Triquinanes rearrangement

The same process shown in Scheme 88 starting from different 2-substituted oxetanes and using biphenyl as the electron-carrier catalyst under THF reflux has been used to prepare regioselectively substituted primary alcohols. On the other hand, the combination of a DTBB-catalyzed ca 20%) lithiation with triethylaluminium in TFIF at —78 °C has been used for the transformation of strained oxetanes to substituted di- and triquinanes through a rearrangement process . An example is given in Scheme 89 for the transformation of oxetane 299 into the product 302 through radicals 300 and 301. 

Interestingly, Lewis acid catalyzed rearrangement of 6-methyltricyclo[8.2.0.0, 6]dodecan-9-one gave the spiroannulated triquinane ketone (l/ , 5/ )-l-methylspiro[bicyclo[3.3.0]octane-2,lf-cyclopentane]-6-one as the major product (70%).85... 

The rearrangement of (-)-modhephene (62) to a (-)-triquinane (63) through acid catalysis has been demonstrated (Scheme 18).55 The rearrangement involved protonation, 1,2 s-bond and methyl shifts, and deprotonation. Monitoring by XH NMR suggested the (-)-isocomene (64) as an intermediate, further proved by transformation of natural (64) to (63). 

The photochemical transformations of vinylcyclopropanes are of mechanistic interest, but because of the high energy of the reaction intermediates, especially in the triplet manifold, practical applications are scarce. The regio- and stereo-selective aspects are, however, similar to those of the thermal processes. Additional methods that provide for the cyclopentene rearrangement involve the degenerate photochemical rearrangements observed by Wender during meta-photocycloaddition of arenes and alkenes and utilized extensively in the synthesis of triquinane sesquiterpenes. -" -" (See Section 8.1.9 or ref. 41 for a recent summary.)... 

In a novel combination of Pauson-Khand cycloaddition with vinylcyclopropane chemistry, de Meijere has described an entry to linearly fused triquinanes beginning with cyclopropylalkynes. Cyclopentenone formation has been carried out with a variety of substitution patterns on the cyclopropane, and moderate yields achieved with both norbomene and cyclopentene as substrates. Thermal vinylcyclopropane-cy-clopentene rearrangement of the cycloaddition products leads to the final tricyclic system (Scheme... 

In the laboratory of K. Fukumoto, the stereoselective total synthesis of (+)-A -capnellene was carried out using an intramolecular Diels-Alder reaction to obtain a tricyclic 5-5-6 system. Since the target molecule was a triquinane, the six-membered ring had to be converted to a five-membered one, a transformation achieved by a Wolff rearrangement. The required a-diazo ketone was prepared via a deformylative diazo transfer reaction and was photolyzed in methanol. The ring-contracted methyl ester was isolated as a 3 1 mixture of separable isomers favoring the a-isomer. 

Isomerizations. A synthetically useful transformation is the rearrangement of linear triquinanes to the angular isomers. For the conversion of propargyl ethers to allenyl ethers microwave promotion is advantageous. ... 

In a representative example of this type, complexed l-ethoxy-2-ethynylcyclopropane 7 was used in the Pauson-Khand reaction with cyclopentene to give the cyclopropyl-substituted bi-cyclo[3.3.0]octenone 8, which on thermal rearrangement of the vinylcyclopropane unit gave the triquinane ring system 9 ... 

Acetone- or acetophenone-sensitized irradiation of the enones yields the reeurranged ketones (213, 214) which are described as useful substrates for the synthesis of polyquinane natural products. The enones (215) can be converted by photosensitized irradiation into the diquinanediones (216). Irradiation of (215) in acetone gives a low yield of the tricyclic ketone (217). This ketone, while stable in the dark, is photochemically converted into the diquinanedione (215). The authors present evidence that this conversion results from fission of the cyclopropyl bond to yield the biradical (218). Demuth and Hinsken have reported the use of the oxardi-ir-methane rearrangement in the synthesis of annelated triquinanes. Thus the photo conversion of the enone (219) affords the tetracyclic ketone (220,72%). In an analogous reaction the enone (221) is converted into the isomeric ketone (222,70%). ... 

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