Palladium Mediated Cyclization Reactions

D.U. Palladium Mediated Cyclization Reactions. It is possible to use organopalladium chemistry for cyclization reactions. Treatment of 386 with the palladium catalyst ([dba]3Pd2, where dba = dibenzylidene acetone) led to cyclization that generated a five-membered ring (387) in 78% yield.227... 

A series of benzo[fc]benzo[2,3-cfjthiophen-6,9-diones 12 has been prepared in modest yields by palladium mediated cyclization of the precursors 13. However, the necessity to use stoichiometric amounts of the palladium source precludes cost effective preparation of the targets. The required substrates 13 may be constructed by palladium catalyzed reactions between the appropriate phenols with 2,3-dimethylbenzoquinone <06SC3319>. 

Finally, fully reduced heterocycles have been prepared either from a sequential azomethine imine cycloaddition-palladium-mediated cyclization process <2003T4451>, or from the reaction of A-( l-benzotriazolylalkyU-AyV-disubstitutcd hydrazine with methyl vinyl ether <1997JOC8210>. 

The synthesis of highly substituted tetrahydrofurans by means of a multicomponent reaction based on this palladium-mediated cyclization process was reported by Balme and co-workers [94]. In this reaction, formation of enolate 103 by the initial 1,4-addition of an allylic alcohol to the conjugate acceptor 102 is followed by a... 

Boger reported studies on palladium-mediated cyclization to form the CDE ring system of lavendamycin, as shown in Eq. (2) [74-76]. These reactions were conducted with stoichiometric amounts of [Pd(PPh3)4] (2). When used in a 1 mol% quantity, 2 failed to catalyze these reactions, presumably because of the absence of a base. Until almost 10 years later, no palladium-catalyzed animation chemistry was reported, and few citations of the early amination chemistry existed. 

Abstract This review gives an insight into the growing field of transition metal-catalyzed cascades. More particularly, we have focused on the construction of complex molecules from acyclic precursors. Several approaches have been devised. We have not covered palladium-mediated cyclizations, multiple Heck reactions, or ruthenium-catalyzed metathesis reactions because they are discussed in others chapters of this book. This manuscript is composed of two main parts. In the first part, we emphasize cascade sequences involving cycloaddition, cycloisomerization, or ene-type reactions. Most of these reaction sequences involve a transition metal-catalyzed step that is either followed by another reaction promoted by the same catalyst or by a purely thermal reaction. A simple change in the temperature of the reaction mixture is often the only technical requirement to go from one step to another. The second part covers the cascades relying on transition metalo carbenoid intermediates, which have recently undergone tremendous... 

Another interesting tandem Michael initiated sequence was developed in our laboratory by combining the conjugate addition of unsaturated alkoxides to alkylidene malonates with a palladium-mediated coupling reaction with an organic halide. In this cyclization reaction, an organopalladium species acts as the electrophilic partner of the cyclization. This reaction results in the trans addition of the organopalladium species and of the nucleophile across the unsaturation, and therefore, in overall difunctionalization of the unsaturated substrates [66,67]. 

Another palladium-mediated cyclization enables 2-iodoaniline to be converted at 140 to quinoline by reaction with a substituted allyl alcohol in the presence of triphenylphosphine, HMPT and hydrogencarbonate. Allyl alcohol itself gives poor results in this reaction. 

Transition metals, especially palladium, have been used for the stereoselective synthesis of these alkaloids with remarkable success. Since Overman s approach to (+)-tazettine (62) using an intramolecular Heck reaction (225), this methodology for the construction of the critical quaternary stereogenic center has been implemented for the preparation of many alkaloids. Grigg later reported access to (R,R)-crinane via regiospecific palladium-mediated cyclization (226). An efficient total synthesis of racemic galanthamine (75) following this approach has been recently disclosed (227). After the key intramolecular Heck reaction, deprotection of the... 

When the reaction conditions were applied to allylic carbonate 8, the palladium-mediated cyclization occurred to form the 11-membered ketone 9 in 86% yield (eq 30). When the palladiun-catalyzed allylation of propargyl malonate 10 in the presence of BSA was combined with the rhodium-catalyzed Pauson-Khand t)fpe reaction, the tandem action of the two catalysts gave an excellent yield (92%) of bicyclopentenone 11 in one-pot (eq 31). ... 

The capacity to effect direct insertion of a C-O or C-N multiple bond into a carbon-palladium bond has been exploited in a Pd(OAc)2-mediated cyclization reaction of alk)fnes containing tethered aldehyde, ketone or nitrile groups. Such processes can result in the formation of tetrahydrofurans incorporating a... 

The intramolecular Heck reaction presented in Scheme 8 is also interesting and worthy of comment. Rawal s potentially general strategy for the stereocontrolled synthesis of the Strychnos alkaloids is predicated on the palladium-mediated intramolecular Heck reaction. In a concise synthesis of ( )-dehydrotubifoline [( )-40],22 Rawal et al. accomplished the conversion of compound 36 to the natural product under the conditions of Jeffery.23 In this ring-forming reaction, the a-alkenylpalladium(n) complex formed in the initial oxidative addition step engages the proximate cyclohexene double bond in a Heck cyclization, affording enamine 39 after syn /2-hydride elimination. The latter substance is a participant in a tautomeric equilibrium with imine ( )-40, which happens to be shifted substantially in favor of ( )-40. 

Banfi L, Basso A, Cemlli V, Guanti G, Riva R (2008) Polyfunctionalized pyrrolidines by Ugi multicomponent reaction followed by palladium mediated Sfj2 cyclizations. J Org Chem 73 1608-1611... 

Ohkubo et al. reported the synthesis of the arcyriaflavins B (346), C (347), and D (348) involving a base-induced indolylation of dibromo-Af-methylmaleimide 1420 and a palladium-mediated oxidative cyclization of the bisindolylmaleimides 1428, 1429, and 1430 as key steps (337). The reaction of 6-benzyloxyindole (1419) and... 

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