Transition metal‐ and acid‐induced transformations of 6‐siloxy‐substituted 5,6‐dihydro‐4<i>H</i>‐1,2‐oxazines: Preparation of pyrroles, nitrones, 1,4‐dicarbonyl compounds and derivatives thereof

Hippeli, Claudia; Zimmer, Reinhold; Reißig, Hans‐Ulrich

doi:10.1002/jlac.199019900189

Cited by 23 publications

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References 20 publications

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“…Pyrroles can be synthesized from dihydrooxazines 145 containing a heteroatomic substituent at C-6, for example, when X is an alkoxy, siloxy, or amino group (Scheme ). Nakanishi , and Reissig , showed that dihydrooxazines 145 are smoothly transformed into pyrroles 146 upon the action of metal carbonyls [Mo(CO) 6 , Fe(CO) 5 , Fe 2 (CO) 9 , and Fe 3 (CO) 12 ] (Scheme ). The best yields are obtained when Mo(CO) 6 or Fe 3 (CO) 12 in the presence of trifluoroacetic acid is employed.…”

Section: Chemical Properties Of Scoementioning

confidence: 99%

“…The intermediate 147 is suggested to result from the elimination of HX molecule from the starting dihydrooxazine 145 . The second variant postulates the initial cleavage of the weak N−O bond in oxazine 145 with carbonyl M(CO) n to form complex 149 . The latter decomposes upon the action of acid HX, giving aldimine 150 , which further is cyclized to the final pyrrole 146 .…”

Section: Chemical Properties Of Scoementioning

confidence: 99%

“…In particular, deoximation of 6-hydroxy-substituted SCOE of type 60 can be considered as an interesting pathway for the synthesis of 1,4-dicarbonyl compounds. The deoximation of SCOE is carried out upon the action of cerium ammonium nitrate (Scheme , eq 1) or upon heating in aqueous formaldehyde (Scheme , eq 2) . In the case of 6-siloxy-substituted SCOE, preliminary removal of the silyl group is performed.…”

Section: Chemical Properties Of Scoementioning

confidence: 99%

“…6-Alkoxy and 6-siloxy substituted dihydrooxazines 1 also can undergo rearrangement to five-membered cyclic nitrones upon the action of acids, as is shown in Scheme . 6-Ethoxydihydrooxazine 181 upon treatment with hydrochloric acid in methanol gives nitrone 182 in good yield.…”

Section: Chemical Properties Of Scoementioning

confidence: 99%

“…Another pathway for the fragmentation of SCOE is also possible, that is, the cleavage of the N−O and C(3)−C(4) bonds in the unstable intermediate 4 H -1,2-oxazines 212 (Scheme ). ,,, In particular, this occurs when bicyclic dihydrooxazines 211 are treated with TFA and leads to high yields of unsaturated ketones 213 and benzonitrile. Obviously, a retro-[4 + 2]-cycloaddition of unstable 4 H -1,2-oxazines 212 takes place, which emerge after elimination of trimethylsilanol from the starting SCOE 211 upon the action of CF 3 CO 2 H.…”

Section: Chemical Properties Of Scoementioning

confidence: 99%

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Chemistry of Six-Membered Cyclic Oxime Ethers. Application in the Synthesis of Bioactive Compounds

