Experimental evidence for a [2 + 2] mechanism in the Lewis acid-promoted formation of α,β-unsaturated esters from ethoxyacetylene and aldehydes. Synthesis and characterisation of 4-ethoxyoxetes.

Oxophosphonium-alkyne cycloaddition reactions: reversible formation of 1,2-oxaphosphetes and six-membered phosphorus heterocycles

“…The metathesis reaction between carbonyls and alkynes is a powerful method for the synthesis of α,ß-unsaturated ketones and has received much attention in the construction of heterocycles and in natural product synthesis. − This well-understood reaction involves an oxete intermediate, which is unstable toward ring opening and the formation of the enone product (Scheme a) . Since carbonyl–alkyne [2 + 2] cycloadditions are orbital-symmetry forbidden in the ground state, either photo-irradiation or the use of a catalyst , has been established as an avenue to overcome the kinetic barriers. Nonetheless, the strained cyclic intermediate can be stabilized by electron-withdrawing substituents, which has allowed the isolation and structural characterization of many oxete derivatives. − …”

Thiophosphonium–Alkyne Cycloaddition Reactions: A Heavy Congener of the Carbonyl–Alkyne Metathesis

“…These reactions have been extensively utilized in synthetic chemistry . The carbonyl–alkyne metathesis proceeds via a four-membered oxete intermediate, which is usually directly transformed into the α,β-unsaturated ketone, − unless it is stabilized by strongly electron-withdrawing groups. − Due to the lower tendency of sulfur to form double bonds, thietes are more stable than oxetes, − and a dynamic equilibrium between the “closed” thiete and “open” α,β-unsaturated thioketone form was observed with thioether substituents. , Given these differences between oxetes and thietes, we became curious to explore how the introduction of another heavy main group element would affect the stability of the four-membered ring species. Although numerous examples for heavy main group carbonyls R 2 E=O and thiocarbonyls R 2 E=S have been synthesized, − the reactivity with alkynes is little developed.…”

Catalytic Asymmetric Synthesis of Stable Oxetenes via Lewis Acid-Promoted [2 + 2] Cycloaddition

“…Photoinduced [2 + 2] cycloaddition of nπ* excited-state carbonyl compounds with alkynes gives labile oxetene intermediates (eq 1 in Scheme b). ,, The corresponding α,β-unsaturated carbonyl compounds are obtained through electrocyclic ring opening of the thermodynamically unstable oxetenes. Lewis acid-promoted [2 + 2] cycloaddition also provides the oxetene intermediates, but ring opening to give α,β-unsaturated carbonyl compounds readily takes place because of the Lewis acid, even at low reaction temperatures (eq 2 in Scheme b). , …”

“…To synthesize more stable oxetenes, the more sterically demanding dimethylphenylsilyl group in alkynylsilane 2c was employed; the combination of 2c and 3 in the presence of 1 (2 mol %) afforded the stable oxetene 4c in excellent yield (93%) and enantioselectivity (99% ee); 4c could be isolated and purified by silica-gel column chromatography without conversion to 5b or α,β-unsaturated ketone 7 (eq 3 in Scheme ). Heat treatment of 4c in toluene- d 8 resulted in electrocyclic ring opening − to gave 7 quantitatively . Monitoring of the reaction at 70 °C revealed a long half-life of 110.6 h. The absolute configuration of 4b was determined to be R by derivatization to the known α-hydroxy-γ,δ-unsaturated ester…”