Ring opening reactions cyclobutenones

Cycloaddition reactions of the indole 2,3-double bond are not limited to alkenes as partners. Acetylenic compounds have also been used in photochemical cycloaddition reactions with indoles to produce cyclobutenone derivatives. There have been extensive studies on the reaction of indoles with dimethyl acetylenedi-carboxylate (61, DMAD), which produce a number of structurally distinct products [31]. By devising a photosensitized cycloaddition reaction of DMAD to activated indoles 60 in the presence of acetophenone, Neckers and Davis were able to produce the cyclobutene derivatives 62 in good yields (Scheme 14) [32]. The resulting cyclobutenes can then be converted to the corresponding benzazepines 63 via thermal ring-opening reactions. 

An analogous vinylketene intermediate (127, see Schemes 57 and 59) as proposed for the Dotz reaction has been assumed in the so-called cyclobutenedione methodology [161]. The key intermediate is a 4-aryl or 4-alkenyl substituted 2-cyclobutenone (128) that can be obtained e.g. by the reaction of the 3-cyclo-butene-1,2-dione (129) with the appropriate lithium reagent or Stille coupling with 4-chloro-3-cyclobutenone. Thermal cyclobutenone ring opening to the vinylketene 130 followed by electrocyclization furnishes the highly substituted aromatic compound 131 (see Scheme 59). 

As a matter of fact, the above preparative reaction to obtain the framework of bicyclo[ 3.2.0 ]heptenone is already in hand. Indeed, the ring closure step after electrocyclic ring opening of 4-hydroxy-2-cyclobutenone is not limited to fully conjugated systems synthetic variants are realizable with other prox-imally placed ketenophUes. When an allyl group was located at C-4, the ketene underwent an intramolecular [2 + 2] cycloaddition reaction with this double bond to give the bicyclo[3.2.0]heptenone derivatives [111, 112[. 

Compound 54 has only been characterized in solution and is very unstable. Compound 55 is formed via ring opening of 3,3-diphenylcyclopropene with the -ethene(di-/wr-butylphosphanyl-/ -ethyl)cyclopentadienylcobalt(i). Unfortunately, this compound has not been isolated. It is worth mentioning that, by reaction with the same cobalt(i) precursor, diphenylcyclopropenone gave a cyclobutenone cobalt compound 56 (characterized by X-ray) in very good yields (91%) (Figure 18). ... 

The reactions that have been described in Schemes 19.40-19.42 are related to a large body of work that focused on developing methods for the synthesis of fused aromatics and quinones that could not be easily prepared by means of conventional aromatic substitution chemistry. Many of these methods make use of the elec-trocyclic ring opening of a cyclobutenone in the key step. The example shown in Scheme 19.43 is from the... 

This concept is particularly flexible in that the corresponding reaction of 4-cyclobu-tyl-2-cyclobutenones opens a route to eight-membered carbocyclic rings. 

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