6‐Nitro‐1,2,3,4‐tetrahydroquinolines by a tandem reductive amination‐SNAr reaction

A tandem reductive amination‐S_NAr reaction has been developed for the synthesis of 6‐nitro‐1,2,3,4‐tetrahydroquinolines. Treatment of 4‐(2‐fluoro‐5‐nitrophenyl)‐2‐butanone or 3‐(2‐fluoro‐5‐nitrophenyl)‐propanal with primary amines and sodium cyanoborohydride in methanol at room temperature provided good to excellent yields of the substituted tetrahydroquinolines. The reaction proceeded best with the ketone substrate using primary amines that were unbranched at the α‐carbon. The aldehyde also produced the target heterocycles, but these were accompanied by 10‐15% of the uncyclized side chain reductive amination products.     

4H‐1‐Benzopyrans by a tandem SN2‐SNAr reaction

Treatment of 2‐fluoro‐5‐nitrobenzyl bromide with active methylene compounds in the presence of excess potassium carbonate in acetone leads to the formation of highly functionalized 4H‐1‐benzopyrans by a tandem S_N2‐S_NAr reaction sequence. The reaction works well with β‐keto esters, β‐keto sulfones, β‐keto phosphine oxides, β‐keto phosphonates and β‐keto nitriles. The reaction is simple to perform and affords products in 50‐92% yields.     

ConFlo III - an interface for high precision delta(13)C and delta(15)N analysis with an extended dynamic range

A newly developed interface coupling a CHN combustion device (elemental analyser 'EA') to an isotope ratio mass spectrometer is described and evaluated. The purpose of the device is to extend the dynamic range of delta(13)C and delta(15)N analysis from less than 2 orders of magnitude to more than 3 orders of magnitude. Carbon isotope ratio measurements of atropine as a model compound have been performed analysing between 1 μg to 5 mg C with acceptable to excellent precision (0.6 to 0.06 per thousand, delta-notation). The correction due to the blank signal is critical for sample amounts smaller than 4 μg C. The maximum sample weight is determined by the combustion capacity of the EA. Larger sample amounts are measured using dilution of a small part of the EA effluent with helium. The dilution mechanism works virtually free of isotope fractionation. Copyright 1999 John Wiley & Sons, Ltd.     

Ethyl 1,4‐Dihydro‐4‐oxo‐1,8‐naphthyridine‐3‐carboxylates by a Tandem SNAr‐Addition‐Elimination Reaction

A series of N‐substituted 1,4‐dihydro‐4‐oxo‐1,8‐naphthyridine‐3‐carboxylate esters has been prepared in two steps from ethyl 2‐(2‐chloronicotinoyl)acetate. Treatment of the β‐ketoester with N,N‐dimethylformamide dimethyl acetal in N,N‐dimethylformamide (DMF) gave a 95% yield of the 2‐dimethylaminomethylene derivative. Subsequent reaction of this β‐enaminone with primary amines in DMF at 120^oC for 24 h then afforded the target compounds in 47–82% yields by a tandem S_NAr‐addition‐elimination reaction. Synthetic and procedural details as well as a mechanistic rationale are presented.     

Shorted operators and the structure of operators with numerical radius one

The theory of the shorted operator is used to prove that if w(T)1, then T can be written in the form T=2(I-C*C)^1/2C. Using this form, Ando has exhibited a matricial form for a unitary 2-dilation of T.     

Diastereoselective synthesis of linear‐fused tricyclic nitrogen heterocycles by a tandem reduction‐reductive amination reaction

A two‐step diastereoselective synthesis of linear‐fused tricyclic nitrogen heterocycles has been developed from cyclic β‐ketoesters. The cyclization substrates are readily prepared by alkylation of the methyl 2‐oxo‐cycloalkanecarboxylates with 2‐nitrobenzyl bromide. Hydrogenation of these substrates initiates a reaction sequence involving (1) reduction of the aromatic nitro group, (2) condensation of the resulting hydroxyl‐amine or aniline nitrogen with the cycloalkanone and (3) reduction of the imine. The products are isolated in high yield as single diastereomers having the trans‐fused ring junction. The observed selectivity is rationalized in terms of a steric effect imposed by the ester group in the final reductive amination step which directs the incoming hydrogen to the opposite face of the molecule. By comparison, reductive cyclizations of substrates lacking the stereodirecting ester group give mixtures of cis and trans products with a preference for the cis‐fused heterocycle.