Dehydrogenation of poly(1,3‐cyclohexadiene)–polystyrene binary block copolymers

The dehydrogenation of poly(1,3‐cyclohexadiene)–polystyrene binary block copolymers obtained by anionic copolymerization with alkyllithium/amine systems was investigated for the first time. The dehydrogenation of the poly(1,3‐cyclohexadiene) block, which was composed of 1,2‐cyclohexadiene (1,2‐CHD) and 1,4‐cyclohexadiene (1,4‐CHD) units, was strongly affected by the polymer chain structure. The existence of 1,2‐CHD units prevented the dehydrogenation of the poly(1,3‐cyclohexadiene) block in the binary block copolymer. The rate of dehydrogenation was fast on a long sequence of 1,4‐CHD units, whereas it was relatively slow for 1,2‐CHD/1,4‐CHD (≈1/1) unit sequences. The bonding of the polystyrene block to the polymer chain effectively improved not only the rate of dehydrogenation of a long sequence of 1,4‐CHD units but also that of the polymer chain with a high content of 1,2‐CHD units. The dehydrogenation of a poly(1,3‐cyclohexadiene) block containing a small number of 1,2‐CHD units progressed via step‐by‐step reactions. The dehydrogenation of a long sequence of 1,4‐CHD units proceeded as the first step. Subsequently, in the second step, the 1,2‐CHD/1,4‐CHD (≈1/1) unit sequences remaining in the polymer chain were dehydrogenated. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3526–3537, 2006     

TiCl3‐Mediated Synthesis of 2,3,3‐Trisubstituted Indolenines: Total Synthesis of (+)‐1,2‐Dehydroaspidospermidine, (+)‐Condyfoline,and (−)‐Tubifoline

2,3,3‐Trisubstituted indolenine constitutes an integral part of many biologically important monoterpene indole alkaloids. We report herein an unprecedented access to this skeleton by a TiCl₃‐mediated reductive cyclization of tetrasubstituted alkenes bearing a 2‐nitrophenyl substituent. The proof of concept is demonstrated firstly by accomplishing a concise total synthesis of (+)‐1,2‐dehydroaspidospermidine featuring a late‐stage application of this key transformation. A sequence of reduction of nitroarene to nitrosoarene followed by 6π‐electron‐5‐atom electrocyclization and a 1,2‐alkyl shift of the resulting nitrone intermediate was proposed to account for the reaction outcome. A subsequent total synthesis of (+)‐condyfoline not only illustrates the generality of the reaction, but also provides a mechanistic insight into the nature of the 1,2‐alkyl shift. The exclusive formation of (+)‐condyfoline indicates that the 1,2‐alkyl migration follows a concerted Wagner–Meerwein pathway, rather than a stepwise retro‐Mannich/Mannich reaction sequence. Conditions for almost quantitative conversion of (+)‐condyfoline to (?)‐tubifoline by way of a retro‐Mannich/1,3‐prototropy/transannular cyclization cascade are also documented.     

Synthesis of (3‐N‐substituted‐piperidin‐4‐ylidene)acetic Acid Ethyl Esters

A convenient preparation of skeletons 2A and 2B (cyclic γ,δ‐diamino‐α,β‐unsaturated esters) is reported by a three‐step synthetic route based on a sequence of NBS‐mediated one‐pot α‐bromination/Wittig olefination of piperidin‐4‐one 3 , nucleophilic addition with NaN₃, and followed by PPh₃‐promoted Staudinger reduction/substitution or CuI‐catalyzed Huisgen 1,3‐dipolar cycloaddition.     

Highly Stereoselective Synthesis of 1,3‐Dienes through an Aryl to Vinyl 1,4‐Palladium Migration/Heck Sequence

An efficient aryl to vinyl 1,4‐palladium migration/Heck sequence was developed for the stereoselective synthesis of 1,3‐dienes. High stereoselectivity was observed not only for 1,3‐dienes bearing two similar aryl groups at terminal positions, but also for those with configurations shown to be unfavorable with previous methods.     

Adsorption of 1,3‐Benzenedithiol and 1,3‐Benzenedimethanethiol on Gold Surfaces

The adsorption characteristics of 1,3‐benzenedithiol (1,3‐BDT) and 1,3‐benzenedimethanethiol (1,3‐BDMT) on Au surfaces are investigated by means of surface‐enhanced Raman scattering, UV/Vis absorption spectroscopy, and cyclic voltammetry (CV). 1,3‐BDMT is found to adsorb via two S–Au linkages at concentrations below monolayer coverage, but to have an upright geometry as the concentration increases on Au nanoparticles. On the other hand, 1,3‐BDT is found to adsorb by forming two S–Au linkages, regardless of concentration, based on the disappearance of the ν(SH)_free stretching band. Because of the absence of the methylene unit, 1,3‐BDT appeares not to self‐assemble efficiently on Au surfaces. The UV/Vis absorption spectroscopy and CV techniques are also applied to check the formation of self‐assembled monolayers of 1,3‐BDT and 1,3‐BDMT on Au. Density functional theory calculations based on a simple adsorption model using an Au₈ cluster are performed to better understand the nature of the adsorption characteristics of 1,3‐BDT and 1,3‐BDMT on Au surfaces.     

