Angewandte
Chemie
Sonogashira coupling but could be combined with Suzuki
coupling or Heck coupling to generate most varieties of p-
conjugated oligomer. This methodology should be applicable
to the construction of diverse insulated molecular wires, with
precisely defined sequences, lengths, and end groups.
Received: May 29, 2007
Published online: August 2, 2007
Keywords: cyclodextrins · molecular wires · rotaxanes ·
.
solid-phase synthesis · supramolecular chemistry
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[14] The synthesis of 1ꢀa-CD and 2ꢀa-CD was also attempted on
hydrophilic swollen gels, but rotaxane formation on these
supports is less efficient than on controlled-pore glass. For
example, we synthesized 2ꢀa-CD using the route shown in
Scheme 3 with oligo(oxyethylene glycol) ether functionalized
polystyrene in place of CPG (phenol ether linked PEG400 on
polystyrene cross-linked with 2 mol% divinylbenzene; poly-
(oxyethylene glycol) polymer (POP) resin, as described in M. C.
McCairn, S. R. Tonge, A. J. Sutherland, J. Org. Chem. 2002, 67,
4847 – 4855). The product was a 50:1 mixture of 2 and 2ꢀa-CD
(HPLC analysis after cleavage of the ester link with LiOH).
Thus it appears that a-CD is excluded from the gel-phase
support; however, it is known that large molecules, such as
enzymes, enter some hydrophilic gels; see: A. Basso, B. A.
Maltman, S. L. Flitsch, G. Margetts, I. Brazendale, C. Ebert, P.
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[15] Abbreviations: DMAP: N,N-dimethyl-4-pyridinamine; HBTU:
O-(1H-benzotriazol-1-yl)-N,N,N’,N’-tetramethyluronium hexa-
fluorophospate; pyBOP: benzotriazol-1-yloxytris(pyrrolidino)-
phosphonium hexafluorophospate; TPPTS: tris(3-sulfonatophe-
nyl)phosphane trisodium salt; dba: trans,trans-dibenzylidene-
acetone; Ts: para-toluenesulfonyl.
Figure 1. a) Partial NOESY spectrum of 3ꢀ(a-CD)2 ([D6]DMSO, 298 K,
500 MHz). Cross-peaks to protons on the two cyclodextrin units (A
and B) are shown in red and blue, respectively; resonances a–i are
assigned to the aromatic protons as shown in Scheme 3. b) Calculated
structure of 3ꢀ(a-CD)2 showing the van der Waals surface of 3 and
the solvent-accessible surface of the a-CDs (1.4-ꢀ probe radius).[17]
bulky phenylene residue.[18] This is the first cyclodextrin
[3]rotaxane to be formed selectively as the head-to-tail
isomer;[19] previous examples have been synthesized as tail-
to-tail isomers (with both 6-rims at the center),[20,21] or as
mixtures of stereoisomers.[22,23] This work represents the first
solid-phase synthesis of a cyclodextrin rotaxane, the first
solid-phase synthesis of any [3]rotaxane, and the first
stereoselective synthesis of a cyclodextrin [3]rotaxane with a
head-to-tail stereochemistry (with the 2,3-rims of both CDs
pointing in the same direction). Use of the large tripodal
anchor 11 as a grafting point during solid-phase synthesis is
critical for avoiding crowding on the solid support, and will
enable the final insulated molecular wires to be anchored to
TiO2 semiconductor nanoparticles.[24] The general synthetic
strategy used to construct [3]rotaxane 3ꢀ(a-CD)2 by repeated
coupling with masked aryl iodide monomers is not limited to
[16] A similar rotaxane to 1ꢀa-CD was reported parallel to this
investigation: J. S. Park, J. N. Wilson, K. I. Hardcastle, U. H. F.
Bunz, M. Srinivasarao, J. Am. Chem. Soc. 2006, 128, 7714 – 7715.
Angew. Chem. Int. Ed. 2007, 46, 6845 –6848
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim