ISSN:0018-019X    

DOI:10.1002/hlca.19960790131

出版商:WILEY‐VCH Verlag GmbH    

年代:1996

数据来源: WILEY

摘要:

AbstractOn the way to the fullerene‐acetylene hybrid carbon allotropes2and6, the oxidative homocoupling of the 2‐functionalized 1‐ethynylated C60derivatives11, 12, 14, and15was investigated. UnderGlaser‐Hayconditions, the two soluble dumbbell‐shaped bisfullerenes17and18, with two C60moieties linked by a buta‐1,3‐diynediyl bridge, were formed in 52 and 82% yield, respectively (Scheme 2). Cyclic‐voltammetric measurements revealed that there is no significant electronic communication between the two fullerene spheresviathe buta‐1,3‐diynediyl linker. Removal of the 3,4,5,6‐tetrahydro‐2H‐pyran‐2‐yl (Thp) protecting groups in18gave in 80% yield the highly insoluble dumbbell19with methanol groups in the 2,2′‐positions of the buta‐1,3‐diynediyl‐bridged carbon spheres. Attempted conversion of19to the all‐carbon dianion6(C 1242−)viabase‐induced elimination of formaldehyde was not successful presumably due toexo‐dig cyclization of the formed alkoxides. The occurrence of this cyclization under furan formation was proven for 2‐[4‐(trimethylsilyl)buta‐1,3‐diyn‐1‐yl][60]fullerene‐1‐methanol (21), a soluble model compound for19(Scheme 3). To compare the properties of ethynylated fullerene mono‐adducts to those of corresponding higher adducts, hexakis‐adducts26and28with an octahedral functionalization pattern resulting from all‐e (equatorial) additions were prepared by the reversible‐template method ofHirsch (Scheme 4). Reaction of the ethynylated mono‐adducts25or13with diethyl 2‐bromomalonate/1,8‐diazabicyclo[5.4.0]undec‐7‐ene (DBU) in the presence of 1,9‐dimethylanthracene (DMA) as reversible template led to26and28in 28 and 22% yield, respectively. Preliminary experiments indicated a significant change in reactivity and NMR spe

ISSN:0018-019X    

DOI:10.1002/hlca.19960790103

出版商:WILEY‐VCH Verlag GmbH    

年代:1996

数据来源: WILEY

ISSN:0018-019X    

DOI:10.1002/hlca.19960790132

出版商:WILEY‐VCH Verlag GmbH    

年代:1996

数据来源: WILEY

摘要:

AbstractReduction of a bis‐radical precursor, 6‐phenyl‐1,1‐bis[3‐(phenylselanyl)propyl]‐3a,4‐dihydro‐1H,3H‐cyclopenta[c]furan‐5‐one (6), with 3 equiv. of Ph3SnH provides mixtures ofcis,cis‐ orcis,trans‐angular triquinane products (3aα,5aβ,8β,8aR*)‐ and (3aα,5aα,8β,8aR*)‐hexahydro‐3a‐propyl‐8‐phenyl‐5H‐dicyclopenta[b,c]furan‐7(8H)‐one (cis,cis‐12/cis,trans‐12), in yields that vary from 50%/50% to 91%/6% depending on the reaction concentration. A mechanistic model for this process is proposed that involves a non‐selective phenylselenium‐group abstraction step followed by successive kinetic resolutions of diastereoisomeric radical intermediates. This reaction shows how yields in group‐selective reactions can be compounded to levels above that

ISSN:0018-019X    

DOI:10.1002/hlca.19960790104

出版商:WILEY‐VCH Verlag GmbH    

年代:1996

数据来源: WILEY

摘要:

Regioselective 1,3‐Dipolar Cycloadditions of a ‘Thiocarbonyl‐methanide’ ((Alkylidenesulfonio)methanide) with Aromatic SulfinesReaction of the spirocyclic 2,5‐dihydro‐1,3,4‐thiadiazole7and thiobenzophenoneS‐oxide (6a) in THF at 45° yielded the spirocyclic 1,3‐dithiolane 1‐oxide8, thiirane9, and the diazane derivative10in a ratio of 61:15:23 (Scheme 2). The formation of8is rationalized by a 1,3‐dipolar cycloaddition of ‘thiocarbonyl‐methanide’1, generated from7by thermal elimination of N2, and the CS bond of sulfine6a. Cyclization of intermediate1leads to thiirane9. Under the same conditions,7and adamantane‐2‐thioneS‐oxide (6b) or 2,2,4,4‐tetramethyl‐3‐thioxocyclobutanoneS‐oxide (4) reacted to give only9and10but no cycloadduct of type8(Scheme 4). With the aim to favor the formation of8, a mixture of6aand 1.1 equiv. of7was heated to 45° without any solvent in a sealed tube. The ratio of products was only slightly different from that of the thermolysis in THF. An analogous experiment with7and 9H‐fluorene‐9‐thioneS‐oxide (6c) yielded cycloadduct13and9(Scheme 5). It is most interesting that the 1,3‐dipolar cycloadditions of1and the sulfines6aand6cproceeded with different regioselectivity. A reaction mechanism for the unexpected formation of10is proposed inScheme 7. The key step is the base‐catalyzed ring opening of7and the nucleophilic addition

ISSN:0018-019X    

DOI:10.1002/hlca.19960790105

出版商:WILEY‐VCH Verlag GmbH    

年代:1996

数据来源: WILEY

摘要:

