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

Abstract

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.

Formal Enantioselective [4+3] Cycloaddition by a Tandem Diels–Alder Reaction/Ring Expansion

“…[5] Most similar to our system is the Lewis acid-catalyzed [4 + 3] cycloaddition of 2-siloxyacroleins with dienes, which has been proposed to involve a stepwise process, in which the zwitterionic intermediate goes directly to the [4 + 3] cycloadduct. [21][22][23] Even though it has been shown that the Diels-Alder cycloadduct derived from cyclopentadiene and a 2-siloxyacrolein rearranges cleanly to the formal [4 + 3] cycloadduct, [18] computational studies suggest that this process occurs by a retro DielsAlder reaction followed by a [4 + 3] cycloaddition. [24] Therefore, we decided to conduct further studies to understand how the [4 + 3] cycloadducts in the reaction of 2-alkylacroleins and cyclopentadiene are formed.…”

“…Such a ring closure of 19 would be much more reasonable when R is siloxy, especially if it is also accompanied by a silyl migration. [21][22][23] …”

(1Z,3Z)-1,4-Di(pyridin-2-yl)buta-1,3-diene-2,3-diol: The Planar Highly Conjugated Symmetrical Enediol with Multiple Intramolecular Hydrogen Bonds

“…9 1 H NMR spectra confirm that butane-2,3-dione is the major tautomeric form. 10 1,3-Butadiene-2,3-diols fit the definition of enediols. 11 Such compounds are more stable if there are bulky aromatic groups present in the molecule 11 or if there is a carbonyl group conjugated with the enolic CdCOH moiety.…”

“…13 On the other hand, titration of the ethanolic solution with bromine shows that diketone is probably contaminated with both mono-and dienol, PhCHdC(OH)C(OH)dCHPh. 10 Further, this highly conjugated compound was obtained in a pure form by recrystallization from benzene. 14 Formation of the strong hydrogen bond is also expected to be responsible for stabilization of 1,6-diketohexa-3,4dienediols (a in Chart 1).…”

The (4+3)-cycloaddition reaction: heteroatom-substituted allylic cations as dienophiles

“…Based on work by Childs 47 and Sasaki, 48 we reported the (4+3)-cycloaddition reaction of a silyloxyacrolein. Thus, treatment of 106 with 10 mol% of Sc(OTf) 3 in the presence of furan afforded the cycloadduct 107 in very high yield and complete diastereoselectivity (eqn ( 16)).…”