摘要:

AbstractIt has been previously shown that the structural analysis of the title compounds by electron‐impact ionization (EI)mass spectrometry is complicated by extensive, if not complete, thermal rottlerone‐type rearrangement and/or fragmentation reactions. The use of a ‘softer’ ionization technique such as field desorption (FD) is, therefore, required in order that these compounds may be characterized in terms of their molecular size and composition. In this report, the FD mass spectra of typical representatives of this class of compounds are described along with the attempted suppression of undesirable thermal reactions by the optimization of the emitter tempe

ISSN:0018-019X    

DOI:10.1002/hlca.19870700402

出版商:WILEY‐VCH Verlag GmbH    

年代:1987

数据来源: WILEY

摘要:

AbstractChemical degradation of sarverogenin (1) to several bile‐acid derivatives of known structures is described. This is a proof that1contains the normal tetracyclic steroid skeleton, and that the structure suggested for1byFuhrer et al.[3] on the basis of spectral data is correc

ISSN:0018-019X    

DOI:10.1002/hlca.19870700403

出版商:WILEY‐VCH Verlag GmbH    

年代:1987

数据来源: WILEY

摘要:

Multistep Redox Systems. 1,3‐Dimethylidenecyclobutanes Substituted by π‐Syntheses and PropertiesAs potential redox systems 1,3‐dimethylidenecyclobutanes are synthesized in which the‘exo’‐methylidene group participates in a cyclopentadienyl (typeC4)or cycloheptatrienyl (typeA5) system. Benzo‐annellated systems are also described. The formation of the CC bond by the reaction of thioketones (3and31) with diazo compounds is extended to extended to1,6,7,21, and24, which so far had not been employed. In some cases, intermediate thiiranes can be isolated (e.g.4) which are smoothly transformed into the target compounds of typesC4andA5by extrusion of sulfur. The new CC bonds may be introduced stepwise, thereby allowing the synthesis of unsymmetrically substituted 1,3‐dimethylidenecyclobutanes (e.g.20and35). Reduction of by Na does not produce bicyclobutanes but compounds43and44with one

ISSN:0018-019X    

DOI:10.1002/hlca.19870700404

出版商:WILEY‐VCH Verlag GmbH    

年代:1987

数据来源: WILEY

摘要:

AbstractCrystal structures and electrical properties of radical‐cation salts of the chiral organic donor TMET (S,S,S,S,‐bis‐(dimethylethylenedithio)tetrathiafulvalene) are described. Two structural types, 2:1 with octahedral anions Pf −6, AsF −6, SbF −6, I −3(incommensurate), and 3:2 with tetrahedral anions BF−4, CIO−4, ReO−4are observed. Resistivity measurements between 2 and 298 K indicate that the 3:2 types are organic metals, while the other compounds are semiconductors. (TMET)3(CIO4)2is metallic down to about 120 K at ambient pressure and remains met

ISSN:0018-019X    

DOI:10.1002/hlca.19870700405

出版商:WILEY‐VCH Verlag GmbH    

年代:1987

数据来源: WILEY

摘要:

Chiral Alkoxytitanium(iv) Complexes for Enantioselective Nucleophilic Additions to Aldehydes and asLewisAcids inDiels‐AlderReactionsA Number of chiral 1,2‐1,3 and 1,4‐diols were prepared and used as alkoxy ligands on Ti for enantioselective nucleophilic transfer of methyl, butyl, cyclopropyl, allyl, alkinyl, and phenyl groups to aromatic aldehydes, as well as for the enantioselective[4+2]cycloaddition of acrylate to cyclopentadiene. The 1,2‐diols were pinane diol7and 1,2:5.6‐diacetonide‐protected mannitol9(Scheme 3) and tartrates. The 1,3‐diols were obtained from the yeast‐reduction products of 2‐oxocyclopentane‐ and 2‐oxocyclohexanecarboxylates and excess MeLi, BuLi, or PhLi (or the correspondingGrignardreagents; see4–6.) As 1,4‐diols, we used the products2and3from tartrate acetals and methyl or Pheny1Grignardreagents, the bis(benzaldehyde) acetal8of d‐mannitol ando,o'‐binaphthol (22). These diols were attached to the Ti‐atom by azeotropic removal of i‐PrOH from a mixture with [TiCi(i‐PrO)3]. Addition of various organometallic reagents R‐metal (metal = Li, BR3, MgX, MnC1, CuLiR) was followed by combination with aldehydes at – 75., a warm up period, quenching with aqueous KF solution, and workup (for results seeTables 1–6andFormulae17–20). The enantiomeric excess of the secondary alcohols obtained varies greatly, certain combinations of chiral ligands, nucleophilic groups, and aldehyde substrates give rise to values as high as 90% ee; seee.g. Table 4.The Ti‐complexes of the general formula [Ti(R*O)2Ci2] or [Ti(R*O)2(i‐PrO)CI] induced theDiels‐Alderaddition of methyl acrylate to cyclopentadience to take place at –30.. The best enantioselectivity (50% ee) was observed with the binaphthol derivative (Table 7). The structures of the complexes involved in these reactions are unknown. The substitution on C(2) of the dioxolanes2and3(derived from tartaric acid) has a pronounced effect on the selectivities of both reactions studied here (Tables 2, 3, and7). This remote effect (1,6‐distance between the stereogenic acetal‐C‐atom and the Ti‐centers) must be caused by conformational changes in the vicinity of the reactive site,i.e.the TiC bond in the nucleophilic addition reactions and