2011

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CONTENTS 1. Introduction 5004 2. Synthesis of Six-Membered Cyclic Oxime Ethers 1 and 2 5005 2.1. [4 þ 2]-Cycloaddition Reactions 5005 2.2. Intramolecular Cyclization of γ-Functionalized Oximes 5009 2.3. Intramolecular Cyclization of Functionalized O-Substituted Hydroxylamines 5012 2.4. Synthesis of SCOE from N-O-Containing Heterocyclic Derivatives 5012 2.4.1. Synthesis of SCOE from Other 1,2-Oxazines 5012 2.4.2. Synthesis of SCOE from Isoxazolines and Five-Membered Cyclic Nitronates 5014 2.5. Other Methods 5014 3. Chemical Properties of SCOE 5016 3.1. Reduction of SCOE 5016 3.1.1. Reduction of SCOE to Tetrahydro-2H-1,2oxazines 5018 3.1.2. Reduction of SCOE with 1,2-Oxazine Ring-Opening 5020 3.1.2.1. Reduction of SCOE to 1,4-Amino Alcohols. 5020 3.1.2.2. Reduction of SCOE to Carbonyl Compounds. 5021 3.1.2.3. Reduction of SCOE to γ-Hydroxyoximes. 5022 3.1.3. Reduction Reactions of SCOE with 1,2-Oxazine Ring Contraction 5022 3.1.3.1. Reduction of SCOE to Pyrrolidines. 5022 3.1.3.2. Reduction of SCOE to N-Hydroxypyrrolidines. 5023 3.1.3.3. Reduction of SCOE to Pyrroles. 5024 3.1.3.4. Reduction of SCOE to Aziridines. 5025 3.1.3.5. Reduction of SCOE to Furan Derivatives. 5025 3.2. Addition Reactions to the CdN Double Bond of SCOE 5026 3.3. 1,2-Oxazine Ring-Opening in SCOE 5027 3.4. Deoximation of SCOE 5028 3.5. 1,2-Oxazine Ring Rearrangements in SCOE 5028 3.6. 1,2-Oxazine Ring Fragmentation Reactions in SCOE 5030 4. SCOE in the Synthesis of Natural Products and Biologically Active Compounds 5031 4.1. Synthesis of Amino Acids 5032 4.2. Synthesis of Polyhydroxylated Pyrrolidines 5033 4.3. Synthesis of Pyrrolizidine and Indolizidine Alkaloids and Their Analogs 5033 4.4. Synthesis of Biologically Active 2-Aryl-Substituted Pyrrolidines and Amines 5035 4.5. Synthesis of Other Natural Compounds 5036 5. Conclusion 5037 Author Information 5037 Biographies 5038 Acknowledgment 5038 References 5038

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Section: Chemical Properties Of Scoementioning

confidence: 99%

Section: Chemical Properties Of Scoementioning

confidence: 99%

Section: Chemical Properties Of Scoementioning

confidence: 99%

Section: Chemical Properties Of Scoementioning

confidence: 99%

Section: Chemical Properties Of Scoementioning

confidence: 99%

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Chemistry of Six-Membered Cyclic Oxime Ethers. Application in the Synthesis of Bioactive Compounds

2011

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On the mechanism of diels‐alder reactions of nitroso alkenes: exo/endo selectivity, stereospecificity, E/Z selectivity, and relative reactivity of various olefins

1990

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The mechanism of the hetero Diels-Alder reaction of nitroso alkenes 2 with silyl enol ethers and other olefins has been investigated. Using the bicyclic nitroso compound 2a a study of the exolendo selectivity has demonstrated that the ex0 approach is preferred with the siloxyethene l a as dienophile. On the other hand, the siloxycyclopentene 1 c gives a mixture of cycloadducts 3c with an excess of endo product (endo: exo 82: 18). The stereospecificity of the nitroso alkene cycloaddition could be demonstrated with the stereochemically homogeneous silyl enol ethers 1 b and 1 d. Experiments with enol ethers 1 f and 1 g also occur stereospecifically. a-Nitrososty-rene 2b reveals surprisingly high kEIz values when EIZ-isomeric olefins are compared in competition experiments. Also, a detailed reactivity scale of 2b including various structurally different silyl enol ethers and other typical dienophiles shows that a complex interplay of electronic and steric effects is operating. The large influence of steric effects is taken as evidence for a highly ordered transition state in the cycloaddition. All mechanistic details for the Diels-Alder reactions of nitroso alkenes 2 with (silyl) enol ethers are in strong accord with a concerted mechanism and exclude the involvement of zwitterions or diradicals as intermediates.In a previous account3' we have reported on the regioselective synthesis of 6-siloxy-substituted 5,6-dihydro-4H-1,2-0xazines (abbreviated as 1,2-oxazines) from silyl enol ethers 1 and nitroso alkenes 2. According to Gilchrist4), reactive intermediates 2 are easily generated in situ by base treatment of the corresponding a-halogen oximes. R G R RSSiO 2Characteristic features of this cycloaddition are high yields and a broad variability with respect to component 1. For this reason it can serve as an ideal model reaction for the study of all relevant mechanistic details of a Diels-Alder reaction with inverse electron demand5). Although several aspects have been investigated by Gilchrist and coworkers6), not all of their results are conclusive (e.g. stereospecificity of the cycloaddition). For the anticipated synthetic use') of 1,2oxazines, however, full understanding of the mechanism is an indispensable prerequisite. In this paper we will deal with the stereospecificity, the exolendo and EIZ selectivity of nitroso alkenes as well as with the relative reactivity of silyl enol ethers 1, or other suitable olefins, towards 2. exolendo Selectivity of 3,CDihydro-1 -nitrosonaphthaleneIn an intramolecular nitroso alkene cycloaddition an endo-selective reaction has been reported (exo: endo = 1 : 3.4)8). To study this stereochemical feature in the geometrically less restricted intermolecular reaction we have employed 3,bdihydro-1 -nitrosonaphthalene (2a). This compound incorporates an E-nitroso alkene moiety and is generated from the bromo oxime 4a, which can easily be prepared from a-tetralone. 3a l a R = H l b CH3 Na2C03 2al \ (exo/endo > 97:3) 4a 3 b (exo/endo -64:36)Cycloaddition of 2a to trimethylsiloxy ethene (1 a) a...

