In this Letter, it is demonstrated that the unusual reactivity of cyclopropenes can increase the scope and utility of intermolecular Pauson-Khand reactions. The well-defined chiral environment of cyclopropenes has a powerful influence on the diastereoselectivity of the reactions and leads to the production of a single cyclopentenone in each of the described cases. The cyclopropane ring strongly influences the stereochemistry of reactions at the enone, and the three-membered ring can subsequently be cleaved under mild conditions. [reaction: see text]
A short, enantioselective synthesis of (-)-pentalenene is described. Catalytic enantioselective cyclopropenation with (R,R)-Rh2(OAc)(DPTI)3 was used to set the absolute stereochemistry, and an intramolecular Pauson-Khand reaction of the resulting cyclopropenyne was used to establish the quaternary center.
Large scale syntheses of 3-(cycloprop-2-en-1-oyl)-oxazolidinones from acetylene and ethyl diazoacetate are described. Unlike other cyclopropenes that bear a single substitutent at C-3, these compounds are stable to longterm storage. Although the cyclopropene derivatives are unusually stable, they are reactive toward cyclic and acyclic dienes in stereoselective Diels-Alder reactions.The Rh-catalyzed reaction of α-diazoesters with alkynes (Scheme 1) is an exceptionally useful and operationally simple method for the preparation of cycloprop-2-ene carboxylates 1 -substances that are useful building blocks for a number of strain-assisted transformations. 2 In theory, the Rh-catalyzed reaction of acetylene with ethyl diazoacetate should provide a particularly inexpensive route to ethyl cycloprop-2-ene carboxylate (1a). 3 Compound 1a and derivatives might serve as useful tools for diastereoselective synthesis. However, the usefulness of cyclopropenes with a single substituent at their sp 3 centers has been limited by their instability. Thus, 3-methylcyclopropene is extremely unstable, whereas 3,3-dimethylcyclopropene is quite stable at room temperature. 4 Although cycloprop-2-ene carboxylates (1) are more stable than 3-alkylcyclopropenes, their longterm storage is also not practical. 3a Herein, we describe scalable syntheses of 3-(cycloprop-2-en-1-oyl)-oxazolidinone derivatives 2-4: crystalline derivatives of cycloprop-2-ene carboxylic acid that are stable over long periods. Although derivatives 2-4 are unusually stable, they are reactive dienophiles that undergo Diels-Alder reactions with high stereoselectivity.Several preparations of cycloprop-2-ene carboxylates have been described in the literature. 3 In an important study, Baldwin and Villerica compared several routes to cycloprop-2-ene-1-carboxylates, and described a scalable, Rh-catalyzed procedure for the preparation of methyl cycloprop-2-ene carboxylate (1b). 3a Despite the considerable merits of the procedure, there were two limitations that detracted from the preparative utility: the need to prepare methyl
Described are the X-ray crystallographic and spectral properties of Co-complexes that were isolated from two Pauson-Khand reactions of chiral cyclopropenes. These are the first examples of isolated Co-complexes derived from the putative alkene-insertion intermediates of Pauson-Khand reactions. The binuclear Co-complexes are coordinated to μ-bonded, five-carbon "flyover" carbene ligands. It is proposed that the complexes result from cyclopropane fragmentation subsequent to alkene insertion. The observation of these metal complexes provides a rationale for the origin of regioselectivity in Pauson-Khand reactions of cyclopropenes.
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