Synthesis and Properties of 2,3‐Diethynyl‐1,3‐Butadienes

The first general preparative access to compounds of the 2,3‐diethynyl‐1,3‐butadiene (DEBD) class is reported. The synthesis involves a one‐pot, twofold Sonogashira‐type, Pd⁰‐catalyzed coupling of two terminal alkynes and a carbonate derivative of a 2‐butyne‐1,4‐diol. The synthesis is broad in scope and members of this structural family are kinetically stable enough to be handled using standard laboratory techniques at ambient temperature. They decompose primarily through heat‐promoted cyclodimerizations, which are impeded by alkyl substitution and accelerated by aryl or alkenyl substitution. An iterative sequence of these unprecedented Sonogashira‐type couplings generates a new type of expanded dendralene. A suitably substituted DEBD carrying two terminal alkyne groups undergoes Glaser–Eglinton cyclo‐oligomerization to produce a new class of expanded radialenes, which are chiral due to restricted rotation about their 1,3‐butadiene units. The structural features giving rise to atropisomerism in these compounds are distinct from those reported previously.     

Direct Conversion of Nitriles into Alkene “Isonitriles”

The sequenced addition of RLi to nitriles, trapping with isopropylformate, and dehydration with phosphoryl chloride provides an efficient, direct synthesis of alkene isocyanides. The one‐pot sequence involves a series of carefully orchestrated steps: addition, formylation, tautomerization, and dehydration, with CuCN catalyzing a key equilibration of a formyl imine to an N‐formyl enamine. The resulting aromatic alkeneisocyanides, that are otherwise challenging to synthesize, engage in an unusual [4+2]‐type cycloaddition/1,3‐H shift/decyanation sequence to afford substituted naphthalenes.     

Synthesis and antimicrobial activity of new benzofuranyl‐1,3‐benzoxazines and 1,3‐benzoxazin‐2‐ones

New benzofuranyl‐1,3‐benzoxazines and 1,3‐benzoxazin‐2‐ones are synthesized in which benzofuran is coupled with 1,3‐benzoxazines and 1,3‐benzoxazin‐2‐ones through ‐CONH‐ and ‐COCH₂‐ bridges, respectively. The antimicrobial activity of these compounds is reported.     

Cycloaddition of bis‐1,3‐dipolar reagents to 1,3‐diarylprop‐2‐en‐1‐ones

Bis pyrazolines and isoxazolines were prepared by 1,3‐dipolar cycloaddition of benzene‐1,3/1,4‐dicarboxaldehyde dihydrazones and dioximes to 1,3‐diaryl‐prop‐2‐en‐1‐ones. © 2003 Wiley Periodicals, Inc. Heteroatom Chem 14:379–383, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.10169     

无气味、实用的缩硫醛/酮化反应试剂：2-（1，3-二噻-2-亚基）-3-羰基丁酸甲酯

a-Oxo ketene dithioacetals, methyl 2-（1,3-dithian/dithiolan-2-ylidene）-3-oxobutanoate （2a/2b） prepared in nearly quantitative yields simply from methyl acetylacetate, carbon disulfide and 1,3-dibromopropane/1,2-dibromoethane in the presence of potassium carbonate, were investigated in the thioacetalization with various carbonyl compounds 3. It has been demonstrated that methyl 2-（1,3-dithian-2-ylidene）-3-oxobutanoate （2a） could act as a nonthiolic, odorless and practical thioacetalization reagent. A range of aldehydes and ketones 3 were converted into the corresponding dithioacetals 4 in high yields （up to 91%） in the presence of 2a. Moreover, 2a showed high chemoselectivity between aldehyde and ketone in thioacetalization.     

α,β‐Double Electrophilic Addition of Allene‐1,3‐Dicarboxylic Esters for the Construction of Polysubstituted Furans by KI/tert‐Butyl Hydroperoxide (TBHP)‐Promoted Oxidative Annulation

An unprecedented KI/tert‐butyl hydroperoxide promoted tandem Michael addition/oxidative annulation of allene‐1,3‐dicarboxylic esters and 1,3‐dicarbonyl compounds has been developed. This procedure provides a new, facile, and transition‐metal‐free synthetic approach to afford polysubstituted furans in moderate to excellent yields (up to 93 %). This method first establishes a α,β‐double electrophilic reaction mode of allene‐1,3‐dicarboxylic esters to form 1,3‐dicarbonyl compounds.     