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Lithiated 5,6‐Dihydro‐4H‐1,2‐oxazines: Synthesis, Highly Diastereoselective Reactions with Electrophiles, and Subsequent Transformations

Reißig

Hippeli

1991

Chem. Ber.

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The 6-(trimethylsilyl)methyl-substituted 1,Zoxazine 1 can smoothly be deprotonated with n-butyllithium at C-4 to give a lithiated species which reacts with a variety of electrophiles to provide the new 1,2-oxazines 5 -16 in good yields. Besides the preparative aspect of these transformations, the high stereoselectivity of many reactions is also interesting from a mechanistic point of view. By deprotonation of the 4-deuterated compound 5a it has been proven that n-butyllithium removes exclusively the proton (or deuteron) cis to the 6-CH2SiMe3 group. Also, in most cases the reaction of lithiated 1 with electrophiles occurs with overall retention of configuration to afford preferentially cis-1,2-oxazines (series a). A mechanistic proposal for this highly stereoselective deprotonation process, which seems to be governed by the 1,2-oxazine oxygen, is discussed including a comparison with a recently reported ab initio calculation dealing with oxime ethers. Similar deprotonation/substitution reactions are described for l,2-oxazines 14, 2, 3, and 4. Possibly due to a differing carbanion structure a deviating behavior is observed in some cases. Several acidinduced and reductive ring-opening reactions of 1,6a, Ea, and 14a demonstrate the potential of 4-substituted 1,Zoxazines for the stereoselective synthesis of polyfunctionalized compounds.5,6-Dihydro-4H-l ,Zoxazines A (herein abbreviated as 1,2-oxazines) are heterocycles with a promising potential for the synthesis of polyfunctional compounds2). They can easily be prepared by the hetero-Diels-Alder reaction of electronrich olefins with nitroso alkenes as developed by Gilchrist and coworkers3). By use of silylated dienophiles we were able to extend the scope of this [4 + 21 cycloaddition considerably'). In this contribution we will include our results on the substitution of 1,2-oxazines A proceeding via the carbanion B to give heterocycles C which incorporate an electrophile El at C-4 and are not easily available otherwise.The analogous sequence is well-known for oxime ethers D5) or the related isoxazolines Eq. However, for the sixmembered heterocyclic system A we are only aware of Shatzmiller's reports dealing with compound F and the regioselectivity of its deprotonation'). We have examined the feasibility of the sequence A-+B-+C using the monocyclic 1,2-oxazines 1 and 2, and after having discovered the surprisingly high stereoselectivity we have extended our investigation to the bicyclic compounds 3 and 4. R = H4 R -SiMq

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Transition metal‐ and acid‐induced transformations of 6‐siloxy‐substituted 5,6‐dihydro‐4H‐1,2‐oxazines: Preparation of pyrroles, nitrones, 1,4‐dicarbonyl compounds and derivatives thereof

Cited by 23 publications

References 20 publications

Chemistry of Six-Membered Cyclic Oxime Ethers. Application in the Synthesis of Bioactive Compounds

Chemistry of Six-Membered Cyclic Oxime Ethers. Application in the Synthesis of Bioactive Compounds

On the mechanism of diels‐alder reactions of nitroso alkenes: exo/endo selectivity, stereospecificity, E/Z selectivity, and relative reactivity of various olefins

Lithiated 5,6‐Dihydro‐4H‐1,2‐oxazines: Synthesis, Highly Diastereoselective Reactions with Electrophiles, and Subsequent Transformations

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