AbstractAcid‐catalyzed cyclic ketal formationvs.hydrolysis of a series of hydroxyalkyl cyclic enol ethers in the presence of 1 equiv. of H2O, and acid‐catalyzed cyclic‐ketal formation (same ketals as above)vs.methanolysis of a series of mixed pent‐4‐enyl hydroxyalkyl ketals withN‐bromosuccinimide in the presence of 4 equiv. of MeOH led to the same result: the intramolecular cyclization processes occur at similar rates as the intermolecular H2O or MeOH attacks independently of the size of the rings formed (five‐, six‐, or seven‐membered), by cyclizations. These results can be explained by the facts that, due to stereoelectronic effects which impose a torsional strain to the sp2hybridized O‐atom, the cyclization activation enthalpy decreases, as the length of the hydroxyalkyl chain increase (ease of cyclization: 7>6>5), whereas the entropy factor favors the cyclization in the reverse fashion (ease of cyclization: 5>6>7). The various reaction pathways have been examined using the semi‐empiricalHamiltonianAM1, and the results obtained confirm that large‐ring formation is enthalpically much favored over the cyclization processes leading to small rings (ease

ISSN:0018-019X    

DOI:10.1002/hlca.19960790106

出版商:WILEY‐VCH Verlag GmbH    

年代:1996

数据来源: WILEY

摘要:

AbstractLeucascandrolide A ((+)‐1), a doubly O‐bridged 18‐membered macrolide of a new type,i.e., showing little C1‐branchingvs.extensive 1,3‐dioxygenation and a peculiar side chain, was isolated from a calcareous sponge of a new genus,Leucascandra caveolataBOROJEVICand KLAUTAUfrom the Coral Sea. Transesterification of (+)‐1gave the methyl ester3, derived from the side chain, and the 5‐hydroxy derivative (+)‐2, derived from the macrolide portion and with the natural configuration at C(5) (axial).Mosher's MTPA esters4and5obtained from (+)‐2showed scattered Δδ = (δ(S) − δ(R)) data. However, inversion of the configuration at C(5) led,viaketone (+)‐6, to the less encumbered 5‐equatorial hydroxy derivative (+)‐7, whose MTPA esters8and9gave consistent Δδ data, allowing the assignment of the absolute configuration of (+)‐7, and hence of (+)‐1. The structural novelty of (+)‐1and its powerful antifungal and cytotoxic activities are likely to renew interest in calcareous sponges, previously limited to scarcely

ISSN:0018-019X    

DOI:10.1002/hlca.19960790107

出版商:WILEY‐VCH Verlag GmbH    

年代:1996

数据来源: WILEY

摘要:

AbstractStable carbenes derived from thiazole, 1H‐imidazole, and 4H‐1,2,4‐triazole are efficient catalysts for benzointype condensations of formaldehyde. Catalysts derived fromN‐substituted thiazolium salts trimerize formaldehyde to dihydroxyacetone (II). Catalysts based on 1,4‐disubstituted 4H‐1,2,4‐triazol‐1‐ium salts give glycolaldehyde (I) as the main product and noII, whereasN,N′‐disubstituted 1H‐imidazol‐3‐ium salts yield mixtures of both products. The isolation of several intermediates in the catalytic cycle provide a better insight

ISSN:0018-019X    

DOI:10.1002/hlca.19960790108

出版商:WILEY‐VCH Verlag GmbH    

年代:1996

数据来源: WILEY

摘要:

AbstractThe reduction of pyrene with sodium in aprotic diethyl ether allows to crystallize the extremely air‐sensitive radical ion pair pyrene‐sodium‐diethylether. The single‐crystal structure determination at 130 K shows that each sodium counter cation, solvated by one diethyl‐ether molecule, is η3‐ and η6‐coordinated to one of the short‐axis six‐membered rings of two pyrene radical anions. The resulting dibenzene‐sodium sandwiches form a string, in which the hydrocarbon planes are canted to each other by 62°. In the pyrene radical‐anion skeleton, no distortion due to its negative charge can be detected relative to that of the neutral molecule. From the temperature‐dependent signal multiplets of preceding ESR investigations, the solvent‐separated pyrene radical anion as well as two different contact radical‐ion pairs had been identified and their structures in solution approximated by potential‐energy estimates. Referring to the recently discovered long‐axis Na⊕ contact ion pair polymorph, crystallized at lower temperatures, the structure reported here represents the second and probably thermodynamically more stable one. Both the ESR and the structural results provide some insight into the multidimensional networks of equilibria in aprotic solution, which are activated by alkali‐metal redu

ISSN:0018-019X    

DOI:10.1002/hlca.19960790109

出版商:WILEY‐VCH Verlag GmbH    

年代:1996

数据来源: WILEY

摘要:

The Sodium Reduction of Azulene to the Contact‐Ion Triple [Disodium‐(1,1′,6,6′‐Tetrahydro‐6,6′‐bi(azulene)‐1,1′‐diide)–bis(diglyme)]∞Colorless air‐sensitive single crystals can be grown from a diglyme solution after the reduction of azulene by a Na‐metal mirror. Structure determination at 150 K reveals a dimer dianion, in which the seven‐membered rings are connected in 6,6′‐positions and doubly diglyme‐solvated Na+counter cations η5‐coordinate to the five‐membered rings. Based on preceding cyclovoltammetric measurements in aprotic azulene solutions as well as on extensive MNDO enthalpy of formation calculations, a proposal is forwarded how possibly the contact ion tr

ISSN:0018-019X    

DOI:10.1002/hlca.19960790110

出版商:WILEY‐VCH Verlag GmbH    

年代:1996

数据来源: WILEY