(−)-Sparteine-Mediated Asymmetric Intramolecular Carbolithiation of Alkenes: Synthesis of Enantiopure Cyclopentanes with Three Consecutive Stereogenic Centers

An asymmetric intramolecular carbolithiation reaction was developed by combining the (−)-sparteine-mediated enantiotopos-differentiating deprotonation and the anionic 5-exo-trig cyclization. Achiral 6-phenylhex-5-enyl carbamates were efficiently cyclized furnishing regio-, diastereo- (dr >99 : 1), and enantioselectively (er >98 : 2) 1,2-trans-substituted cyclopentanes. The intermediate primary benzylic lithium-carbanion pairs were – in spite of their configurative lability – diastereoselectively substituted by versatile electrophiles creating a third consecutive stereogenic center. Additionally, some 4-functionalized 6-phenylhex-5-enyl carbamates were also cyclized in high yield to provide enantiomerically pure cyclopentanes incorporating three adjacent stereogenic centers.     

Dichloro[TADDOLato(2−)-O,O′]titanium/Dichlorobis[1-methylethoxy]titanium-Mediated,Highly Diastereo- and Enantioselective Additions of Silyl Enol Ethers to Nitro Olefins and [3+2] Cycloadditions of Primary Adducts to Acetylenes

The diastereoselective, Ti-Lewis-acid-mediated, low-temperature addition of silyl enol ethers to 1-aryl-2-nitroethenes (Scheme 1) occurs enantioselectively with dichloro[TADDOLato(2−)-O,O′]titanium 3 (TADDOL=α,α,α′,α′-tetraaryl-1,3-dioxolane-4,5-dimethanol) (Scheme 2). At least 3 equiv. of Lewis acid are required for high conversions (yields). However, the chiral Lewis acid 3 can be `diluted' with the achiral Cl₂Ti(OCHMe₂)₂ analog (ratio 1 : 2.5), with hardly any loss of enantioselectivity! Both, the primary (4+2) cycloadducts ( B , 9 ) and the γ-nitro ketones ( A , 1a – h , 5 , 7 ), formed by hydrolysis, can be isolated in good yields and with high configurational purities (Schemes 3 and 4, and Table 1). The relative and absolute configurations (2S,1′R) of the products 1 from cyclohexanone silyl enol ether and 1-aryl(including 1-heteroaryl)-2-nitroethenes (obtained with (R,R)-TADDOLate) are assigned by NMR spectroscopy, and optical comparison and correlation with literature data, as well as by anomalous-dispersion X-ray crystal-structure determination (nitro ketone 1c ; Fig.). The nitro ketone 7 from cyclohex-2-enone and 4-methoxy-β-nitrostyrene was cyclized (via a silyl nitronate C ; Scheme 5) to the nitroso acetal 8 , and one of the bicyclic nitronate primary adducts 9 underwent a [3+2] cycloaddition to phenylacetylene and to ethyl 2-butynoate to give, after a ring-contracting rearrangement, tricyclic aziridine derivatives with five consecutive stereocenters ( 10 , 11 ; Scheme 5 and Table 2), in enantiomerically pure form. With an aliphatic nitro olefin, the Ti-TADDOLate-mediated reaction with (silyloxy)cyclohexene led to a moderate yield, but the product 4 was isolated in a high configurational purity.     

β-Thiopeptides: Synthesis,NMR Solution Structure,CD Spectra,and Photochemistry

To test the effect of NH−C=S groups (Scheme 1) on the stability of β-peptide secondary structures, we have synthesized three β-thiohexapeptide analogues of H-(β-HVal-β-HAla-β-HLeu)₂-OH ( 1 ) with one, two, and three C=S groups in the N-terminal positions (cf. 2 – 4 and model in Fig. 1). The first C=S group was introduced selectively by treatment with Lawesson reagent of Boc-β-dipeptide esters ( 6 and 8 ). A series of fragment-coupling steps (with reagents as for the corresponding sulfur-free building blocks) and another thionation reaction led to the title compounds with a C=S group in residues 1, 1, and 3, as well as 1, 2, and 3 of the β-hexapeptide (Schemes 2 and 3). The sulfur derivatives, especially those with three C=S groups, were much more soluble in organic media than the sulfur-free analogues (>1000-fold in CHCl₃; Table 1). The UV and CD spectra (in CHCl₃, MeOH, and H₂O) of the new compounds were recorded and compared with those of the parent β-hexapeptide 1 (Figs. 2 – 4); they indicate the presence of more than one secondary structure under the various conditions. Most striking is a pronounced exciton splitting (Δλ ca. 20 nm, amplitude up to +121000) of the ππ*_C=S band near 270 nm with the β-trithiohexapeptide (with and without terminal protecting groups), and strong, so-called `primary solvent effects', in the CD spectra. The CD spectrum of the β-dithiohexapeptide 3 undergoes drastic changes upon irradiation with 266-nm laser light of a MeOH solution (Fig. 5). The NMR structure in CD₃OH of the unprotected β-trithiohexapeptide 4 was determined to be an (M)-3₁₄-helix (Fig. 7), very similar to that of the non-thionated analogue (cf. 1 ). NMR and mass spectra of the β-hexapeptides with C=S and with C=O groups are compared (Figs. 6 and 8).     

