Ring opening of cyclobutenones

The ring opening of cyclobutenone derivatives leads to the formation of vinylketenes (equation 3). The potential of these compounds in the synthesis of complex molecules has been investigated in the 1980s by a number of research groups, most notably those of Moore, Liebeskind and Danheiser. An excellent review describing work in this area prior to 1986 is available. ... 

Substituted phenols are synthesized by the nickel(0)-catalyzed ring opening of cyclobutenones and subsequent [4+2]cycloaddition with alkynes [106]. 

Scheme 4.2 Calculated (MP4SDQ/6-31G ) relative energies (kcal mol b for ketene formation by ring-opening of cyclobutenone (6), oxetenone (8), and azetinone (10).

A rhodium-catalyzed ring opening of the cyclobutenones 637 forms the r 4-vinylketene intermediates 638, which can dimerize to afford (/. )-6-vinyl-2//-pyran-2-oncs in excellent yield (Scheme 146) <2004AGE5369>. A ruthenium-mediated ring opening and dimerization of cyclobutenones forms a mixture of (E)- and (Z)-6-vinyl-2//-pyran-2-ones in high yield (Equation 257) <2004AGE5369>. 

The preparation of two A,A-diethylaminofurans 200 (R = Me, Et) by ring-opening of the cyclobutenone 199 using a Grignard reagent has been described (Eq. (34)) (73TL3353). 

An unusual synthesis of ( )-septicine (866) from dimethyl squarate (878) exploits rearrangement of a 4-(l-pyrrolo)cyclobutenone 879 to the indolizine-5,8-dione 880, presumably via a ketene intermediate formed by electrocyclic ring opening of 879 (Scheme 113) (584). Partial reduction of 880 yielded the 4-hydroxypyridone 881, the triflate ester of which was deoxygenated with a palladium(II)-formic acid system to yield the l,2,3,5-tetrahyi oindolizin-5-one 882. Mild reduction with aluminum hydride completed the synthesis of ( )-866 in an excellent overall yield of 27% from 878. 

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 chiller is restricted to a short, but by no means complete, review of key synthetic routes to cyclobutenes, bmzocyclobutenes and cyclobutenones and a generally qualitative discussion of the way in which substituents control both the ease of ring opening and the stereochemistry of the products obtained. The reader should thus be in position to make useful predictions. Finally we have included pertinent synthetic applications which illustrate in useful and often very imaginative ways the value of the... 

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). 

Photochemical ring-opening in benzene solution to the allenic keten (201) explains the conversion of the cyclobutenone (202) into the lactone (203 80%).118 A similar route explains the formation of (204) from (205). Such ring-opening... 

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. 

Rodrigues and Verardo [33] were able to effect the formation of similar cyclobutenone derivatives 65 from indoles 64 and DMAD at 20°C, using a catalytic amount of BF3-OEt2. The resulting cyclobutenones 65 were then converted to the ring-opened benzazepines 66, upon heating at reflux in benzene (Scheme 15). 

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