摘要:

AbstractQualitative conformational analysis of the entirety of conceivable hexo‐ and pentopyranosyloligonucleotide systems derived from the diastereoisomeric aldohexoses (CH2O)6and aldopentoses (CH2O)5predicts the existence of a variety of pairing systems which have not been experimentally investigated so far. In particular, the analysis foresees the existence of a ribopyranosylisomer of RNA (‘p‐RNA’), containing the phosphodiester linkage between the positions C(4′) and C(2′) of neighboring ribopyranosyl units. Double strands of p‐RNA oligonucleotides are expected to have a linear structure and to show purine‐pyrimidine and purine‐purine (Watson‐Crick) pairing comparable in strength to that observed in homo‐DNA. Experimentally, synthetic β‐D‐ribopyranosyl (4′→2′)‐oligonucleotides derived from adenine and uracil confirm this prognosis:adenine‐uracil pairing in p‐RNA duplexes is stronger than in the corresponding RNA duplexes. Importantly, adenine in p‐Ribo(A8) does not show (reverse‐Hoogsteen) self‐pairing, in sharp contrast to its behavior in the homo‐DNA series. The sheer existence of strong and selective pairing in a system that isconstitutionally isomericto RNA and can be predicted to have a linear structure has implications for the problem of RNA's origin. In this context, a comprehensive experimental study of the pairing properties of p‐RNA, of its potential for constitutional assembly, self‐replication, and

ISSN:0018-019X    

DOI:10.1002/hlca.19930760602

出版商:WILEY‐VCH Verlag GmbH    

年代:1993

数据来源: WILEY

摘要:

AbstractThe synthesis of the regioisomeric 8‐azaguanineN7‐,N8‐, andN9‐(β‐D‐2′,3′‐dideoxyribonucleosides) (1, 2, and3, respectively) and of the diamino derivative13is described. The anion of 5‐amino‐7‐methoxy‐3H‐1,2,3‐triazolo[4,5‐d]pyrimidine (5) was glycosylated with 5‐O‐[(tert‐butyl)dimethylsilyl]‐2,3‐dideoxy‐D‐glycero‐pentofuranosyl chloride (6; anomeric mixture), yielding the regioisomeric 2′,3′‐dideoxyribofuranosides as anomeric mixtures7a/10a, 8a/11a, and9a/12a. They were desilylated with Bu4NF in THF affording the 5‐amino‐7‐methoxy‐nucleosides7b–12b. Treatment with aqueous NaOH gave the 8‐azaguanine β‐D‐2′,3′‐dideoxynucleosides1–3and their α‐D‐anomers14–16. The reaction of7bwith NH3/MeOH yielded the diamino compound13. The N‐glycosylic bond of 8‐aza‐2′,3′‐dideoxyguanosine (1) is four‐times more stable against acid than that of 2′,3′‐dideoxyguanosine. Compounds1, 2, and13were converted to

ISSN:0018-019X    

DOI:10.1002/hlca.19930760603

出版商:WILEY‐VCH Verlag GmbH    

年代:1993

数据来源: WILEY

摘要:

AbstractThe bicyclic ketones1–6entered into diastereoselective (>95% d.e.) aldol reactions with a variety of aldehydes (Scheme 1andTable 1). A representative series of aldols was converted (E)‐selectively into α,β‐unsaturated ketones by (i) spontaneous base‐promoted dehydration (Scheme 1andTable 2) and also by (ii) conversion into brosylate and base‐mediated elimination with lithium diisopropylamide/N,N,N′,N′‐tetramethylethylenediamine (LDA/TMEDA;Scheme 2). The simple α‐methylidene ketones17aand18awere obtainedviaoxidation of the phenylselenides19and20, respectively (Scheme 4). The tertiary aldol27was synthesized best by treatment of 1,3‐diketone26with Me4Zr (Table 4). In this fashion, the facileretro‐aldol reaction of27wa

ISSN:0018-019X    

DOI:10.1002/hlca.19930760604

出版商:WILEY‐VCH Verlag GmbH    

年代:1993

数据来源: WILEY

摘要:

AbstractThe fluxionality of [Ir4(CO)8(μ2‐CO)3L] (L = Br−, I−, SCN−, NO2−, P(4‐ClC6H4)3, PPh3, P(4‐MeOC6H4)3, P(4‐Me2NC6H4)3), as studied by 2D‐13C‐NMR in solution, is due to two successive scrambling processes: the merry‐go‐round of six basal CO's and CO bridging to alternative faces of the Ir4tetrahedron. The basicity of the ligand L has no significant effect on the activation parameters. The scrambling process of lowest activation energy in [Ir4(CO)7(μ2‐CO)3(PMePh2)2] correspond to the two possible synchronous CO bridging about a unique face of the metal tetrahedron swapping the relative axial and radial positions of the ligands L. The disubstituted clusters [Ir4(CO)10(μ2‐LL)] with one edge‐bridging ligand have a ground‐state geometry with three edge‐bridging CO's (LL = bis(diphenylphosphino)methane, bis(diphenylarsino)methane, bis(diphenylphosphino)propane) or with all terminal CO's (LL = CH3SCH2SCH3). In all cases, the fluxional process of lowest activation energy in the merry‐go‐round of six CO's about a unique triangular face. For the P and As donor ligands, this process is followed by the rotation of terminal CO's bonded to two Ir‐atoms residing o

ISSN:0018-019X    

DOI:10.1002/hlca.19930760605

出版商:WILEY‐VCH Verlag GmbH    

年代:1993

数据来源: WILEY

摘要:

