The Total Synthesis of Iboga Alkaloids<sup>1</sup>

Büchi, G.; Coffen, David L.; Kocsis, K.; Sonnet, Philip E.; Ziegler, Frederick E.

doi:10.1021/ja00965a039

Cited by 99 publications

(37 citation statements)

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“…The structures of 22 and 23 were determined by Taylor and co-workers in 1957 who employed both chemical degradation and X-ray crystallographic analysis [17], and the absolute configuration of 22 was established by Bla¬ha and co-workers [18]. Following the first synthesis of (AE)-ibogamine by the B¸chi group in 1965 [19], numerous routes to the racemic alkaloid have been published [20], including one by Sallay [21], which began with a DielsÀAlder cycloaddition to 1,4-benzoquinone. Only one asymmetric pathway to ibogamine has been reported, and this concluded at a 80 : 20 mixture favoring the ()-enantiomer [22].…”

mentioning

confidence: 99%

Catalyzed Asymmetric DielsAlder Reactions of Benzoquinone. Total Synthesis of (−)‐Ibogamine

White

Choi

2002

Helvetica Chimica Acta

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Dedicated to Professor Dieter Seebach on the occasion of his 65th birthday and in recognition of his numerous contributions to asymmetric synthesis.The DielsÀAlder reaction of 1,4-benzoquinone with 1,3-dienes catalyzed by Mikami×s [Ti{(S)-binol(2 À)}Cl 2 ] complex (binol [1,1'-binaphthalene]-2,2'-diol) gives cycloadducts in good yield and in high enantiomer excess. A model is proposed to explain the observed absolute configuration of cycloadducts, and the reaction is used as the key step in an asymmetric synthesis leading to the alkaloid (À)-ibogamine.1. Introduction. ± The asymmetric DielsÀAlder reaction has become one of the most widely used constructs for the assembly of enantiomerically enriched chiral carbocycles [1]. In its catalyzed version, the reaction offers an especially powerful synthetic method, which can lead to adducts from achiral materials in good yield and high enantiomer excess (ee) at or below room temperature [2]. Most catalyzed cycloadditions have relied on two-point ligation of an achiral dienophile with the chiral catalyst to form a relatively rigid, highly asymmetric, activated complex [3]. Typical dienophiles in this class are b-dicarbonyl compounds, where chelation with a metal such as titanium bound to a chiral ligand creates a complex in which only one face of the dienophile is exposed for reaction with its diene partner. As a result, asymmetric DielsÀAlder additions in this mode can be very efficient. The asymmetric DielsÀAlder reaction with a dienophile such as a simple a,b-unsaturated ketone or a 1,4-quinone raises the question of whether stereocontrol can be exercised effectively by single-point ligation of a chiral catalyst with the lone pair of electrons of only one carbonyl group. This question was first answered in the affirmative by Mikami and co-workers who showed that the reaction of 1,4-naphthoquinone with 1-methoxybuta-1,3-diene in the presence of a [Ti IV {binol(2 À)}] catalyst (binol [1,1'-naphthalene]-2,2'-diol) led to diketone 1 in high ee (Scheme 1) [4]. Subsequently, we found that benzoquinone in the presence of the Mikami catalyst underwent DielsÀAlder cycloaddition with a 1,3-diene derivative in similarly high ee [5]. Other examples from the laboratories of Corey [6] and Nicolaou [7] illustrate the participation of substituted benzoquinones in asymmetric DielsÀAlder reactions catalyzed by Mikami×s [Ti{binol(2 À)}Cl 2 ] system, although the argument for single-point ligation in these cases is less clear. Recently, it was shown that a,b-unsaturated ketones, for which single-point ligation is obligatory, undergo DielsÀAlder addition to cyclopentadiene in the presence of a chiral imidazolidinone to give cycloadducts in moderate to high ee; however, the generality

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confidence: 99%

Catalyzed Asymmetric DielsAlder Reactions of Benzoquinone. Total Synthesis of (−)‐Ibogamine

White

Choi

2002

Helvetica Chimica Acta

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“…40,41 The tosylhydrazone of the endo compound was separated out as a crystalline solid from a mixture of endo/exo products. The endo-isomer was wrongly assigned to exo-isomer by Krow and co-workers and the configuration of the ethyl group was confirmed by Hodgson's group based on the single-crystal X-ray structure.…”

Section: Methodsmentioning

confidence: 99%

“…Büchi's synthetic route toward ibogamine following a cationic cyclization of 64 by PTSA and hot acetic acid was simple but it proceeded via the formation of rearrangement product 65 that was discussed in a subsequent paper. 41 The rearranged product 65 was then reduced with LiAlH 4 , followed by oxidation at C-19; a subsequent elimination afforded the α,β-unsaturated ketone 66. Interestingly, compound 66 underwent rearrangement in the presence of Zn/HOAc to yield the desired scaffold which was then converted to ibogamine and its epimer by Wolff-Kishner reduction (Scheme 2).…”

Section: Electrophilic C2-c16 Cyclization Approachmentioning

confidence: 99%

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Progress in the Synthesis of Iboga-alkaloids and their Congeners

Jana

Paul

Sinha

2011

Organic Preparations and Procedures International

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“…Ibogamine (360) and ibogaine (361) were isolated from Tabernanthe iboga, and their structures were determined by X-ray analysis [346] and total synthesis [347]. …”

Section: Ibogaminementioning

confidence: 99%