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Diastereoselective Allylation of Carbonyl Compounds and Imines: Application to the Synthesis of Natural Products Miguel Yus,* Joséc

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Diastereoselective Allylation of Carbonyl Compounds and Imines: Application to the Synthesis of Natural Products Miguel Yus,* Joséc Review pubs.acs.org/CR Diastereoselective Allylation of Carbonyl Compounds and Imines: Application to the Synthesis of Natural Products Miguel Yus,* JoséC. Gonzalez-Gó mez,́ and Francisco Foubelo* Departamento de Química Organica,́ Facultad de Ciencias and Instituto de Síntesis Organicá (ISO), Universidad de Alicante, Apdo. 99, 03080 Alicante, Spain 4.2.1. Substrate Control 5651 4.2.2. Reagent Control 5652 5. Synthesis of Natural Products 5655 5.1. Substrate Control 5655 5.1.1. Chiral Carbonyl Compounds 5655 5.1.2. Chiral Imines and Imine Derivatives 5662 5.1.3. Carbonyl Compounds with Chiral Auxil- iaries Attached to the Oxygen Atom 5664 5.2. Reagent Control 5667 5.2.1. Chiral Nucleophiles with Stereogenic Centers in the Transferred Hydrocarbon CONTENTS Backbone 5667 5.2.2. Chiral Nucleophiles with Stereogenic 1. Introduction 5595 Centers in Nontransferred Ligands 2. Substrate Control 5597 Bonded to the Metal Center 5669 2.1. Chiral Carbonyl Compounds 5597 5.2.3. Double Allylation under Reagent Con- 2.1.1. Allyl Stannanes 5597 trol 5687 2.1.2. Allyl Silanes and Boranes 5599 5.2.4. Allyl Transfer from a Chiral Homoallyl 2.2. Chiral Imines and Imine Derivatives 5600 Amine 5688 2.2.1. Imines and Imine Derivatives from 5.3. Propargylation and Allenylation Reactions 5688 Chiral Carbonyl Compounds 5600 5.3.1. Propargylation Reactions 5688 2.2.2. Imines and Imine Derivatives with a 5.3.2. Allenylation Reactions 5691 Chiral Framework Attached to the 6. Conclusions and Outlook 5691 Nitrogen 5602 Author Information 5691 2.3. Carbonyl Compounds with Chiral Auxiliaries Corresponding Author 5691 Attached to the Oxygen Atom 5612 Notes 5691 3. Reagent Control 5614 Biographies 5691 3.1. Chiral Nucleophiles with Stereogenic Cen- Acknowledgments 5692 ters in the Transferred Hydrocarbon Back- Abbreviations 5692 bone 5614 References 5693 3.1.1. α-Substituted Allyl Boranes 5614 3.1.2. Cyclic α-Substituted Allyl Boronates 5615 3.1.3. Acyclic α-Substituted Allyl Boronates 5617 3.1.4. α-Substituted Allyl Stannanes 5620 1. INTRODUCTION 3.1.5. Allyl Zinc Reagents 5620 The addition of an allylic organometallic compound to a 3.2. Chiral Nucleophiles with Stereogenic Cen- carbonyl compound or an imine is of great synthetic interest ters in Nontransferred Ligands Bonded to because in this reaction together with a new functionality the Metal Center 5622 (hydroxyl or amino group, respectively), a carbon−carbon 3.2.1. Allyl Boron Reagents 5622 bond is formed. In addition, the double bond of the allylic 3.2.2. Allyl Silanes 5629 moiety can participate in a number of further synthetically 3.2.3. Allyl Titanium Reagents 5637 useful transformations: cycloaddition, dihydroxylation, epox- 3.3. Double Stereoselection 5637 idation, hydroboration, hydroformylation, hydrogenation, hy- 3.4. Chiral Donors in Allyl-Transfer Reactions 5638 dration, olefin metathesis, ozonolysis, etc.1 Importantly, if the 3.4.1. Chiral Homoallyl Alcohols 5638 allylations are carried out in a stereoselective fashion, 3.4.2. Chiral Homoallyl Amines 5641 enantioenriched homoallylic alcohols and amines would be 4. Propargylation and Allenylation Reactions 5643 produced, which are valuable building blocks.2 Among the 4.1. Propargylation Reactions 5643 stereoselective methodologies, the catalytic enantioselective 4.1.1. Substrate Control 5643 4.1.2. Reagent Control 5647 Received: January 10, 2013 4.2. Allenylation Reactions 5651 Published: March 29, 2013 © 2013 American Chemical Society 5595 dx.doi.org/10.1021/cr400008h | Chem. Rev. 2013, 113, 5595−5698 Chemical Reviews Review Scheme 1 allylations3 rely on the use of both chiral Lewis acids,4 which the allylation step is often performed on chiral substrates (chiral bind to the electrophile activating it toward nucleophilic attack, pool or advanced intermediates) that can override the chiral and chiral Lewis bases.5 Double activation could be also induction of the catalyst. Some of these reasons are behind of achieved by using chiral bifunctional catalysts.6 In this case, the the fact that in the synthesis of complex organic molecules, simultaneous activation of both electrophilic and nucleophilic including natural products, the stereoselective allylations are reaction partners occurs ideally through a cooperative action of more commonly performed with stoichiometric amounts of different functionalities of the ligand. Although the develop- chiral reagents. In these reactions, the stereochemical ment of catalytic enantioselective allylation reactions is a very information can be transferred by substrate diastereocontrol attractive growing field, it is not always straightforward to find (substrate control), including chiral auxiliaries, or through the an efficient and practical protocol for any synthetic application. use of chiral reagents (reagent control). Sometimes a double Some challenges that still limit the applicability of catalytic induction could be involved in the process, a match/mismatch enantioselective allylation are as follows: (a) some of the effect being possible (Scheme 1). reported catalytic methods make use of large excess of reagents In addition to the face selectivity under the influence of a to ensure the turnover of the catalyst, which are not always chiral substrate or reagent, substituted allyl organometallics cheap and green; (b) when the activation mode does not display high levels of diastereoselection because they react significantly increase the reaction speed, the noncatalytic usually at the γ-position through an ordered acyclic or cyclic allylation causes a lower enantioselection; (c) the “plausible transition state, depending mainly on the metal of the allylic mechanisms” reported for the existing catalytic methods do not organometallic nucleophile. For Si and Sn derivatives, the always allow good predictions of the stereochemical results of addition is commonly explained using acyclic models where the new substrates; (d) in the construction of complex molecules, major approach (antiperiplanar or synclinal) takes place 5596 dx.doi.org/10.1021/cr400008h | Chem. Rev. 2013, 113, 5595−5698 Chemical Reviews Review through the conformation where destabilizing gauche inter- Table 1. Allylation of α-Benzyloxyaldehydes with Stannane 1 actions are minimized. In contrast, for Mg, Ti, B, and In allylic in the Presence of SnCl4 derivatives, a cyclic six-membered Zimmerman−Traxler7 type transition state is usually invoked. In the cyclic model, it is generally proposed that aldehydes locate the H (R2 = H) in the axial position, while aldimines with E-configuration place the H (R2 = H) in the equatorial position (Scheme 1),8 consequently, opposite relative configurations (anti/syn)aregenerally observed in the reaction of aldehydes and aldimines with the same γ-substituted allyl organometallic. The goal of this review is to highlight diastereoselective allylations involving the use of chiral reagents, emphasizing recent developments of synthetic interest. The review is organized according to the source of stereocontrol: first substrate control and after that reagent control allylations will be studied [stereogenic center(s) could be in the allyl unit or in a ligand bonded to the metal atom]. The last section of this review will be dedicated to related propargylation/allenylation processes, and to the application of these methodologies to some selected synthesis of natural products. Diastereoselective allylations leading to racemic products will not be considered in this review. The present work will comprehensively cover the most pertinent contributions to this important research area from 2003 to the end of 2011. We regret in advance that some contributions are excluded in order to maintain a concise transition state, which is formed after transmetalation, operates format, especially concerning the natural product synthesis when 1 equiv of SnCl4 is used. The tin atom would be chelated section. to the benzyl ether and to the carbonyl oxygen of the aldehyde, and the carbamate would thus adopt a pseudoaxial orientation 2. SUBSTRATE CONTROL in order to interact with the tin. The syn−anti triols would be Nucleophilic addition to chiral carbonyl compounds and imines produced after allyl transfer through this transition state (Figure is governed by steric and electronic factors and occurs 1). On the other hand, an open transition state would operate predominantly to the less hindered face of the prostereogenic unit. Efficiency, regarding the stereoselectivity, depends strongly on the bulkiness of the reactants and the nature of the nucleophilic species. Felkin−Anh and Cram-chelate models9 have been recurrently used in order to explain the stereochemical outcomes of these processes, and in many cases, the configuration of the newly created stereogenic center could be successfully predicted. 2.1. Chiral Carbonyl Compounds 2.1.1. Allyl Stannanes. Allylic stannanes are not reactive enough to add to aldehydes, and for that reason the allylation of carbonyl compounds with these nucleophiles must be performed in the presence of a Lewis or a Brønsted acid in order to increase the reactivity of the electrophile. ́ α Marko and co-workers found that the reaction of - Figure 1. Hypothetical transition state structures for the allylation of benzyloxyaldehydes with the functionalized stannane 1 in the α -benzyloxyaldehydes with 1 in the presence of SnCl4. presence of SnCl4 proceeded with remarkable levels of stereocontrol producing syn−anti and syn−syn configured triol units in almost enantiomerically pure form.10 The reactions when the reaction is performed
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