ISSN:0018-019X    

DOI:10.1002/hlca.19870700406

出版商:WILEY‐VCH Verlag GmbH    

年代:1987

数据来源: WILEY

摘要:

Pigments from Leaf‐Glands from Labiatae: 17 Quinonoid Abietanoids fromPlectranthus sanguineusBRITTENFrom the glandular trichomes of the title plant from Nyasaland, Malawi, 17 abietanoid quinones and hydroquinones were isolated and identified, the most interesting amongst them being the novel tricyclic, sevenmembered anhydride8. A detailed HPLC separation of the compounds is presente

ISSN:0018-019X    

DOI:10.1002/hlca.19870700407

出版商:WILEY‐VCH Verlag GmbH    

年代:1987

数据来源: WILEY

摘要:

AbstractThe briarane diterpenoid verecynarmin A (=(−)).(8R,9S,11R,12aR,4Z,6Z)‐11,12‐epoxy‐8,8a,9,10,12,12a,13‐octahydro‐1,5,8a,12‐tetramethylbenzo[4,5]cyclodeca[1,2‐b]furan‐9‐yl acetate; ((−)‐1) is shown to be contained in both the Mediterranean nudibranch molluscArmina maculata(RAFINESQUE) and its prey, the pennatulacean octocoralVeretillum cynomorium(PALLAS). The structure is mainly solved by 1D‐NMR, 2D‐NMR, and NOE studies of (−)‐1. However, the relative configurations at the epoxide and angular methyl centres, as well as the preferred conformation, are established by shift‐reagent and NOE studies of diol(−)‐4 obtained by epoxide‐ring opening of (−)‐1 with LiA1H4. The absolute configuration is establishedviadeacetylation

ISSN:0018-019X    

DOI:10.1002/hlca.19870700408

出版商:WILEY‐VCH Verlag GmbH    

年代:1987

数据来源: WILEY

摘要:

Formation of Thietane DerivativesviaIntramolecular (2+2) CycloadditionOn irradiation, the two 4‐vinyl‐1,3‐thiazole‐5(4H)‐thiones1a, b, synthesized from thiobenzoic acid and the corresponding 3‐amino‐2H‐azirines2a,b, undergo an intramolecular (2+2)‐cycloaddition reaction of the CS and CC bonds to give the tricyclic thi

ISSN:0018-019X    

DOI:10.1002/hlca.19870700409

出版商:WILEY‐VCH Verlag GmbH    

年代:1987

数据来源: WILEY

摘要:

AbstractPhotolysis (λ = 254 nm) of 4‐(tert‐butyl)‐4,6,7,8‐tetrahydro‐7,7‐dimethy‐2H‐1‐benzopyran‐2,5(3H)‐dione (3a) int‐BuOH affords 2‐(2,2‐dimethylpropylidene)‐5,5‐dimethyl‐1,3‐cyclohexanedione (1a) in 80% yieldviahomolysis of the lactone OCO

ISSN:0018-019X    

DOI:10.1002/hlca.19870700410

出版商:WILEY‐VCH Verlag GmbH    

年代:1987

数据来源: WILEY

摘要:

Oxidation Reactions of Methyl Substituted 2,1,3‐Benzothiadiazoles and 2,1,3‐Benzoselenadiazoles with Selenium DioxideThe synthesis5–8by oxidation of 4‐methyl‐(1),5‐methyl(2), 5,6‐dimethyl‐2,1,3‐benzoselenadiazole (3), and 5,6‐dimethyl‐2,1,3‐benzothiadiazole (4), respectively, with selenium dioxide as well as their spectroscopic p

ISSN:0018-019X    

DOI:10.1002/hlca.19870700411

出版商:WILEY‐VCH Verlag GmbH    

年代:1987

数据来源: WILEY