Synthesis of Functionalized Pyrazoles via 1,3‐Dipolar Cycloaddition of α‐Diazo‐β‐ketophosphonates,Sufones and Esters with Electron‐Deficient Alkenes

1,3‐Dipolar cycloaddition of diazo compounds with olefinic substrates is a promising atom‐economic strategy for the construction of functionalized pyrazoles. Over the last few years, our group has been engaged in the synthesis of phosphonyl/sulfonylpyrazoles and pyrazole esters by employing Bestmann‐Ohira Reagent (BOR) and its sulfur and ester analogs as 1,3‐dipole precursors with various dipolarophiles. This account describes the novel synthetic methods developed in our laboratory, in the perspective of closely related work by others, for the synthesis of phosphonyl/sulfonylpyrazoles, pyrazole esters and the total synthesis of Withasomnine, a natural product, by using 1,3‐dipolar cycloaddition as the key step.     

1,3‐Dipolar Cycloadditions of α‐Diazo Ketones Derived from N‐Protected (S)‐Proline with Aromatic and Cycloaliphatic Thioketones

Enantiomerically pure α‐oxo diazo compounds derived from (S)‐proline were used for 1,3‐dipolar cycloaddition with aryl and hetaryl thioketones, as well as with cycloalkanethiones. Whereas the reactions with hetaryl thioketones in boiling THF yield α,β‐unsaturated ketones via a cascade of cycloaddition, 1,3‐dipolar electrocyclization, and desulfurization, the analogous reactions with thiobenzophenone and cycloalkanethiones result in the formation of 1,3‐oxathiole derivatives. In the latter case, the 1,5‐dipolar electrocyclization of the intermediate thiocarbonyl ylide is the key step of the reaction sequence. In all cases, the isolated products are optically active, i.e., the multistep processes occur with retention of the stereogenic center incorporated via the use of (S)‐proline as the precursor of the diazo compounds.     

Synthesis and characterization of multiblock copolymers composed of poly(5‐methyl‐5‐benzyloxycarbonyl‐1,3‐dioxan‐2‐one) outer blocks and poly(L‐lactide) inner blocks

Ethylene glycol (EG) initiated, hydroxyl‐telechelic poly(L ‐lactide) (PLLA) was employed as a macroinitiator in the presence of a stannous octoate catalyst in the ring‐opening polymerization of 5‐methyl‐5‐benzyloxycarbonyl‐1,3‐dioxan‐2‐one (MBC) with the goal of creating A–B–A‐type block copolymers having polycarbonate outer blocks and a polyester center block. Because of transesterification reactions involving the PLLA block, multiblock copolymers of the A–(B–A)_n–B–A type were actually obtained, where A is poly(5‐methyl‐5‐benzyloxycarbonyl‐1,3‐dioxan‐2‐one), B is PLLA, and n is greater than 0. ¹H and ¹³C NMR spectroscopy of the product copolymers yielded evidence of the multiblock structure and provided the lactide sequence length. For a PLLA macroinitiator with a number‐average molecular weight of 2500 g/mol, the product block copolymer had an n value of 0.8 and an average lactide sequence length (consecutive C₆H₈O₄ units uninterrupted by either an EG or MBC unit) of 6.1. For a PLLA macroinitiator with a number‐average molecular weight of 14,400 g/mol, n was 18, and the average lactide sequence length was 5.0. Additional evidence of the block copolymer architecture was revealed through the retention of PLLA crystallinity as measured by differential scanning calorimetry and wide‐angle X‐ray diffraction. Multiblock copolymers with PLLA crystallinity could be achieved only with isolated PLLA macroinitiators; sequential addition of MBC to high‐conversion L ‐lactide polymerizations resulted in excessive randomization, presumably because of residual L ‐lactide monomer. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6817–6835, 2006     

Eine cyclische Methylendiphosphinsäure: 1,3‐Dihydroxy‐1,3‐dioxo‐1,2,3,4‐tetrahydro‐1λ5,3λ5‐[1,3]diphosphinin

A Cyclic Methylenediphosphinic Acid: 1,3‐Dihydroxy‐1,3‐dioxo‐1,2,3,4‐tetrahydro‐1λ⁵,3λ⁵‐[1,3]diphosphinine Strong acids protonate 1,3‐bis(dimethylamino)‐1λ⁵,3λ⁵‐[1,3]diphosphinine ( 5 ) to give the corresponding cation. The protonation is followed by hydrolytic cleavage of the dimethylamino groups resulting in the formation of the cyclic methylenediphosphinic acid ( 6 ).     

