FULL PAPER DOI: 10.1002/ejoc.200700373 1,3-Diethynylallenes: Stable Monomers, Length-Defined Oligomers, Asymmetric Synthesis, and Optical Resolution Matthijs K. J. ter Wiel, [a][‡] Severin Odermatt, [a][‡] Patrick Schanen, [a][‡] Paul Seiler, [a] and François Diederich* [a] Keywords: Allenes / Carbon-rich scaffolds / Enynes / Oligomerization / Stereoselective synthesis A series of differently substituted 1,3-diethynylallenes (DEAs) have been synthesized, confirming that the pre- viously introduced construction protocols tolerate a variety of functional groups. The new DEAs bear at least one polar group to facilitate enantiomer separations on chiral station- ary phases and to allow further functionalization. They are thermally and environmentally stable compounds since bulky substituents next to the cumulene moiety suppress the tendency to undergo [2+2] cyclodimerization. A series of length-defined oligomers were obtained as mixtures of ste- reoisomers by oxidative coupling of a monomeric DEA under Glaser–Hay conditions. The electronic absorption data indi- cate a lack of extended π-electron conjugation across the Introduction For a long time, since the first discussion by Van’t Hoff, [1] allenes have been regarded as a curiosity and considered as unstable. However, the discovery of cumulenic bonds in natural products has risen the attention given to these struc- tures. [2] Moreover, allenes are very interesting reactive inter- mediates. [3] It is therefore not surprising that much progress has been achieved in the synthesis and the understanding of the properties of this important class of compounds. [4] Thus, allenes bearing two different substituents at each of the two termini (RR'C=C=CRR') are chiral and the op- tical stability of enantiomers is high. For 1,3-dimethylallene, the activation free enthalpy for rotational isomerism leading to racemization was determined by Roth and co-workers as ΔG ≠ = 44.2 kcal mol –1 . [5] Our group has been involved in the exploration of ethyn- ylated allenes as building blocks for the construction of lin- ear and macrocyclic carbon-rich scaffolds through oxidative acetylenic coupling. [6] Particularly interesting in this regard are 1,3-diethynylallenes (DEAs). The synthesis of DEAs [a] Laboratorium für Organische Chemie, ETH-Zürich, Hönggerberg, HCI, 8093 Zürich, Switzerland Fax: +41-44-632-1109 E-mail: diederich@org.chem.ethz.ch [‡] M. K. J. t. W., P. S., and S. O. made equal contributions to this paper. Supporting information for this article is available on the WWW under http://www.eurjoc.org or from the author. Eur. J. Org. Chem. 2007, 3449–3462 © 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 3449 oligomeric backbone due to the orthogonality of the allenic π-systems. Remarkably, even complex mixtures of stereoiso- mers only yield one single set of NMR signals, which under- lines the low stereodifferentiation in acyclic allenoacetylenic structures. Optical resolution of DEAs represents an amazing challenge, and preliminary results on the analytical level are reported. Asymmetric synthesis by Pd-mediated S N 2'-type cross-coupling of an alkyne to an optically pure bispropar- gylic precursor opens another promising route to optically active allenes with stereoselectivities currently reaching up to 78% ee. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) proved however challenging as they tend to quickly dimer- ize by [2+2] cycloaddition; furthermore, facile isomeriza- tions have been observed. [6d] We were able to circumvent these problems by introducing sterically bulky substituents close to the cumulene moiety. [6b] Following the synthesis of a first series of stable DEAs, we proceeded with the prepa- ration of shape-persistent allenoacetylenic macrocycles with unusual shapes and symmetries and potential host-guesting properties. [6a,7] Although 1,3-diethynylallenes are axially chiral, we used in all previous work racemic mixtures for acetylenic scaf- folding. [6] The reason for this is the still valid finding that the optical resolution of the dialkynylated allenes represents a formidable challenge, even for a research group with large experience in diverse optical resolutions. As a result, acety- lenic scaffolding led to complex mixtures of stereoisomers, with diastereoisomer (but not enantiomer) separation suc- ceeding only at the stage of the rigid macrocyclic allenoace- tylenes. [6a] Clearly, a versatile access to enantiomerically pure DEAs would be highly desirable, in particular since semi-empirical calculations predict the formation of helical foldamers with distinct conformational preferences upon oxidative acetylenic coupling of enantiomerically pure DEAs. While the synthesis of optically pure allenes has been well studied, [4f,8] most of the reported routes seem difficult to apply to the preparation of enantiomerically pure DEAs, due to the fact that the two acetylenic substituents provide