Claisen rearrangement of allyl esters
01 Aug 1972-Journal of the American Chemical Society (American Chemical Society)-Vol. 94, Iss: 16, pp 5897-5898
About: This article is published in Journal of the American Chemical Society.The article was published on 1972-08-01. It has received 303 citations till now. The article focuses on the topics: Carroll rearrangement & Sigmatropic reaction.
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TL;DR: The main findings are: Lanthanide(II) Triflates in Organic Synthesis inorganic Synthesis 2295 10.2.1.
Abstract: 2.1.8. Michael Reaction 2245 2.1.9. Others 2247 2.2. Cyclization Reactions 2248 2.2.1. Carbon Diels−Alder Reactions 2248 2.2.2. Aza-Diels−Alder Reactions 2252 2.2.3. Other Hetero-Diels−Alder Reactions 2255 2.2.4. Ionic Diels−Alder Reaction 2256 2.2.5. 1,3-Dipolar Cycloadditions 2256 2.2.6. Other Cycloaddition Reactions 2258 2.2.7. Prins-type Cyclization 2259 2.3. Friedel−Crafts Acylation and Alkylation 2259 2.4. Baylis−Hillman Reaction 2263 2.5. Radical Addition 2264 2.6. Heterocycle Synthesis 2267 2.7. Diazocarbonyl Insertion 2270 3. C−X (X ) N, O, P, Etc.) Bond Formation 2271 3.1. Aromatic Nitration and Sulfonylation 2271 3.2. Michael Reaction 2272 3.3. Glycosylation 2273 3.4. Aziridination 2275 3.5. Diazocarbonyl Insertion 2276 3.6. Ring-Opening Reactions 2277 3.7. Other C−X Bond Formations 2280 4. Oxidation and Reduction 2280 4.1. Oxidation 2280 4.2. Reduction 2281 5. Rearrangement 2283 6. Protection and Deprotection 2285 6.1. Protection 2285 6.2. Deprotection 2288 7. Polymerization 2291 8. Miscellaneous Reactions 2291 9. Lanthanide(II) Triflates in Organic Synthesis 2295 10. Conclusion 2295 11. Acknowledgment 2295 12. References 2295
923 citations
753 citations
TL;DR: Trisubstituted alkenes have been prepared for the first time via intermolecular olefin cross-metathesis, using 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene ruthenium alkylidene complexes 3a,b in good yields with moderate E selectivity.
282 citations
TL;DR: While the chemistry of other heteroatom-substituted allenes is of high impact and value to organic synthesis, allenamines should prove to be more attractive for developing stereoselective methodologies as well as rapid assembly of structural complexity.
Abstract: In the past four decades, allenes have progressively risen from an unenviable status of being a structural curiosity to becoming one of the most powerful and versatile synthetic building blocks in organic synthesis.1–3 Although the focal theme of this review is centered on chemistry of allenamides, a proper introduction would need to commence with allenamines. Allenamides are functionally derived from allenamines,4 which along with structurally related systems such as allenol ethers5 and allenyl sulfides,6 can be classified as heteroatom-substituted allenes. Allenamines have been known for more than forty years since the first documentation of their preparations and characterizations in 1968 by Viehe.7 It is noteworthy that Viehe was at the time developing a based-catalyzed isomerization of propargyl amines as a useful protocol for synthesizing ynamines (Scheme 1), which had just come onto the scene as a useful synthetic building block.8–10 Allenamines were postulated as an intermediate en route to ynamines in this prototropic isomerization that follows essentially the zipper-type mechanism.
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Scheme 1
The π-donating ability of nitrogen atom renders allenamines more electron-rich than simple allenes, thereby predisposing them to electrophilic activations. An electronic bias can be exerted through delocalization of the nitrogen lone pair toward the allenic moiety as demonstrated in the resonance form of allenamines. Accordingly, highly regioselective transformations can be achieved with consecutive addition of electrophiles and nucleophiles (Scheme 2). In addition to aforementioned regiochemical control, allenamines also offer a number of other advantages over simple allenes. The trivalent nature of the nitrogen atom allows: (1) Tethering of a chirality-inducing unit for providing stereochemical induction; concomitantly with the inclusion of a coordinating unit to provide conformational rigidity; (2) a much greater flexibility in designing intramolecular reactions or tandem processes than with oxygen- or sulfur-substituted allenes; and last but not the least, (3) a novel entry to alkaloids if the nitrogen atom can be preserved throughout the entire transformative sequence. Moreover, intramolecular reaction manifolds as shown with a possible diastereoselective cyclopropanation reaction (Scheme 2) can greatly manifest these remarkable features, particularly the latter two. Therefore, while the chemistry of other heteroatom-substituted allenes is of high impact and value to organic synthesis, allenamines should prove to be more attractive for developing stereoselective methodologies as well as rapid assembly of structural complexity.1,2
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Scheme 2
Without illustrating any specifics here on allenamine chemistry given all the comprehensive reviews,1,2 elegant precedents adopting allenamines in a range of transformations have indeed been documented to further support their synthetic potential and provoke interest from the synthetic community. Unfortunately, further developments had been severely thwarted because allenamines are also highly sensitive toward hydrolysis with a tendency to polymerize even at low temperatures (Scheme 3), thereby creating serious difficulties in their preparation and experimental handling.1,2 Consequently, the great potential of chemistry of nitrogen-substituted allenes could only be partially realized. Therefore, efforts to identify an allenamine-equivalent should be of high significance if it can strike the right balance between stability and reactivity.
