Heck cylization

The amines 43, 46, and 47 constituted dehydro analogs of two intermediates in Rawal s strychnine synthesis, 49 and 50 (Scheme 20).7h This suggested that a simple reduction of the C ring double-bond, while conserving a C14-C15 unsaturation, would access synthetic relay points only two (via a Heck cylization of 50) or three (via alkylation of 49 with side chain 45, then Heck reaction) steps from 1, and allow the claim of a formal synthesis. 

The transformations of o-iodoalkenylbenzenes in the presence of CO and palladium catalysts may involve either carbonylative cylization, or intramolecular Heck reaction, as well as a number of intramolecular pathways leading to oligomeric byproducts. Non-carbonylative pathways can be reasonably suppressed by applying elevated pressures of GO. Under such conditions, the products are formed in good yields, and the predominant termination stage is Pd hydride / -elimination to form exo-cycWc double bond (Equation (27)). 

An interesting case of 6-endo- versus 5- j o-cylization has been studied by Bombrun and Sageot [129], who investigated intramolecular Mizoroki-Heck cycUzation of enamides in a thiophene series (Scheme 6.50). If the reaction was carried out under Jeffery conditions, then it proceeded nearly exclusively via the 6- iio-cychzation mode however, when run under reductive Mizoroki-Heck conditions (e.g. in the presence of a hydride source), a 5-exo-cyclization is mostly achieved and the lack of a /3-hydrogen in this case leads to product 179. 

Even selective 7-exo-cylizations [25] have been reported via intramolecular Mizoroki-Heck reactions on solid supports. One synthesis deUvers bicyle 25 in 60% overall yield (Scheme 14.7) [25]. 

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See also in sourсe #XX -- [ ]

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