Formation of aryl‐bis (6‐amino‐1,3‐dimethyluracil‐5‐yl) methanes by reaction of 6‐amino‐1, 3‐dimethyluracil with aromatic aldehydes

The synthesis of aryl‐bis(6‐amino‐1,3‐dimethyluracil‐5‐yl)‐methanes 3a‐m by condensation of 6‐amino‐1,3‐dimethyluracil ( 1 ) with aromatic aldehydes 2a‐m at room temperature is reported. The structures of the compounds were established using various spectroscopic analyses and X‐ray crystallography. The crystal structures of two aryl‐bis (6‐amino‐1,3‐dimethyluracil‐5‐yl) methanes are presented.     

Methyl 3‐substituted‐isoxazole‐5‐ carboxylates syntheses on solid supports via wang resin‐bound 2,3‐dibromopropionate

A novel Wang resin‐bound 2,3‐dibromopropionate reagent has been developed and used as a potential dipolarophile for the facile preparation of methyl 3‐substituted‐isoxazole‐5‐carboxylates through triethylamine, promoting a sequence of reactions involving the in situ generation of 2‐bromoacrylate resin and nitrile oxide, regioselective 1,3‐dipolar cycloaddition, and loss of hydrogen bromide in one pot, and then cleavage from the resin with sodium methoxide. The advantages of this method include simple operation and mild conditions with good yield and high purity of the products. © 2012 Wiley Periodicals, Inc. Heteroatom Chem 23:444–448, 2012; View this article online at wileyonlinelibrary.com . DOI 10.1002/hc.21035     

Diethylbis(2,2′‐bipyridine)iron/MAO. A Very Active and Stereospecific Catalyst for 1,3‐Diene Polymerization

Diethylbis(2,2′‐bipyridine)Fe/MAO is an extremely active catalyst for the polymerization of 1,3‐dienes. Polymers with a 1,2 or 3,4 structure are formed from butadiene, isoprene, (E)‐1,3‐pentadiene and 3‐methyl‐1,3‐pentadiene, while cis‐1,4 polymers are derived from 2,3‐dimethyl‐1,3‐butadiene. The 1,2 (3,4) polymers obtained at 25°C are amorphous, while those obtained below 0°C are crystalline, as was determined by means of X‐ray diffraction. Mechanistic implications of the results are briefly discussed.     

Reaction of 6‐Methyl‐2‐Thiouracil and 6‐Phenyl‐2‐Thiouracil with Chloro‐β‐Dicarbonyl and Bromo‐β‐Dicarbonyl Compounds and Their Nitrile Analogs

Derivatives of 2‐methylidene‐1,3‐dihydropyrimidin‐4‐ones 2a , 2b , 2c , 2d , 2e , 2f , 2g were synthesized by interaction of 6‐methyl‐2‐thiouracil and 6‐phenyl‐2‐thiouracil 1a , 1b with some activated halogenides: diethyl bromomalonate, ethyl 2‐chloro‐3‐oxobutanoate, ethyl 2‐bromocyanoacetate, 2‐bromo‐5,5‐dimethylcyclohexan‐1,3‐dione, and bromomalononitrile. The boiling of 1a with ethyl 2‐bromocyanoacetate in mixture of ethanol and EtONa results in intramolecular cyclization and formation of thiazolo[3,2‐a]pyrimidin‐5‐one 3 . Interaction of 1a with 3‐chloropentane‐2,4‐dione and 2‐bromo‐1,3‐diphenylpropane‐1,3‐dione yielded corresponding S‐substituted thiopyrimidines 4a , 4b . In general, the products of 1b S‐alkylation are less prone to sulfur extrusion. Reaction of 1b with diethyl bromomalonate in the absence of EtONa stops at the S‐alkylation step, while in the presence of EtONa in ethanol or PPh₃ in dioxane 2‐(ethoxycarbonylmethyl)thio‐6‐phenyl‐1,3‐dihydropyrimidin‐4(1H)‐one 6 is formed exclusively. Molecular structure and crystal structure of 2‐(1,1‐diethoxycarbonylmethyliden)‐6‐methyl‐1,3‐dihydropyrimidin‐4(1H)‐one 2a are discussed.     

Reactions of unsymmetrical α‐diazo‐β‐diketones with imines: Syntheses of 4H‐1,3‐oxazin‐4‐ones

Cycloaddition reactions of an unsymmetrical α‐diazo‐β‐diketone, 2‐diazo‐1‐phenyl‐1,3‐butanedione, with a series of imines having various substituents were studied. The results indicated that only cycloadducts derived from acetylphenylketene, which was generated by the thermal Wolff rearrangement of 2‐diazo‐1‐phenyl‐1,3‐butanedione with phenyl migration, and imines were obtained. © 2002 Wiley Periodicals, Inc. Heteroatom Chem 13:165–168, 2002; Published online in Wiley Interscience (www.interscience.wiley.com). DOI 10.1002/hc.10015