摘要:

Direct photolysis of alkyl-substituted cyclobutene derivatives in solution with monochromatic far-UV light sources results in competing ring opening to the corresponding substituted 1,3-butadiene derivatives and fragmentation to the corresponding alkene and alkyne via formal [2 + 2] cycloreversion. The excited states leading to the two types of products have been identified. Cycloreversion occurs stereospecifically and arises as a result of excitation of the π,R(3s) Rydberg state, which is of comparable energy to the π,π* state in alkylcyclobutenes. In spite of its stereospecificity, the reaction is nonconcerted; evidence is presented to suggest that it occurs by a mechanism involving [1,2] migration (ring contraction) to yield a cyclopropylcarbene intermediate, which then fragments to yield the alkyne and alkene. The stereochemistry at C3 and C4 in the starting cyclobutene is retained in the alkene produced in the reaction. Ring opening proceeds nonstereoselectively (from the π,π* singlet state) in well over a dozen systems that have been investigated, and possible mechanisms to account for this are discussed. It is proposed that orbital symmetry plays a role in the reaction, but only in the early stages of ring opening. Bicyclic cyclobutene derivatives in which the cyclobutene double bond is located across the [0] bridge undergo ring opening with a high degree of disrotatory stereoselectivity. Possible reasons for this unusual behavior are discussed in light of recentabinitiotheoretical results.

ISSN:0008-4042    

DOI:10.1139/v93-021

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

Brassinolide (1) and castasterone (2) were prepared from aldehyde7, in turn available from stigmasterol (3). The addition of (E)-1-propenyllithium to7, followed by diastereoselective Sharpless epoxidation of allylic alcohol8using (L)-(+)-diethyl tartrate, and regioselective epoxide-opening of (threo) epoxy alcohol15with isopropylmagnesium chloride in the presence of a catalytic amount of cuprous cyanide afforded diol17. The epoxidation and cuprate addition steps were investigated in greater detail with the simpler model aldehyde4. Hydrolysis of17provided2, which was converted preferentially into1or its regioisomer22by Baeyer–Villiger oxidation with trifluoroperoxyacetic acid or peroxyseleninic acids, respectively.

ISSN:0008-4042    

DOI:10.1139/v93-022

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

A series of 2-N-quinolyl- and 2-N-isoquinolyl-substituted 1,1,3,3-tetramethylguanidines has been synthesized and the pKaof each has been measured in acetonitrile and water solvents, together with the pKaof the corresponding aminoquinoline or aminoisoquinoline. The pKavalues in acetonitrile and water fall on the same correlation line as previously determined for 2-N-pyridylguanidines and aminopyridines. Replacement substituent constants (σa) are reported for several quinolyl and isoquinolyl groups in the two solvents.

ISSN:0008-4042    

DOI:10.1139/v93-023

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

A kinetic study is reported on the oxidation of diphenylamine (DPA) by peroxodisulfate (PDS) anions in H2SO4and buffered acetic acid – acetate 20% v/v methanol–water media. The primary detectable product of the reaction isN-phenyl-p-benzoquinoneimine (PBQ), which undergoes acid-catalyzed hydrolysis giving different decomposition products. At pH > 4 (acetic acid – acetate buffers) the hydrolytic decomposition is very slow and it does not interfere with PBQ formation, but in H2SO4media of pH < 2.5, PBQ decomposes as it is formed. In these media, the reaction obeys the kinetic rate law:a, the kinetic parameter, being acid dependent. Neither radical promoters (Ag+) nor radical traps (allyl alcohol) influence the reaction rate. In acetic acid – acetate buffers of pH > 4 the kinetic rate law is simple: first order in each one of the reactants, and acid independent. These results have been interpreted by assuming the formation, in a preequilibrium step, of anN-diphenylhydroxylamine-O-sulfonate intermediate, which further rearranges to yield PBQ. The rate-determining step of the reaction changes with the protonation state of the intermediate, whose protonation pKahas been kinetically estimated as 2.4.

ISSN:0008-4042    

DOI:10.1139/v93-024

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

A quality test is proposed for SCF atomic orbitals,approximated as finite linear combinations of suitable basis functionsThe key is in a function, readily derived from the Hartree–Fock equationwhich is identically zero for true Hartree–Fock spin orbitals and not so for approximate orbitals. In this way, our test measures how closely approximate orbital descriptions approach the true Hartree–Fock limit and thus provides a quality ordering of orbital bases with respect to one another and with respect to that limit, in a scale uniquely defined by the latter. Moreover, this analysis also holds for atomic subspaces of our choice, e.g., the valence region. Examples are offered for representative collections of Slater- and Gaussian-type orbital expansions.

