Synthesis of the first 3,6,9,15,18,18-hexa-substituted-1,2,4,5,7,8,10,11,13,14,16,17-dodecadehydro[18]annulenes with D6h-symmetry
作者:Yoshiyuki Kuwatani、Naoto Watanabe、Ikuo Ueda
DOI:10.1016/0040-4039(94)02181-a
日期:1995.1
3,6,9,12,15,18-Hexaphenyl-1,2,4,5,7,8,10,11,13,14,16,17-dodecadehydro[18]annulene and related compounds have been synthesized. These compounds were found to be stable both in crystalline forms and chloroform solution at room temperature, to have aromatic character by the existance of a diamagnetic ring current on the basis of the nmr spectral data, and to conform to the D6h-symmetry group.
used [11+7] strategy, the [14+4] and [8+10] strategies were thus explored. They proved to be efficient, affording seven novel hexaoxy[6]pericyclynes corresponding to six different substitution patterns. These compounds were obtained in 7-15 steps as mixtures of stereoisomers. Thus, by using dibenzoylacetylene as the C(4) electrophile, a [14+4] strategy allowed the synthesis of two hexaphenyl representatives
Since the concept of aromaticity has been proposed to be generalizable to acetylenic rods (‘linear ring’ of [2]annulene), p-diisopropyl-tetraphenyl-carbo-benzene (C48H34) and an indenone-fused isopropyl-triphenyloctadehydro[14]annulene (C42H26O) can be regarded as based on heptacyclic aromatic cores. The formation and X-ray crystal structures of both products are described. The latter has been obtained
Three different kinds of ring carbo-mers of [5]cyclitol ethers were targeted as challenging examples of functional [5]pericyclynes. Three tertiary pentaaryl-carbo-[5]cyclitol methyl ethers were synthesized through a [11 + 4] ring-closing double addition of triphenyl- and tri-p-anisyl-undecatetrayn-diides to dibenzoyl acetylene. These compounds, obtained as oily mixtures of stereoisomers, are stable and can behave as acetylenic ligands of one or two Co-2(CO)(6) units. NMR analysis reveals that the broad diasteroisomeric dispersity of a triether, is consistently reduced in the symmetrized pentaether. Three bis-secondary triaryl-carbo-[5]cyclitol methyl ethers with adjacent CH(OR) vertices were synthesized through a similar [11 + 4] ring-closing process, where the same tetrayn-diides add to both the carbaldehyde ends of the (eta(2)-OCH-C=C-CHO)Co-2(CO)(6) complex. Despite the possibility of tautomeric isomerization, the occurrence of two adjacent bis-propargylic carbinol vertices does not diminish the stability of the [5]pericyclyne framework. Finally, two bis-secondary carbo-[5]cyclitol methyl ethers with non-adjacent CH(OH) vertices were synthesized through an alternative [10 + 5] ring-closing process. The bis-secondary carbo-[5]cyclitols are regarded as isohypsic equivalents of the challenging [C,C](5)carbo-cyclopentadienyl cation. A diphenyl-hexaoxy-[5]pericyclyne with two non-adjacent secondary carbinol vertices was also prepared through a [10 + 5] ring-closing strategy: this molecule is an isohypsic equivalent of the previously calculated zwitterionic carbo-cyclopentadien one, which could be observed as a DCI/NH3-MS fragment after treatment with SnCl2/HCl. Analytical HPLC showed that the C-11 triphenyl-undecatetrayne precursor of the [11 + 4] strategy was obtained as a statistical 1:2:1 mixture of the three possible diastereoisomers. Semi-preparative HPLC allowed for the resolution of this mixture. The pure major diastereoisomer was employed to prepare a partly resolved sample of pentamethoxy-pentaphenyl-[5]pericyclyne. Analytical HPLC showed that the latter corresponds to the statistical distribution of the expected three residual diastereoisomers. Semi-preparative HPLC finally afforded samples of diastereoisomerically pure pentamethoxy[5]pericyclyne as crystalline solids. (C) 2004 Elsevier Ltd. All rights reserved.
Solid-Phase Synthesis of Homogeneous Ruthenium Catalysts on Silica for the Continuous Asymmetric Transfer Hydrogenation Reaction
作者:Albertus J. Sandee、Daniëlle G. I. Petra、Joost N. H. Reek、Paul C. J. Kamer、Piet W. N. M. van Leeuwen
The solid-phase synthesis of new asymmetric transfer hydrogenation catalysts as well as the use of these silica supported systems in batch and flow reactors is reported. The ruthenium complex of NH-benzyl-(1R,2S)-(-)-norephedrine covalently tethered to silica showed a high activity and enantioselectivity in the reduction of acetophenone. In three consecutive batchwise catalytic runs, we obtained ee values 88%. In a continuous flow reactor, a very constant catalytic activity was observed; no catalyst deactivation occurred over a period of one week, This has been ascribed to successful site isolation. Using optimized conditions in this flow reactor, the ee was as high as 90% at 95% conversion, The supported catalysts generally show the same trend in catalyst performance as in solution. The viability of our approach was further shown in one example, the ruthenium(II) complex of (1S,2R)-(+)-2-amino-1,2-diphenylethanol, for which an enantiomeric excess of 58% was observed, which is nearly three times higher than its homogeneous analogue.