Cyclobutane ring reductive

Opening of a cyclobutane ring fused to a quinolizine system under reductive conditions has been described. Thus, the previously mentioned compound 128 was obtained by treatment of 132 with samarium diiodide (Equation 8)... 

The cyclobutane rings of 1,2-ethano[2.n]cyclophanes can be opened in a Birch reduction at — 60°C in tetrahydrofuran and liquid ammonia to give [4.n]cyclophanes.212-214 In this manner, macrocyclic rings with twelve, thirteen, fourteen, fifteen, sixteen, seventeen, eighteen and twenty-seven members have been synthesized. 

The initial scission of cyclobutane ring of available pagodane derivative 37 is achieved as a result of homolytic bromination. The tribromide 38, thus formed, underwent facile bromine elimination-fragmentation (and hydrogenolysis of the C-Br bond at the a-methoxycarbonyl center) under the action of metals to form diene 39. Diimide reduction of 39 affected only one double bond and gave product 40, which served as a common precursor for both 35 and 36. The... 

Hydrosilylation of (-)-p-pinene (54) under radical conditions results in opening of the strained cyclobutane ring to give only (53 equation 44). However, Pt-catalyzed hydrosilylation gives the expected product, which, after conversion of Si—Cl to Si—H, is used for the asymmetric reduction of ketones. ... 

Quinolyl cyclobutylmethyl ketone 72 undergoes C-C bond cleavage by rho-dium(I) to form the inserted intermediate 73, as mentioned in Eq. 21. Sequential (3-carbon elimination leads to ring-opening of the cyclobutane ring. Addition of pyridine and P(OMe)3 induces reductive elimination to give a mixture of linear 8-quinolyl ketones 74 and 75 [86]. 

The relative instability (or greater reactivity) of small (three and four membered) rings, as compared with their medium (five through seven membered) counterparts, is evidenced (Chapter 3) by, for example, the higher heat of combustion per methylene (-CHj-) for the compounds with the smaller rings. Furthermore, the small-ring compounds cyclopropane and cyclobutane undergo reduction to the... 

With Ni° as a catalyst, an intermolecular [4+2] cycloaddition [45] reaction with cyclobutanone 52 and 4-octyne 53a produced cyclohexenone 54a in 95% yield. The proposed reaction mechanism is illustrated in Scheme 9. Presumably the reaction of 52 and 53a with Ni° would proceed through oxidative cyclization to give oxanicke-lacyclopentene (55). P-C elimination cleaves the cyclobutane ring to generate 56 and leads to formation of product 54a after reductive elimination. Overall, a formal [4+2] cycloaddition was accomplished with Ni° via p-C elimination. In contrast, Rh was not an effective catalyst for this transformation. 

A novel variant on this theme is contained in Coates synthesis of (13), which contains the ring skeleton of the sesterterpene ophiobolin. In this case reductive fission of the cyclobutane ring [(12) - (13)] was used instead of the more usual retro-Aldol cleavage. 

Electrophilic Cleavage and Oxidation of Cyclobutane Derivatives. Periodic acid in aqueous THF cleaves a 1,2-dioxygenated cyclobutane to a 1,4-diketone in a new synthesis of cis-jasmone. Mono- and bis-l,2-dioxetans (232) and (233) may be made by photosensitized oxygenation of the dibenzylidenecyclo-butane (231). Subsequent reductive reactions can be used to open the cyclobutane ring of (233). 

Those [2.n]phanes possessing cyclobutane ring(s) can be transformed into [4.n]phanes by Birch reduction and can also be transformed into benzocyclobutene analogs. 

Humulene reacts with electrophiles initially at the trisubstituted 6,7 double bond in a Markovnikov fashion. With N-bromosuccinimide in aqueous acetone, the attack of bromonium ion triggers a double cyclization to tricyclic bromohydrin (104) 147). The formation of the /mn -fused cyclobutane ring by electrophilic cyclization onto the 2,3-double bond is similar to the second ring closure in the caryophyllene biogenesis. By dehydration and reductive cleavage of the three-membered ring, it was possible to convert (104) into caryophyllene. 

Bond constructions similar to those just discussed can be achieved using an alkylidene malonate which is tethered to an alkyl bromide [72]. Of particular interest in this context is the controlled potential reductive cyclization of 263. As illustrated, the method provides a reasonably facile and modestly efficient entry to cyclobutanes 264. Presumably, the process is initiated by reduction of the alkylidene malonate rather than the alkyl halide, since alkyl bromides are more difficult to reduce. The same substrate, when reduced with L-Selectride undergoes conjugate addition of hydride and a subsequent cyclization leading to the five-membered ring 265. The latter transformation constitutes an example of a MIRC reaction [71-73], a process which is clearly complementary to the... 

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