Cycloisomerization of Aromatic Homo- and Bis-homopropargylic Alcohols via Catalytic Ru Vinylidenes: Formation of Benzofurans and Isochromenes

Abstract

Ru-catalyzed cycloisomerizations of aromatic homo- and bis-homopropargylic alcohols effectively afford benzofurans and isochromenes. These processes proved to be chemo- and regioselective (5-, and 6-endo cyclizations) derived from key Ru vinylidene intermediates. The presence of an amine/ammonium base-acid pair is crucial for the catalytic cycle.

Osmium‐Catalyzed 7‐endo Heterocyclization of Aromatic Alkynols into Benzoxepines

“…However, nonterminal alkynol 1 i failed to cyclize with either ruthenium or osmium catalysts, indicating that the cyclization occurs via a catalytic metal-vinylidene complex. [2, 11d] The superior capacity of [CpOs-(py) 3 ]PF 6 as the catalyst for this reaction is also shown in the challenging regioselective 7-endo heterocyclization of benzylic-type alkynol 3. The pharmacologically interesting 2-benzoxepine 4…”

“…[4] More recently, the efficient preparation of indoles by the rhodiumcatalyzed cycloisomerization of 2-(ethynyl)anilines, [5] and the smooth preparation of benzofurans and benzopyrans by the ruthenium-catalyzed 5-endo and 6-endo heterocyclization reactions of substituted (2-ethynyl)phenols and benzylic alcohols, respectively, have been also described. [6,7] On the other hand, the heterocyclization of alkynols into the sevenmembered oxepines, a framework commonly found in complicated polycyclic marine natural products, [8] has only been achieved from specific acetonide-protected alkynol substrates via tungsten-vinylidene complexes. [9] Osmium is more reducing than ruthenium, prefers to be saturated by coordination, and redox isomers with more metal À carbon bonds.…”

A Simple and Efficient Access to Naphtho[b]furans by Claisen Rearrangement/Cyclization of Bromonaphthyl 3‐Phenylallyl Ethers

“…By using K 2 CO 3 as base and DMFa ss olvent, we have synthesized av ariety of functionalized naphthofurans in good to high yields (49-92 %) and with satisfactory selectivities.Keywords: bromonaphthyl ethers;C laisen rearrangement;c yclization;n aphthofurans;t ransitionmetal-free Naphtho [b]furans are important structural motifst hat find presence in manyn atural products [1] and pharmaceutical agents.[2] As ac onsequence,n umerous efficient synthetic strategies have been developed for the construction of this structure.A mong the protocols reported to date,m ost involve the use of versatile starting substrates [3] in combination with as uitable transition-metal catalyst, such as Pd, [4] Pt, [5] In, [6] Ru, [7] Au, [8] Ag, [9] Fe, [10] Cu.[11] Despite of theirs ynthetic versatility,t he cost of transition-metal catalysts and the difficulty associated with removing transition-metal impuritiesf rom finalp roductsm ay restrict the practical applicability of these methods.Not surprisingly,t ransition-metal-free approaches to prepare naphthofurans have also been pursued. Fore xample,i n2 012, Thomas et al [12] reported that naphtho [b]furans could be obtained in high yields by af acile microwave-assistedC laisen rearrangement of naphthyl 2-propynyl ethers (Scheme 1a).…”

Bimolecular coupling reactions through oxidatively generated aromatic cations: scope and stereocontrol

“…Substituted isochromenes are not as readily accessible as substituted chromenes, but chloroisochromene 20 (entry 6) could be prepared through a known protocol. 14 This compound required slightly higher temperatures than 4 for the oxidation step but the intermediate cation reacted smoothly with allyl tributyltin to yield 21 .…”