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Scheme 3
Toward this end, allenamides should represent ideal candidates as a stable allenamine-equivalent. Delocalization of the nitrogen lone-pair into the electron-withdrawing amido group should diminish its donating ability toward the allenic moiety, thereby leading to improved stability (Scheme 4). In short, the very simple fact that allenamides can champion an extra resonance form speaks volume of its superior stability over allenamines. It could be a great story if allenamides were a result of some clever design in search for a stable allenamine-equivalent. However, this is not true and the story is much less dramatic. Allenamides have co-existed along side of allenamines for all of the last four plus decades after Dickinson's first preparation and concise characterizations of 1,2-propadienyl-2-pyrrolidinone in 1967 (Scheme 5).11
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Scheme 4
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Scheme 5
In fact, Dickinson coined the term “allenamide” to describe 1,2-propadienyl-2-pyrrolidinone based the analogy of using enamides17 for Stork’s N-acylated enamines. To clarify reports by Cho12 and others,13 Dickinson concisely demonstrated that treatment of 2-pyrrolidinone with NaH and propargyl bromide had indeed led to the allenamide as the major and stable product also via the same prototropic isomerization pathway. Intriguingly, unlike Viehe’s work, allenamide did not undergo further isomerization to the respective ynamide, although with further treatment of NaOMe and pyrrolidine, ynamide was postulated as an intermediate en route to the N-acyl-pyrrolidine product. Nevertheless, this documentation of ynamide actually predated Viehe’s 1972 account,14 and chemistry of ynamides has indeed generated an immense amount of interest from the synthetic community in the last 15 years.15,16
To align with the history, our foray into this field coincides with both of Viehe and Dickinson’s work. In search of a useful synthetic method to construct chiral ynamides 16 years ago,10 we found that based-catalyzed prototropic isomerization of propargyl amides reliably arrested at the allenic stage and gave none of the desired ynamides10 (Scheme 6) regardless of nature of the base used, temperature, and solvents (also see Schemes 24 and and2525 vide infra). More importantly, to properly acknowledge a critical person in our entire endeavor in allenamide chemistry, I owe everything to the very first postdoctoral research fellow in my group, Dr. Lin-Li Wei [Ph.D. with Professor Teck-Peng Loh at National University of Singapore]. Dr. Wei, who was working on these isomerizations, pointed out that these allenamides that she had obtained could prove to be an excellent allenamine-equivalent, and evolve into highly versatile synthetic building blocks in organic synthesis.
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Scheme 6
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Scheme 24
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Scheme 25
Given the precedent, the ease of preparation, and stability, the most critical question would be whether these allenamides could possess sufficient reactivity. A survey of the literature indicates that although it was far from a blank page, allenamides have been much less explored relative to allenamines.4,18–20 Precise reasons are not very clear, but there were very few citations on synthesis and applications of allenamides before 1989. While few more reports appeared from late 1980’s to mid-1990’s, the real outburst in chemistry of allenamides came 16 years ago, just as we also became deeply involved in the development of allenamide chemistry. Such sustained emergence strongly suggests that allenamides have set the gold standard for balancing reactivity and stability. They are becoming proven allenamine-equivalents that can be employed in a diverse array of stereoselective and intramolecular reactions that were not possible with traditional allenamines. They represent the ideal platform for pushing the limit of synthetic potential of nitrogen-substituted allenes.
It is the purpose of this review to provide proper illustrations of the elegant chemistry involving allenamides that has come to pass, thereby eliciting a greater amount of interests from the synthetic community to create new allenamide chemistry. Lastly, this perspective that advancement of any field requires collective creativity and innovation from many people and not just a few individuals rings hollow here. On that note, although we are trying our very best to be comprehensive, it is likely that we have inadvertently missed some beautiful work for which we express our regret here in advance.
267 citations
TL;DR: A practical method for the diastereoselective generation of two adjacent quarternary stereocenters by combining a copper-catalyzed C-O coupling reaction with a subsequent Claisen rearranged in a one-pot domino process is described.
Abstract: A practical method for the diastereoselective generation of two adjacent quarternary stereocenters by combining a copper-catalyzed C−O coupling reaction with a subsequent Claisen rearrangement in a one-pot domino process is described. Furthermore, an experimentally simple method for the stereoselective synthesis of a variety of different types of vinyl ethers is delineated using the same catalyst system.
167 citations