Allenes furan-2 acetate

Attempts to liberate l-methyl-l-aza-2,3-cyclohexadiene (329) from 3-bromo-l-methyl-l,2,5,6-tetrahydropyridine (326) by KOtBu in the presence of [18]crown-6 and furan or styrene did not lead to products that could have been ascribed to the intermediacy of 329 (Scheme 6.70) [156], Even if there is no doubt as to the allene nature of 329 on the basis of the calculations on the isopyridine 179 and 3d2-lH-quinoline (257), it is conceivable that the zwitterion 329-Za is only a few kcal mol-1 less stable than 329. This relationship could foster the reactivity of 329 towards the tert-butoxide ion to an extent that cycloadditions to activated alkenes would be too slow to compete. On the other hand, the ultimate product of the trapping of 329 by KOtBu could have been an N,0-acetal or a vinylogous N,0-acetal, which might not have survived the workup (see, for example, the sensitivity of the N,0-acetal 262 [14], Scheme 6.57). 

Furan has served as a starting material for the synthesis of methyl rt-tetradeca-frans-2,4,5-trienoate (177), a compound presumed to be a sex attractant of the male dried bean beetle (72TL3777). Conversion of furan to 4,4-dimethoxy-frans-but-2-en-l-al (174) was accomplished in two steps by a previously reported procedure. Intermediate (174) was then ethynylated, acetylated and reacted with lithium dioctylcuprate to yield allene (176). Hydrolysis of the acetal and Mn02-NaCN oxidation of the aldehyde afforded the methyl ester (177 Scheme 37). 

Chromene (2,3-didehydro-2/7-l-benzopyran) 156 can be generated by treatment of 3-bromo-27/-chromene 157 with potassium /rz -butoxide (Scheme 18). When done in the presence of styrene or furans, trapping is possible to give products such as 158 and 159, respectively. 352-Chromene could be expected to behave as an allene, a diradical, or an ylide. In the absence of 7t-nucleophiles, acetal 160 was formed, which is suggestive of the importance of the ylide in terms of structure <1998EJ0237>. 

Craft and Gung developed a paUadium-catalyzed transannular [4+3] cycloaddition route in which all of the rings of cortistatins are prepared in one step from a single macrocyclic precursor (Scheme 19.50) [114]. Exposure of macrocyclic allene 233 to a catalytic amount of palladium (II) acetate in the presence of excess lithium bromide resulted in the formation of 238 as a single isomer in 37% yield. This is the first report of a transannular [4+3] cycloaddition. The proposed mechanism is shown in Scheme 19.50. The formation of allene-palladium complex 234 affords a a-allylpalladium intermediate, which rapidly isomerizes to the 7i-allylpalladium intermediate 235. This can then undergo intramolecular cycloaddilion via an endo (compact) transition strucmre 236 to give bromonium ion 237. The loss of a proton results in the formation of the observed product 238. Cycloadduct 238 was readily converted into the tetracyclic core skeleton of cortistatins 239 by selective reduction of the olefin formed by cycloaddition with furan, followed by reductive debromination. 

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