Reactions of enamines of cyclic ketones with methyl propiolate. Reinvestigation and application in a tandem ring expansion with four carbon atoms

Abstract

Enamines of cyclic ketones 1 with ring sizes ranging from seven to twelve react with methyl propiolate via (2 + 2) cycloaddition and subsequent conrotatory ring opening of the cyclobutene moiety in 2. The resulting cis,trans-cycloalkadienes 3 rearrange further via a thermal [ 1,5] hydrogen shift to the corresponding &,ciscycloalkadienes 4. The structures previously described in the literature of several of these cycloalkadienes are shown to be incorrect. For a representative cis,trans-cycloalkadiene (3g, R = H) and a &,cis-cycloalkadiene (5a, R = H) the structures were proven by single-crystal X-ray analysis. The reactive "enamine" moiety in 4 allows a tandem ring-expansion reaction of enamines of cyclic ketones with four C atoms. Reaction of 4 with a second electron-deficient acetylenic ester gives the cis,cis,trans-cycloalkatrienes 6.

Photochemical, Thermal, and Base-Induced Access to Hydroazulene Derivatives via Two-Carbon Ring-Enlargement Reactions of Condensed Electrophilic Cyclobutenes

“…Compound 10 arose from a [1,5] sigmatropic proton-shift of the primary ring-expansion product 9 (Scheme ). This result proved to be in agreement with those of Reinhoudt et al for related compounds. When the cyclobutene 3 was treated under the same thermal conditions, a complex mixture of compounds was obtained.…”

Synthetic and Mechanistic Aspects of the Reaction of 1,1-Difluoro-2,2-bis(dimethylamino)ethene with Ethyl Propiolate

“…In general, reactions of propiolates with enamines involve initial [2 + 2] cycloaddition reactions to form cyclobutene derivatives, with these cycloadducts undergoing a subsequent electrocyclic ring opening, quite often spontaneously. As is the case in the example given below, such electrocyclic reaction is often followed by a rapid 1,5-hydrogen shift to alleviate the strain of the originally formed cis,trans -diene . In the case of alicyclic enamines, the net result is a ring-enlarged product two carbon atoms larger than the original .…”

Diethyl 3-Piperidino-2,7-cycloheptadiene-1,2-dicarboxylate

“…The C1---C2 and C6--C7 bonds are somewhat shorter than a single C---C bond ( Table 2). The torsion angles and bond lengths may be compared with those of methyl 2-(1-pyrrolidinyl)-cis,cis-1,6-cycloheptadiene-l-carboxylate (Vos et al, 1986) in which the corresponding torsion angles about the double bonds are 4.0 and -20.3 ° , and the double-bond lengths are 1.334 (2) and 1.396 (2) ~,.…”