Enantioselective Preparation of γ-Amino Acids and γ-Lactams from Nitro Olefins and Carboxylic Acids,with the Valine-Derived 4-Isopropyl-5,5-diphenyl-1,3-oxazolidin-2-one as an Auxiliary

Titanium enolates of acyl-oxazolidinones 1 , derived from acetic, propanoic, 3-methylbutanoic, and 4-methylpentanoic acids and 4-isopropyl-5,5-diphenyl-1,3-oxazolidin-2-one, are added to aliphatic and aromatic nitro olefins in the presence of TiCl₄ (Schemes 2 – 4). The products, 4-nitro carboxylic-acid derivatives 2 , are formed in high diastereoselectivities (ds 80 to >99%) and in good yields (50 – 75% of purified samples of ds >98%). Hydrogenation over Raney-Ni of the NO₂ group in the adducts leads directly to the corresponding γ-lactams ( 3 and 8 ; 80 – 92%), with recovery of the insoluble auxiliary (ca. 95%). Ring opening is achieved through the N-Boc-lactams ( 4 ), which are converted to N-Boc-protected γ-amino acids 5 or to their benzyl and methyl esters ( 6 and 7 ; Scheme 5). The configuration of the products (containing up to three new stereogenic centers; Scheme 1) is assigned by comparison with literature data, by X-ray crystal-structure analysis (for 2c , g , f , 8 , Fig.), and by analogy. Thus, the (S)-auxiliary gives rise to combination of the trigonal centers of enolate and nitro olefin with Si/Si topicity (relative topicity all-lk; cf. A ).     

Umweltchemie 1998

Nachdem im Trendbericht des letzten Jahres ausgewählte Beispiele aus Bereichen der Umweltchemie besprochen wurden,¹⁾ sollen in diesem Jahr Ergebnisse aus Ökotoxikologie and Humantoxikologie behandelt werden. Zunächst werden einige Arbeiten aus den Bereichen der Entsorgungsverfahren und neue Entwicklungen bei Spurenstoffen vorgestellt. Da das Gebiet der nachhaltigen Chemie immer mehr in den Brennpunkt des Interesses rückt, werden auch neue umweltfreundliche Synthesen und Produkte ausführlich diskutiert.     

Characterization of a poly(ethersulphone) by on-line coupling of size exclusion chromatography with NMR and FTIR spectroscopy

An on-line detection „on the flow”︁ during the size exclusion chromatography run using FTIR and NMR spectrometers was applied to investigate the liquid chromatographic separation process of oligomers and to identify the separated chromatographic compounds. The FTIR and NMR detectors give real-time information about the chemical structure and functional groups of the eluated species. The preferences and limits of the on-line coupling were discussed using an oligomeric hydroxyl terminated poly(ethersulphone) as an example. Molecular weights could be calculated from the ¹H-NMR spectra of the separated species using the intensities of end-group and main-chain signals. Residues of N-methylpyrrolidone which was used as solvent in the oligomer synthesis were identified by the FTIR detector.     

Unusual Optical Properties of Mn‐doped ZnO: The Search for a New Red Pigment—A Combined Experimental and Theoretical Study

A pigment of your imagination : A range of polycrystalline solid solutions of a zinc‐rich Zn_x?1Mn_xO system (see figure) have been prepared and studied in terms of their colour, diffuse reflectance spectra, Mn valence state and electronic structure. The intense optical absorption arises from Mn²⁺ doping and is thought to be due to forbidden or partially forbidden transitions between the valence and the conduction band.

     

Novel triphenylarsinyl-functionalized N-heterocyclic carbene ligands in palladium-catalyzed C-C coupling reactions

Synthesis of novel triphenylarsinyl-functionalized N-heterocyclic carbene pre-ligands starting from N,N-dimethylbenzylamine, chlorodiphenylarsine and different 1-substituted imidazoles and their characterization by NMR and X-ray analysis is reported. Furthermore, these precursors are applied to different palladium-catalyzed reactions such as Heck-, hydro-Heck, π, σ domino-Heck and Suzuki reactions to give the C-C coupling products in good to excellent isolated yields.