摘要:

AbstractThe terms “chiral” and “achiral” are sharply defined when applied to geometric figures or models. The same terms are also commonly used to refer to the real systems to which these models have been adjoined, e.g., molecules, solvents, or reagents. Here, the terms are not sharply defined but depend upon conditions of measurement. The contrast between the geometric and operational usages is discussed in

ISSN:0021-2148    

DOI:10.1002/ijch.197600002

出版商:WILEY‐VCH Verlag    

年代:1976

数据来源: WILEY

摘要:

AbstractThe question as to whether two species of the same molecular composition are identical or isomeric in the conventional sense cannot be answered in the absence of stated experimental criteria of conditions and methods of observation. A new definition is proposed which minimizes such dependence on experimental criteria. “Residual isomerism” is defined as an auxiliary term to make the new definition practically use

ISSN:0021-2148    

DOI:10.1002/ijch.197600003

出版商:WILEY‐VCH Verlag    

年代:1976

数据来源: WILEY

摘要:

AbstractThe reaction of borane with α‐pinene in tetrahydrofuran attains equilibrium in 3 days at 0°. With 15% excess (+)‐α‐pinene ([α]D26.5+ 47.99°), the formation of diisopinocampheylborane is essentially quantitative (‐99.5%). The longer reaction time is accompanied by a selective incorporation of the major isomer into the reagent. Thus, (+)‐α‐pinene, dehydroborated from diisopinocampheylborane with triethylamine, exhibits an optical rotation of [α]D23.5+ 51.0°, indicating an optical purity of 99.8%, and the isopinocampheol, obtained following the oxidation of the reagent, showed [α]D23−35.1° (c10, benzene), a higher rotation than any realized previously. The hydroboration ofcis‐2‐butene with this diisopinocampheylborane in tetrahydrofuran at −25° provided 2‐butanol of high optical purity, [α]D22.5−13.29°, an optical purity of 98.4%. Thus this reaction proceeds with

ISSN:0021-2148    

DOI:10.1002/ijch.197600004

出版商:WILEY‐VCH Verlag    

年代:1976

数据来源: WILEY

摘要:

AbstractZ‐(−)‐(1R, 3R, 4S)‐menthyl‐α‐acetamidocinnamate was hydrogenated with neutral diphosphine‐rhodium(I) complexes containing (+)‐(2S, 3S) or (−)‐(2R, 3R)‐2,3‐isopropylidene‐2,3‐dihydroxy‐1,4‐bis(diphenylphosphino)butane(DIOP) to give a diastereomeric mixture of N‐acetylphenylalanine (1R, 3R, 4S)‐menthyl ester (76.8% diastereomeric excess [d.e.] (S, 3R) and 52.5% d.e. (R, 3R), respectively). The behavior of the unsaturated (−)‐menthyl ester substrate was also investigated with neutral diphosphine‐rhodium(I) complexes containing symmetrical α, ω‐bis(diphenylphosphino)alkanes {Ph2P(CH2)nPPh2} 6.6% d.e. (S, 3R) [n = 3]; 12.0% d.e. (R, 3R) [n = 4]; 7.2% d.e. (R, 3R) [n = 5]; and 7.4% d.e. (R, 3R) [n = 6]. Z‐(−)‐(1S, 2R, 4S)‐bornyl‐α‐acetamidocinnamate was similarly studied: 49.8% (S, 2R)[(+)‐(2S, 3S)‐DIOP]; 64.1% d.e. (R, 2R) [(−)‐(2R, 3R)‐DIOP]; 4.9% d.e. (S, 2R) [n = 3]; 9.1% d.e. (R, 2R

ISSN:0021-2148    

DOI:10.1002/ijch.197600005

出版商:WILEY‐VCH Verlag    

年代:1976

数据来源: WILEY

摘要:

AbstractSome N‐acylated α, β‐unsaturated peptides (dehydropeptides) containing one, two or three dehydrophenylalanine (dehydro‐Phe) residues and a C‐terminal L‐amino acidic residue have been prepared and characterized by NMR, IR and UV absorption. For N‐actyl‐(dehydro‐Phe)2‐L‐Ala the crystal and molecular structure have been determined by X‐ray diffraction analysis. CD spectra of N‐acetyl‐(dehydro‐Phe)‐ϵ‐Cbzo‐L‐Lys and N‐acetyl‐(dehydro‐Phe)2‐ϵ‐Cbzo‐L‐Lys have been measured in different solvents and, of N‐acetyl‐(dehydro‐Phe)2‐L‐Val, in the presence of different amounts of urea. The results have been compared with those for N‐acetyl‐(dehydro‐Phe)‐(dehydro‐Tyr)‐L‐Glu, for the tetrapeptide N‐acetyl‐(dehydro‐Phe)3‐L‐Val, and for analogous compounds already reported [J. Am. Chem. Soc.,97, 5820 (1975)]. Crystalline molecular structure and chiroptical properties are consistent with the hypothesis that the dehydropeptides assume in solution a chiral conformation based on a 4 → 1 hydrogen‐bonded ten‐membered ring, also referred to as β‐turn. The importance of these systems for the unde

