Cyclobutane fused

Unusual heterocyclic systems can be obtained by photodimerizations and for five-membered heterocycles with two or more heteroatoms such dimerizations need be effected on their ring-fused derivatives. Cyclobutanes are usually obtained as in the photodimerization of the s-triazolo[4,3-a]pyridine (540) to the head-to-head dimer (541). These thermally labile photodimers were formed by dimerization of the 5,6-double bond in one molecule with the 7,8-double bond in another (77T1247). Irradiation of the bis( 1,2,4-triazolo[4,3-a]pyridyl)ethane (542) at 300 nm gave the CK0icage-like structure (544) was formed (77T1253). 

Unlike coumarin, chromone (206) undergoes efficient unsensitized photoaddition to tetramethylethylene, cyclopentene, ketene dimethyl acetal, and but-2-yne.180 The major product of such an addition to tetramethylene is the cis-fused cyclobutane derivative (207) the formation of the two minor products (208 and 209) is easily rationalized. Added benzophenone has no visible effect on this cycloaddition, which is therefore believed to involve the attack of triplet chromone on the ground-state alkene. Photoaddition to furo-chromones has also been studied,179 and the photosensitized cyclo-... 

Keywords a-adamanthyl ketone, prolinol, Norrish type II photoreaction, fused cyclobutane... 

NMR data for the major air-fused cyclobutane isomers are presented... 

Thiocoumarin photodimerizes more efficiently than does coumarin in solution and yields the head-to-head product. The photocycloaddition with 2,3-dimethylbut-2-ene affords a 4 1 mixture of the as- and trans-fused adducts, but terminal alkenes give only the m-fused cyclobutane in which the unsubstituted C atom has added to C-3 of the thiocoumarin (Equation 68) <1991JPH(57)231 >. Thiocoumarin reacts ca. 5 times faster under irradiation with tetrachloroethene than does isothiocoumarin (Scheme 70). Both thiocoumarins afford the appropriate m-fused adduct (Equation 69) <1997HCA1865>. 

Marine natural products Spiro-fused cyclobutanes... 

Welwitindolinone A isonitrile also has a number of structural features that make it a very challenging target, most notably the sterically crowded spiro-fused cyclobutane ring, which at first sight might seem thermodynamically unstable (Fig. 3). 

Highly strained spin-fused cyclobutane ring... 

Fused cyclobutane derivatives can yield 6-6 heterocycles. Thus, compound (434) gives the dihy-dropyranopyridine derivative (436) (28% yield) in xylene at reflux temperature via the intermediate (435) <82CL1361>, cyclobutane (437) gives heterocycle (438) (25% yield) as the major product when treated with SnCl4 <82CPBl53l) and compounds (439) and (440) afford products (275) and (441), respectively, with DBU and either methanol or ethanol <92JHC387>. 

Dihydrothiophene-1,1 -dioxide (2-sulfolene) (131) underwent a photochemical [2+2] cycloaddition with maleic anhydride to yield cycloadduct 132 which contained a m-fused cyclobutane (Scheme 36).83 Simple reactions of the anhydride function provided access to a wide range of novel bi- and tricyclic sulfones 133-135 containing the novel 2-thiabicyclo[3.2.0]heptane-2,2-dioxide ring system. 

Meerwein rearrangements, a i7a-hydroxyandrostane (21) is apparently dehydrated easily with migration of the Cps) methyl group from C(i3> to Cd ) [124]. The stereoelectronic requirements in the I7 3 epimer (22) would be satisfied by migration of the C(i4)- C(ig) bond to C(i4>"-C(i7), but the contraction of ring D to a highly strained tmns fused cyclobutane structure must be less favourable than the observed cis ... 

Several instances of product control of the second type have been described. The most straightforward are the skeletal rearrangements which occur when the mono-tosylates of 4a,5 - (4) or 5a,6a-dihydroxy cholestanes 5) are treated with bases [ 132]. More remarkable is a recently observed contraction of ring D to give what must be a very highly strained yreaction occurred in excellent yield when the i7a-hydroxy-i6a-mesyloxy-20-... 

The elegant route to (-)-punctatin A (125) developed by Paquette nicely illustrates the synthetic utility of photochemically mediated intramolecular C—H insertion (Scheme 3). The excellent stereoselectivity observed in the four-membered ring forming process (123) — (124) was, in fact, predicted. The intermediate 1,4-diradical could cyclize to the equatorially fused rran.r-cyclobutane, as illustrated. The alternative cyclization mode, leading to the c/5-fused cyclobutane, has a bad steric interaction with the angular alkoxymethyl group, and so is less favored. 

In studies where cholesteric phase order appears to influence the course of bimolecular reactions, solute diffusion and collisional orientations appear to be affected by solvent anisotropy. An example is the stereoselective photodimerization of 1,3-dlmethylthymine(24). While all four possible cis-fused cyclobutane photodimers are produced in isotropic solutions and disordered glasses, the cis-syn dimer is formed almost exclusively in liquid-crystalline media. Furthermore, selectivity of reaction products in the mesophases is greatly decreased upon addition of an isotropic diluent which disturbs local solvent order (e.g., dioxane or DMSO). Since all of the... 

Dehydrotestosterone acetate adds butadiene under u.v. irradiation, giving as one product the remarkable 4a,5a,6a,7a-adduct (637), containing three fused cyclobutane rings despite very considerable strain, this product was stable to basic hydrolysis and to acetylation at C-17. The major photo-addition product with butadiene was the trans-fused 4a,5)5-adduct (638), which isomerized in a... 

The fused cyclobutane derivatives produced via the above cycloaddition reactions have been utilized as synthetic precursors for the preparation of indole-2-acetonitriles (Scheme 6) [23]. First, the acetyl-substituted cyclobutanes 28 were converted to the cyclobutanone derivatives 29, which were in turn treated with hydroxylamine to provide the corresponding oximes. Beckmann fission of oximes 30 in the presence of thionyl chloride then produced the l-benzoylindole-2-aceto-nitrile derivatives 31 in good yield. 

The C4 ring can exist as part of complex organic compounds, and its fission is aided by the existence of strain and unsaturated substituents thus for example 1,2-diphenylbenzocyclobutene (Scheme 11.5) was easily reduced to 1,2-dibenzylbenzene. Two fused cyclobutane rings (i.e. bicyclo[2.2.0]hexane) reacted to form cyclohexane, and where there are three fused rings, as in the molecule in Scheme 11.5 that defies simple naming, this reaction occurred, but one ring remained intact. ... 

Ladderanes are molecules composed of fused cyclobutane rings (Fig. 12.23) [4]. Like nanotubes and the todlike molecules of [3], they have been considered as possibly having electronic applications. Ladderanes have been made by oligomerization of a (transient) cyclobutadiene [68], and by solid-state photooligomerization of polyenes [4] (Fig. 12.23). 

---