| 1. |
- Blid, Jan, 1970-
(författare)
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Asymmetric [2,3]-Sigmatropic Rearrangement of Allylic Ammonium Ylides
- 2005
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The thesis describes the realization of an asymmetric [2,3]-sigmatropic rearrangement of achiral allylic amines. It is divided into two parts; the first part deals with the development of a Lewis acid-mediated [2,3]-sigmatropic rearrangement and the second the asymmetric version thereof. Quaternization of an -amino amide with various Lewis acids established BBr3 and BF3 to be the most appropriate ones. Various allylic amines were subsequently rearranged into the corresponding homoallylic amines in good to excellent syn-diastereoselectivities, revealing the endo-transition state to be the preferred pathway. The structures of the intermediate Lewis acid-amine complexes were confirmed by NMR spectroscopy studies and DFT calculations. Based on this investigation a chiral diazaborolidine was chosen as Lewis acid and was shown to efficiently promote the asymmetric [2,3]-sigmatropic rearrangement furnishing homoallylic amines in good yields and excellent enantiomeric excesses. In contrast to the achiral rearrangement mediated by BBr3 and BF3, the asymmetric version gave the opposite major diastereomer, revealing a preference for the exo-transition state in the asymmetric rearrangement. To account for the observed selectivities, a kinetic and thermodynamic pathway was presented. On the basis of a deuterium exchange experiment on a rearranged Lewis acid-amine complex and an NMR spectroscopic investigation, the kinetic pathway was shown to be favored.
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| 2. |
- Blid, Jan, et al.
(författare)
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Asymmetric [2,3]-sigmatropic rearrangement of allylic ammonium ylides
- 2005
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 127:26, s. 9352-9353
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Tidskriftsartikel (refereegranskat)abstract
- An asymmetric Lewis acid-mediated [2,3]-sigmatropic rearrangement of allylic amines has been developed, affording the corresponding homoallylic amines in good yield and excellent enantioselectivities. The rearrangement proceeds by complexation of the chiral Lewis acid to the amine followed by deprotonation and rearrangement.
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| 3. |
- Blid, Jan, et al.
(författare)
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Lewis acid mediated asymmetric 2,3 -sigmatropic rearrangement of allylic amines. Scope and mechanistic investigation
- 2007
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Ingår i: Journal of Organic Chemistry. - : American Chemical Society (ACS). - 0022-3263 .- 1520-6904. ; 72:4, s. 1294-1300
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Tidskriftsartikel (refereegranskat)abstract
- [GRAPHIC] The first asymmetric [2,3]-sigmatropic rearrangement of achiral allylic amines has been realized by quaternization of the amines with an enantiomerically pure diazaborolidine and subsequent treatment with Et3N. The resultant homoallylic amines were obtained in good yields and excellent ee's. The observed diastereo- and enantioselectivities were rationalized by invoking a kinetically controlled process, and support for this model was obtained from an NMR spectroscopic investigation of the chiral Lewis acid-substrate complex. The structure of the Lewis acid-product complex was established by X-ray crystallographic analysis and supported the proposed mechanism.
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| 4. |
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| 5. |
- Dressel, Martina, et al.
(författare)
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Total synthesis of (+)-alexine by utilizing a highly stereoselective 3+2 annulation reaction of an N-tosyl-alpha-amino aldehyde and a 1,3-bis(silyl)propene
- 2008
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Ingår i: Chemistry - A European Journal. - : Wiley. - 0947-6539 .- 1521-3765. ; 14:10, s. 3072-3077
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Tidskriftsartikel (refereegranskat)abstract
- A novel route towards the polyhydroxylated pyrrolizidine alkaloid (+)-alexine has been developed. A key step in this synthesis is a highly stereoselective [3+2] annulation reaction of N-Ts-alpha-amino aldehyde 7a (Ts=tosyl) and 1,3-bis(silyl)propene 8a for the construction of the polyhydroxylated pyrrolidine subunit of the target molecule. Previous synthetic strategies rely on carbohydrates that require several protecting-group manipulations, thereby making the total number of steps relatively high. The [3+2] annulation strategy compares favorably with carbohydrate-based syntheses and constitutes a highly efficient entry to polyhydroxylated alkaloids.
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| 6. |
- Hirner, Sebastian, et al.
(författare)
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Microwave-Assisted Rearrangement of Vinylaziridines to 3-Pyrrolines : Formal Synthesis of (-)-Anisomycin
- 2005
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Ingår i: Synlett. - : Georg Thieme Verlag KG. - 0936-5214 .- 1437-2096. ; :20, s. 3099-3102
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Tidskriftsartikel (refereegranskat)abstract
- An efficient microwave-assisted rearrangement of activated vinylaziridines to 3-pyrrolines is described. The rearrangement proceeds in good to excellent yields and is mediated by NaI or LiI in MeCN at elevated temperatures. The synthetic utility of this reaction is shown in an efficient formal total synthesis of the antibiotic (-)-anisomycin.
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| 7. |
- Hirner, Sebastian, 1979-
(författare)
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New Methodologies in Organic Chemistry: Applications to the Synthesis of α-Amino Acids and Natural Products
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis deals with the development and application of new synthetic methodology in organic chemistry. The first part describes the development of a new protocol for the synthesis of 3-pyrrolines by means of a microwave-assisted ring-expansion reaction of 2-vinylaziridines. In addition, this methodology is implemented as a key-step in a formal total synthesis of the antibiotic (-)-anisomycin. In the second part, a new methodology for the synthesis of arylglycines from Weinreb amides is described. In this procedure, a Grignard reagent is added to the iminium ion formed from the Weinreb amide upon treatment with a base. When a chiral amide is used, the nucleophilic addition proceeds with high diastereoselectivity. Finally, an easy and straightforward synthesis of α-amino amides via a base-mediated rearrangement of modified Weinreb amides into N,O-acetals is presented. Subsequent arylation, alkylation, alkenylation or alkynylation of this intermediate affords the corresponding α-amino amides in excellent yields. Furthermore, a more generalized protocol for the α-arylation of Weinreb amides lacking an α-amino moiety is also discussed.
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| 8. |
- Hirner, Sebastian, et al.
(författare)
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Synthesis of alpha-Amino Acids by Umpolung of Weinreb Amide Enolates
- 2008
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Ingår i: European Journal of Organic Chemistry. - : Wiley. - 1434-193X .- 1099-0690. ; :33, s. 5583-5589
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Tidskriftsartikel (refereegranskat)abstract
- An efficient and diastereoselective synthesis of alpha-amino acids from readily available starting materials has been developed. The key feature of this reaction is an umpolung of a glycine-derived enolate, providing an alternative approach for the synthesis of alpha-amino acids.
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| 9. |
- Hirner, Sebastian, et al.
(författare)
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Synthesis of alpha-Amino Amides via N,O-Acetals Derived from Weinreb Amides
- 2009
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Ingår i: Journal of Organic Chemistry. - : American Chemical Society (ACS). - 0022-3263 .- 1520-6904. ; 74:20, s. 7798-7803
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Tidskriftsartikel (refereegranskat)abstract
- An easy and straightforward synthesis of alpha-amino amides via a base-mediated rearrangement of modified Weinreb amides into N,O-acetals is presented. Subsequent arylation, alkylation, alkenylation, or alkynylation of this intermediate affords the corresponding alpha-amino amides in excellent yields. Furthermore, a more generalized protocol for the alpha-arylation of Weinreb amides lacking an alpha-amino moiety is also discussed.
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| 10. |
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