ISSN:0021-2148    

DOI:10.1002/ijch.197600006

出版商:WILEY‐VCH Verlag    

年代:1976

数据来源: WILEY

摘要:

AbstractA systematic investigation of chemically induced asymmetric photosyntheses has been carried ou in the helicene series. The following points have been examined: the influence of the nature and of the position of the chiral inducing group in the precursor, the influence of the structure of the precursor and the effect of a double induction. The best asymmetric stereoselectivities were obtained: (a) when the photocyclisation gave rise to a helicene skeleton substituted at position 1‐; (b) at position 2‐, when an ester of optically pure 2‐hydroxymethyl[6]helicene was used as the chiral inducing

ISSN:0021-2148    

DOI:10.1002/ijch.197600007

出版商:WILEY‐VCH Verlag    

年代:1976

数据来源: WILEY

摘要:

AbstractStereoelective polymerizations performed on substituted oxiranes and thiiranes can yield recovered monomers of high optical purity. The efficiency of the process depends on the chemical nature, the geometry, and the configuration of the chiral ligand of the initiator. It also depends on the nature of the monomer and its enantiomeric composition. When enantiomerically enriched thiiranes were used the stereoelectivity was strongly increased and monomers which were almost optically pure could be isolated using a stepwise procedure. The resolution could be also improved by modifying the initiator with external chiral additives. This stereoelective process seems to be of special interest for the preparation of optically active monomers which are not easily obtained by conventional synthetic methods.

ISSN:0021-2148    

DOI:10.1002/ijch.197600008

出版商:WILEY‐VCH Verlag    

年代:1976

数据来源: WILEY

摘要:

AbstractUV, ORD and CD spectra of (−)poly[N‐(sec‐butyl)‐N‐methyl acrylamide] (II) in methanol, dioxane and hexafluoroiso‐propanol were investigated. CD spectra were resolved into gaussian Cotton bands. Three Cotton effects were found in the 185–250 nm spectral range, a small one at about 230 nm which was assigned to a n → π* transition and two larger ones, of opposite signs, respectively located at ≃ 205 and ≃ 195 nm. For such a non‐stereoregular and non‐rigid polymer, exciton splitting cannot be taken into consideration to explain the presence of two opposite sign Cotton effects in the usual π → π* transition spectral range. Chiroptical properties of polymer (II) were compared to those of (+)N‐(sec‐butyl)‐N‐methyliso‐butyramide taken as a suitable model for the monomeric unit of the polymer. The corresponding secondary amide, (+)N‐(sec‐butyl)iso‐butyramide, was also considered in order to show the effect of the nitrogen atom substitution on the UV and CD spectra and thus on the electronic transitions of the amide chromophore. According to the chiroptical properties of the model compounds and to a discussion based on the literature, the negative Cotton effect found at about 205 nm in the three solvents was assigned to the π1→ π* transition. The positive one found at about 195 nm was considered as resulting from either the π2→ π* or a n → σ* transition usually located at shorter wavelengths in the far UV but red‐shifted in the present case by the disubstitution of the nitrogen atom of the ami

ISSN:0021-2148    

DOI:10.1002/ijch.197600009

出版商:WILEY‐VCH Verlag    

年代:1976

数据来源: WILEY

摘要:

AbstractThe CD spectra of phorbol and of some of its derivatives, such as its 12, 13, 20‐triacetate 1, show at least four bands. Three of them (called I, III, and IV) correspond to the R‐, K‐, and (perhaps) second n → π*‐transition, respectively, of the enone system; the fourth band around 260 nm (band II) is of π → π*‐origin and could be ascribed to an interaction of the lone pair of electrons on the oxygen atom at C(4) with the C=C‐C=O system. The signs of the R‐band as well as the K‐band Cotton effects are very sensitive to chemical modifications around the chromophoric system in the cyclopentenones investigated. The absolute configuration of phorbol may be derived from the CD spectra of some of its derivatives, and, in particular, from the application of the “dibenzoate rule” to 12, 13‐di‐O

ISSN:0021-2148    

DOI:10.1002/ijch.197600010

出版商:WILEY‐VCH Verlag    

年代:1976

数据来源: WILEY

摘要:

AbstractChirality functions for cyclopentane‐derivatives with idealized D5hsymmetry are developed and tested experimentally. The actual conformations of disubstituted cyclopentane‐derivatives are discussed, a series of compounds synthesized and their optical activity measured. A good correlation is obtained between experiment and theory for the 1, 2‐derivatives but not for the 1, 3‐disubstituted cyclopentane compounds. It could be shown that this method cannot be applied to higher substituted deri

ISSN:0021-2148    

DOI:10.1002/ijch.197600011

出版商:WILEY‐VCH Verlag    

年代:1976

数据来源: WILEY