Cycloaddition precursor cyclobutene

Intramolecular 2 + 2 cycloaddition.b A recent synthesis of coronafacic acid (4) proceeds through a cyclobutene (2) to obtain a trienone (3), the immediate precursor ot 4 11 rider the most satisfactory conditions, A1C13 in CH2C12 at 25°, the yield of this Inlramolucular cycloaddition is only 16%. However, since the remaining steps to 4 call he conducted without purification of the intermediates, the overall yield for a rallier lengthy synthesis is reasonable. 

A major initial limitation of the benzocyclobutene approach to o-quinodimethanes was the lack of efficient, large-scale syntheses for many specifically substituted derivatives. Fortunately, recent developments have lemov much of this impediment. Q>nceptually, the synthesis of benzocyclobutenes from aromatic precursors can be envisaged in only a limited number of ways. These include [2 -i- 2] cycloadditions involving benzynes and alkenes, intramolecular cyclization on to a benzyne, cyclizations involving arene anions, and electrocyclic closure of o-quinodimethanes. Benzocyclobutene derivatives can also be prepared by aromatization of bicyclo[4.2.0]octanes. Detailed discussion of variations to these approaches can be found in the cited reviews. The cobalt catalyzed co-oligomerization of 1,5-hexadiynes with al-kynes, especially bis(trimethylsilyl)acetylene, has also been employed for the preparation of specifically substituted benzocyclobutenes. In the latter case the cyclobutenes are often not isolated but converted directly to o-quinodimethanes and subsequent products. ... 

A combination of a photochemical [2 -i- 2] cycloaddition of dichloroethylene to enone (4), followed by a reductive elimination, was used to prepare the tricyclic cyclobutene (5), which served as a precursor to the highly strained 8-carboxy[6]paracyclophane (6 Scheme 2). A similar strategy has been used to prepare other strained propellanes. ... 

Photochemical [2 + 2]-cycloaddition of unbridged bis(alkenyl-Cp)zirconocene complexes provides a unique approach for the synthesis of 6 -hydrocarbyl-bridged -precursor leads to the corresponding meso-ansa-zhconoccne. 

Periplanone B is the most active sex pheromone found in the alimentary tract and excreta of the American cockroach Periplaneta americana. An elegant total synthesis of this germacrane sesquiterpene was achieved by SCHREIBER and SANTINI Cyclodecatrienone 1 is an obvious precursor. One of the oxirane rings arises from epoxidation of the enone CC double bond, the other from [2-1-1]-cycloaddition of a carbene to the carbonyl bond of the enone. Oxidation of the methylene group introduces the additional carbonyl double bond. The CC double bond of the enone results from an elimination of HX in the a-X-substituted cyclodecadienone 2, which, on its part, is feasible by substitution of cyclodecadienone 3. An electrocyclic opening of the cyclobutene ring in 4 provides the 1,3-diene substructure in 3. 

One Sulphur Atom.— The synthesis of the 2,3- and 2,7-dihydrothiepins (96) and (98) by cycloaddition of DMADC to (95) and (97), respectively, has been described, reaction occurring via isolable cyclobutene intermediates. Unlike the parent compoiind (99 X = lone pair) which ring-contracts rather than give the 1-benzothiepin, the sulphone (99 X = Og) may be dehydrated on acid treatment to the corresponding 1-benzothiepin dioxide. The sul-phoxide (99 X = O) undergoes SO-elimination under the same conditions, consistent with the 1-benzothiepin monoxide as an intermediate however, this compound could not be isolated. A successful route to substituted 1-benzothiepins is enol-acetylation of keto-precursors such as (100). In the... 

---