AbstractThe synthesis and X‐ray structure of the binuclear complex tetrakis[(4S)‐4‐phenyloxazolidin‐2‐one]‐dirhodium(II) ([Rh2{(4S)‐phox}4]) are reported. Structure‐selectivity comparisons are made for typical metal carbene transformations, such as inter‐ and intramolecular cyclopropane formation, intermolecular cyclopropene formation and intramolecular C–H insertions of diazoacetates and diazoacetamides. The enantioselectivity achieved in the [Rh2{(4S)‐phox}4]‐catalyzed reactions is intermediate between that of [Rh2{(5S)‐mepy}4] and [Rh2{(4R)‐bnox}4], which were described previously (mepy = methyl 5‐oxopyrrolidine‐2‐carboxylate; bnox = 4‐benzyloxazolidin‐2‐one). In contrast to other catalyzed intermolecular cyclopropane formations, those using [Rh2{(4S)‐phox}4] result preferent

ISSN:0018-019X    

DOI:10.1002/hlca.19930760606

出版商:WILEY‐VCH Verlag GmbH    

年代:1993

数据来源: WILEY

摘要:

AbstractThe molecular structure of a chlorophyllacatabolite isolated from the culture medium of bleachedChlorella protothecoidescells has been determined by X‐ray diffraction; the crystal structure shows a distorted helical arrangement of the pyrrole rings and consists of symmetrical dimers located at a crystallographic diad, which are stabilized by six intermolecular H‐bo

ISSN:0018-019X    

DOI:10.1002/hlca.19930760607

出版商:WILEY‐VCH Verlag GmbH    

年代:1993

数据来源: WILEY

摘要:

AbstractA series of dicationic PdII‐acetonitrile complexes containing bi‐ and tridentate nitrogen and bidentate phosphine ligands (some of which are chiral) has been prepared as their BF4salts. The molecular structures for two of these, [Pd(CH3CN)2(bipy)] (BF4)2(4) and [Pd(CH3CN)((pybox)(i‐Pr))] (BF4)2((S,S)‐pybox(i‐Pr) = 2,6‐bis[(S)‐4′‐isopropyloxazolin‐2′‐yl]pyridine,5) have been determined by X‐ray diffraction. All of these complexes are shown to be effective homogeneous catalysts for the aldol‐type condensation of the isonitrile, methyl isocyanoacetate, with benzaldehyde. Two isonitrile complexes, [Pd(2,2′‐bipyridyl)(CNCH2COOCH3)2] (BF4)2and [Pd((S,S)‐pybox(i‐Pr))(CNCH2COOCH

ISSN:0018-019X    

DOI:10.1002/hlca.19930760608

出版商:WILEY‐VCH Verlag GmbH    

年代:1993

数据来源: WILEY

摘要:

AbstractThe hetero‐Diels‐Alderaddition of SO2to (E)‐hexa‐1,3‐diene (4) gives first 6‐ethyl‐3,6‐dihydro‐1,2‐oxathiin 2‐oxide (= 6‐ethylsultine) with the Et group occupying a pseudoaxial position, and then the more stable stereoisomer6with the Et substituent in a pseudoequatorial position. The SO2additions to 1‐methoxybuta‐1,3‐diene (7) and to 1‐methoxy‐3‐[(trimethylsilyl)oxy]buta‐1,3‐diene (8) give the 6‐methoxysultines9and10, respectively, with the MeO groups in pseudoaxial positions and which do not equilibrate with sultines having pseudoequatorial MeO substituents (anomeric effect). A lower limit of ΔΔG= 3.9 kcal/mol was evaluated at −60° for the stabilizing effect arising from the geminal vicinity of a MeO and sulfinate moiety in 3,6‐dihydro‐6‐

ISSN:0018-019X    

DOI:10.1002/hlca.19930760609

出版商:WILEY‐VCH Verlag GmbH    

年代:1993

数据来源: WILEY

摘要:

AbstractUV Irradiation of (Z)‐ and (E)‐1(10)‐unsaturated 5,10‐secosteroids1–4in acetone solution effected, besides (Z/E)‐isomerization, (i) a stereospecific epoxidation (only in the presence of O2), which, depending on the configuration ((Z) or (E)) in the starting steroid, gavecis‐epoxides5and8(from the (Z)‐compounds1and3) ortrans‐epoxides6,9, and10(from the (E)‐compounds2and4), and (ii) oxidative acetone addition to the olefinic double bond producing 1‐acetonyl

ISSN:0018-019X    

DOI:10.1002/hlca.19930760610

出版商:WILEY‐VCH Verlag GmbH    

年代:1993

数据来源: WILEY

摘要:

AbstractThe three new optically active triamines derived from 2,6‐bis(aminomethyl)pyridine,i.e.2,6‐bis{[(2S)‐2‐(hydroxymethyl)pyrrolidin‐1‐yl]methyl}pyridine (3a), 2,6‐bis{[(2S)‐2‐(methoxymethyl)pyrrolidin‐1‐yl]methyl}‐pyridine (3b), and 2,6‐bis{{(2S)‐2‐[(benzyloxy)methyl]pyrrolidin‐1‐yl}methyl}pyridine (3c), were synthesized. The equilibrium behaviour in solution and CuIIcomplex formation were studied by acidimetric titration and by UV/VIS and CD measurements. It was found that in aqueous solution,3acan coordinate one of the CH2OH groups to CuIIupon deprotonation. X‐Ray crystal structures were determined for the complexes [Cu(3a)Cl]ClO4·H2O, [Cu(3b)Cl]ClO4, and [Cu(3c)Cl]ClO4. In the solid state, coordination of both the OH and the alkoxy groups was observed, and th

ISSN:0018-019X    

DOI:10.1002/hlca.19930760611

出版商:WILEY‐VCH Verlag GmbH    

年代:1993

数据来源: WILEY