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The structure of the quadruple hydrogen bond formed by ureido-pyrimidinone moieties is investigated in dimerised model compounds, as well as in a supramolecular polymer, by solid-state 1 H double-quantum (DQ) NMR spectroscopy under fast magic-angle spinning (MAS). This NMR method combines the sensitivity of 1 H NMR chemical shifts to the strengths of hydrogen bonds with quantitative information about dipole-dipole couplings between pairs of protons. Thus, two-dimensional 1 H DQ MAS spectra provide particularly detailed insight into the arrangement of hydrogen bonds and allow proton-proton distances to be measured. For the supramolecular polymer, a thermally induced irreversible tautomeric rearrangement of the hydrogenbonded moieties is elucidated in the bulk material. This process is associated with an Arrhenius activation energy of (145 AE 15) kJ mol À1 , which can be rationalised in terms of hydrogen-bond dissociation and the reorientation of the supramolecular polymer chain.

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Document VersionPublisher's PDF, also known as Version of Record (includes final page, issue and volume numbers) Please check the document version of this publication:• A submitted manuscript is the author's version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. People interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publisher's website.• The final author version and the galley proof are versions of the publication after peer review.• The final published version features the final layout of the paper including the volume, issue and page numbers. Kuijpers, W. H. A., Genderen, van, M. H. P., ... Buck, H. M. (1989). Synthesis of phosphate-methylated DNA fragments using 9-fluorenylmethoxycarbonyl as transient base protecting group. Journal of Organic Chemistry, 54(7), 1657-1664. DOI: 10.1021/jo00268a030 Link to publication Citation for published version (APA):General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.• Users may download and print one copy of any publication from the public portal for the purpose of private study or research.• You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal ? Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.Download date: 11. May. 2018 J. Org. Chem. 1989Chem. ,54, 1657Chem. -1664 1657Merck silica gel, 230-400 mesh, was used for flash chromatography, and Whatman Magnum-9 Partisil-10 and ODs-3 columns were used for preparative HPLC.Isolation and Extraction. Fresh leaves of C. dichogamus were collected in Kenya in mid-1987 (a voucher sample has been deposited a t the herbarium in the Botany Dept., UBC). The airdried leaves (225 g) were soaked in methanol (500 mL) overnight. The methanol extract was concentrated in vacuo to a gum, water was added, and the resulting suspension was exhaustively extracted with CH2C12. Evaporation of the combined organic extracts in vacuo gave a residue, which was fractionated via silica gel flash chromatography (step gradient: CH2C12 to EtOAc) to give crude crotoxides A (1) and B (2).Crotoxide A (1). The flash fractions containing crude crotoxide A (1) were combined and further purified by silica gel preparative TLC (2:l CH2C12/EtOAc). The preparative TLC fraction containing crotoxide A was dissolved in methanol (5 mL), palladium on charcoal was added (20 mg), and the suspension was stirred at room temperature under 1 atm of hydrogen for 2 days. Filtration of t...

DOI to the publisher's website. • The final author version and the galley proof are versions of the publication after peer review. • The final published version features the final layout of the paper including the volume, issue and page numbers. Link to publication General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal. If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the "Taverne" license above, please follow below link for the End User Agreement: