Cyclobutane moieties

Dihydropyran-4-ones are a source of phenols via an intramolecular [2+2] photocycloaddition reaction and a Lewis-acid catalysed cleavage of the cyclobutane moiety <96TL1663>. 

Compared with the parent system and those with identical substitution in all four carbons, the structure of other derivatives should be affected by the substitution pattern and by the nature of the substituents. For 1,2-disubstituted derivatives, structure type C, in which the doubly substituted cyclobutane bond is weakened (and lengthened), or a related structure type in which the bond is cleaved, should be favored. This is born out by several observations mentioned earlier. For example, the geometric isomerization of 1,2-diaryloxycyclobutane (Sect. 4.1) can be rationalized by one-bond rotation in a type C radical ion. Similarly, the fragmentation of the anti-head-to-head dimer of dimethylindene (Sect. 4.4) may involve consecutive cleavage of two cyclobutane bonds in a type C radical ion. The (dialkylbenzene) substituents have a lower ionization potential (IP 9.25 eV) [349] than the cyclobutane moiety (IP 10.7 eV) [350] hence, the primary ionization is expected to occur from one of the aryl groups. 

High yields of intramolecular (2 + 2)-cycloadducts are obtained from irradiation of (159). The success of the intramolecular additions is reputedly due to the flexibility of the ether linkages. The use of the (2 + 2)-photocycloaddition reactions in the synthesis of so-called paddlanes (160) has been explored. Irradiations of (161) are carried out through a Pyrex filter and are best in cyclohexane as the solvent. The yields of the adducts with two cyclobutane moieties (160) is very good. These products are accompanied by small amounts of the mono-cycloaddition product (162). 

Preparation of photocross-linkable co- and terpolymers of A -isopropylacrylamide, 2-(dimethy-Imaleimido) A -ethylacrylamide as the photosensitive component, and 3-acryloylaminopropionic acid or A -(2-(dimethylamino)ethyl)-acrylamide as ionizable comonomers was reported [166]. Here too, cross-linking takes place through formation of cyclobutane moieties ... 

The C-4 substituted tetronates 159 also undergo intramolecular photocycloaddition to give the tricyclic adducts 160 with the heterocyclic ring directly anellated to the cyclobutane moiety. In consistency with the rule of fives, these photocycloadditions proceed exclusively to furnish the straight adducts. 

This method in the content of microwave-assisted cycloaddition reactions with relevant dienophiles via electrocyclic ring-opening which facilitates generation of a reactive diene 31 for an inter-molecular Diels-Alder [4 + 2] cycloaddition (Scheme 3.29) has also been demonstrated. The overall scope of this method is broad and amenable to a variety of functional group substitutions on the aromatic as well as the cyclobutane moiety. The reported yields range from modest to excellent and the overall diastereoselectivity is impressive. 

The palladium-catalyzed asymmetric allylic substitution of 1,3-diphenyl-2-propenyl pivalate was investigated with dimethyl mal-onate with various C2-symmetric diphosphines ligands having different spacer derived from valine. The cyclobutane moiety gave high selectivity for the (5)-product (eq 45). A reversal of the configuration of the substituted product has been observed in this reaction on changing the spacer. A stereochemical model was proposed to explain this phenomenon. ... 

Benzosceptrins A-C are other dimeric bromopyrrole alkaloids with a unique benzo-cyclobutane moiety. Benzo-sceptrin A was isolated from a Halichondrid sponge of the genus Phakellia collected around Solomon Islands, and benzosceptrins B and C were isolated from a New Caledonian and a Japanese (Okinawa) Agelas sp. (Appenzeller et al, 2009 Kubota et al, 2009). 

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