Bicyclization-anion capture

Bicyclization-anion capture Strained bicyclic systems can be prepared by a tandem cyclization-anion capture process using a Pd catalyst in combination with sodium formate, tricthylaminc, or K2C02... 

In an extension of this work, the Shibasaki group developed the novel transformation 48—>51 shown in Scheme 10.25c To rationalize this interesting structural change, it was proposed that oxidative addition of the vinyl triflate moiety in 48 to an asymmetric palladium ) catalyst generated under the indicated conditions affords the 16-electron Pd+ complex 49. Since the weakly bound triflate ligand can easily dissociate from the metal center, a silver salt is not needed. Insertion of the coordinated alkene into the vinyl C-Pd bond then affords a transitory 7t-allylpalladium complex 50 which is captured in a regio- and stereocontrolled fashion by acetate ion to give the optically active bicyclic diene 51 in 80% ee (89% yield). This catalytic asymmetric synthesis by a Heck cyclization/ anion capture process is the first of its kind. 

The same palladium-catalyzed domino cyclization-anion capture sequence involving carbopalladation of allene (69), which was employed by Grigg et al. for the synthesis of diene 70 (Scheme 10) can yield 1,3,5-hexatriene 194, when starting from 2-bromo-l-ene-6-yne 193 instead of the aryl iodide 68 (Scheme 32) [52], Under the conditions of its formation, 194 immediately undergoes thermal 67r-electrocyclization to give the bicyclic product 195. 

Asymmetric Heck reaction.3 Treatment of the vinyl triflatc I with tclrabulylam-monium acetate (1.7 equiv.) and a catalytie amount of Pd(OAc) and (S)-BINAP in DMSO at 20° results in the fused bicyclic pentanoid 2 in 80% ee, formed by a Heck reaction followed by an anion capture process. This product can be converted into 3, an... 

Differentiation of the prochiral cyclopentadienyl system in 310 by intramolecular AHR generates 7r-allylpalladium 312, which was trapped intermolecularly with the malonate 311 in regio- and stereoselective maimer to provide the bicyclic system 313 with 87 % ee in the presence of NaBr in DMSO. The regio- and stereoselective anion capture can be understood by considering the steric effect as shown by 312. The total synthesis of c nellene 314 has been achieved from 313 [128],... 

In another example, Shibasaki and co-workers applied an asymmetric Heck reaction/anion-capture cascade process to the total synthesis of marine sesquiterpenes capnellenols 14 and capnellene 17 (Schemes 13.5 and 13.6). The cascade involved the use of prochiral cyclo-pentadienyl systems for the generation of bicyclic 71-allyl-palladium species, which were then trapped by a suitable nucleophile. Initial studies in the total synthesis of capnellenols described the trapping of the 71-allylpalladium... 

They proposed that [l,4]-sulfanyl participation gave a [2.2.1] bicyclic sulfonium salt intermediate 10a. This could be captured by the chloride anion to give either 19 (by path a) or 20 (by path b) as outlined in Scheme 3. It should be noted that the usual [l,2]-sulfanyl participation, via spirocyclic sulfonium ion 21, would only give the unrearranged chloride 19. 

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