step is not fast relative to the first.