ISSN:0008-4042    

DOI:10.1139/v93-025

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

The preparation and crystal and molecular structure of the monofunctional 1,2,3,5-diselenadiazolyl radical [RCN2Se2] (R = 3,5-dicyanophenyl) are described. Crystals of [(CN)2C6H3CN2Se2] are triclinic, space groupa = 7.197(2),b = 10.753(3),c = 13.345(5) Å, α = 88.00(3)°, β = 82.05(2)°, γ = 81.00(2)°, MW = 311.1,Z = 4. The structure consists of stacked arrays of radical dimers laced together by networks of short CN---Se contacts, but the steric bulk of the organic ligand leads to a pairwise aggregation of [CN2Se2] columns. These structural features are compared with those observed in related dimers, and the results interpreted in the light of extended Hückel band calculations on a series of model structures. The results provide a qualitative correlation between the size and dimensionality of the intermolecular Se---Se network and the degree of crystal orbital dispersion.

ISSN:0008-4042    

DOI:10.1139/v93-026

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

Treatment of 5′-O-(tert-butyldiphenylsilyl)-2′(and 3′)-deoxy-2′(and 3′)-methyleneuridine (and adenosine) derivatives with phenyl chlorothionocarbonate gave the 3′(and 2′)-O-phenoxythiocarbonyl intermediates, which underwent spontaneous [3,3]-sigmatropic rearrangement to give the 2′,3′-didehydro-2′,3′-dideoxy-2′(and 3′)-(phenoxycarbonylthio)methyl analogues. These allylic thioesters were subjected to tributylstannane-mediated hydrodesulfurization and deprotection to give 2′,3′-didehydro-2′,3′-dideoxy-2′(and 3′)-methyluridine (and adenosine). Tributylstannane-mediated hydrodehalogenation of a 2′-chloro-2′,3′-dideoxy-3′-methyleneuridine derivative afforded the 2′,3′-didehydro-2′,3′-dideoxy-3′-methyl product of allylic transposition exclusively.

ISSN:0008-4042    

DOI:10.1139/v93-027

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

Treatment of the 6′(E)-tosylvinyl homonucleoside1a with Bu3SnH/AIBN/toluene/Δ gave separable mixtures of 6′-vinylstannanes2a(E/Z) in high yields. Stereospecific halodestannylations withN-iodosuccinimide, bromine, andN-bromosuccinimide proceeded smoothly to give the 6′(EorZ)-iodo(and bromo) vinyl compounds with retention of configuration. Chlorine or iodobenzene dichloride effected moderately stereoselective chlorodestannylation. Treatment of2awith NH4F/MeOH/Δ resulted in carbon–tin bond cleavage to give the free vinyl derivative4a. Aqueous trifluoroacetic acid effected concomitant protiodestannylation and deprotection of2ato give4b. Treatment of2a(E) with lead tetraacetate/acetonitrile and deprotection afforded acetylenic derivative3b.

ISSN:0008-4042    

DOI:10.1139/v93-028

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

trans-Dichlorobis(2,3-dihydro-4- and -7-methylbenzo[b]thiophene)palladium(II),1band1c, have been prepared by reaction of 2,3-dihydro-4- and -7-methylbenzo[b]thiophenes with palladium(II) chloride. Both1band1cexhibit complex1H and13C NMR spectra as a result of inversion of the sulfur-containing ligands. Their structures were confirmed by single crystal X-ray studies.1bcrystallized in the triclinic space group(a = 7.219(2),b = 8.441(5),c = 8.494(7) Å, α = 61.47(5)°, β = 88.85(4)°, γ = 84.40(3)°, andR = 0.067 for 1372 observed of 1760 unique reflections).1calso crystallized in the triclinic space group(a = 7.730(3),b = 8.056(3),c = 8.516(4) Å, α = 62.87(3)°, β = 82.31(3)°, γ = 88.82(3)°, andR = 0,040 for 1425 observed of 1825 unique reflections),1band1cand the complex1a(prepared from 2,3-dihydrobenzo[b]thiophene) were unstable in the presence of molecular hydrogen at ambient conditions, liberating the parent dihydrobenzothiophenes and forming elemental palladium.1awas also unstable to the organic hydrogen donor tetrahydronaphthalene, again forming Pd and 2,3-dihydrobenzothiophene. Complexes1a–cwere shown to promote the shift of CO/H2O to hydrogen and CO2although most of the hydrogen was consumed by the reduction of Pd(II) to Pd0.

ISSN:0008-4042    

DOI:10.1139/v93-029

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

A phosphonic acid analogue of platelet activating factor (PAF) bearing a backbone that is isosteric with and has stereochemistry corresponding to that of natural PAF has been synthesized. The synthesis uses a route maintaining stereochemical integrity.

ISSN:0008-4042    

DOI:10.1139/v93-030

出版商:NRC Research Press    

年代:1993

数据来源: NRC