Synthesis Of Seven-Membered Rings Research Articles

Recent advances in transition-metal-catalyzed Büchner reaction of alkynes.

Medium-sized ring-containing organic molecules, especially seven-membered rings, are significant structural motifs. However, such frameworks are considered difficult structures to access owing to entropic effects and transannular interactions. Compared to the construction of five and six-membered rings, the synthesis of seven-membered rings can be more challenging through traditional cyclization pathways. Büchner reactions are particularly attractive and efficient synthetic strategies to construct functionalized seven-membered ring products from the benzenoid double bond with carbene. In recent years, the field of transition-metal-catalyzed Büchner ring expansion reactions of alkynes has experienced a speedy development and a diverse array of efficient synthetic procedures have been disclosed under mild experimental conditions, as the synthesis of synthetically challenging seven-membered rings is easily achieved. In this review, we will focus on the recent progress in the transition-metal-catalyzed Büchner reaction of alkynes and the mechanistic rationale is depicted where possible, with the reactions being sorted according to the type of catalyst.

Cobalt-mediated two-carbon ring expansion of five-membered rings. Electrophilic carbon-carbon bond activation in the synthesis of seven-membered rings.

Electrophilic cobalt(III) mediates an unprecedented two-carbon ring expansion of coordinated five-membered rings, leading to a remarkably general new strategy for the synthesis of seven-membered carbocycles from readily available five-membered ring substrates. The reaction, a metal-mediated [5 + 2] cyclopentenyl/alkyne cycloaddition, proceeds via initial protonation of a cobalt(I) cyclopentadiene complex, followed by rearrangement to an agostic eta3-cyclopentenyl intermediate. The cyclic eta3-allyl residue then undergoes migratory coupling with alkyne followed by carbon-carbon bond activation of the unstrained five-membered ring and recyclization to the ring expanded product, although the order of events and intimate mechanism has not been conclusively established. The reaction is highly selective with respect to which five-membered ring ligand undergoes activation, presumably a consequence of rapid cobalt-mediated interannular hydride transfer and kinetic preference for alkyne insertion into the less substituted cyclopentenyl ring. The alkyne insertion is itself highly regioselective, proceeding via migration to the sterically smaller end of the alkyne. The reaction is sensitive to both the cobalt counterion and the ancillary eta5-cyclopentadienyl substituent but proceeds for a considerable range of alkyl-, aryl-, and trialkylsilyl-substituted terminal and internal alkynes.

ChemInform Abstract: A New Concept for the Synthesis of Seven‐Membered Rings.

AbstractThe 4,4‐bis(trifluoromethyl) substituted hetero‐1,3‐dienes (I) react with epichlorohydrin (II) in the presence of 4‐dimethylaminopyridine to give the heterooxazepines (III).

Synthesis of Seven‐Membered Rings by [(σ2+π2)+π2] Cycloaddition to Homodienes

Synthesis of Seven‐Membered Rings by [(_σ2+_π2)+_π2] Cycloaddition to Homodienes

Lewis acid complexes of 1,2-diketones and their derivatives. The synthesis of seven-membered rings

The structures and charge distributions of Lewis and complexes of butane-2,3-dione, 1, cyclohexane-1,2-dione, 2, and 2-methoxycyclohex-2-en-1-one, 5, have been investigated by 1H nmr spectroscopy. A 1:1 complex was formed between 1 and SnCl4 which appeared to have a bridged structure. In the case of 2 and 5 both oxygens were again involved in complex formation with TiCl4 or SnCl4. In each case the large downfield shift of the vinyl proton resonance, H3, indicated extensive charge delocalization within the complexes and that these systems can be regarded as complexed α,β-unsaturated enones. The reactions of Lewis acid complexes of 2, 5, and the silyl ethers 2-(trimethylsiloxy)cyclohex-2-en-1-one, and 3-(trimethylsiloxy)-but-3-en-2-one, 11, with butadiene have been investigated. The acyclic enol ether 11 reacted with butadiene in the presence of either TiCl4 or SnCl4 to give products corresponding to both 4C + 3C and 4C + 2C cycloaddition reactions; in all other cases 4C + 2C products only could be isolated.