Carbohydrate-auxiliary assisted preparation of enantiopure 1,2-oxazine derivatives and aminopolyols

• Marcin Jasiński,
• Dieter Lentz and
• Hans-Ulrich Reissig

Beilstein J. Org. Chem. 2012, 8, 662–674, doi:10.3762/bjoc.8.74

• diiodide; Introduction During the last few decades carbohydrate-derived nitrones have turned out to be particularly attractive tools for the synthesis of structurally complex compounds [1][2][3][4]. Employed mainly as 1,3-dipoles in cycloadditions [5][6] or as imine analogues in nucleophilic additions [7

• ][8], these nitrones very often furnish the corresponding products in a highly selective manner. In this context, reactions of lithiated alkoxyallenes with enantiopure nitrones are particularly of interest since they lead by a [3 + 3] cyclisation process to 1,2-oxazine derivatives with excellent

• ][12]. Although the reactions of lithiated alkoxyallenes, with nitrones bearing substituents with stereogenic centres at the carbon atom, were studied in our group in great detail [13], N-glycosyl-substituted nitrones have so far not been used as electrophiles for this purpose. This type of nitrone has

Recent developments in gold-catalyzed cycloaddition reactions

• Fernando López and
• José L. Mascareñas

Beilstein J. Org. Chem. 2011, 7, 1075–1094, doi:10.3762/bjoc.7.124

• ) cycloaddition of related 2-(1-alkynyl)-2-alken-1-ones 10 with nitrones [47]. The reactions provided a very practical entry to bicyclic oxazine derivatives 11, which were obtained in good yields and excellent selectivities (Scheme 5). A plausible mechanism, proposed by the authors, consists of an initial

• furanyl intermediate featuring an iminium ion (XI). This species undergoes an intramolecular cyclization to yield the observed azepine product, and regenerates the gold catalyst. Very recently, the same group reported a related gold-triggered formal (4 + 3) cycloaddition between nitrones and 1-(1-alkynyl

• -catalyzed synthesis of polycyclic, fully substituted furans from 1-(1-alkynyl)cyclopropyl ketones [45]. Gold-catalyzed 1,3-dipolar cycloaddition of 2-(1-alkynyl)-2-alken-1-ones with nitrones [47]. Enantioselective 1,3-dipolar cycloaddition of 2-(1-alkynyl)-2-alken-1-ones with nitrones [48]. Gold-catalyzed

Synthetic applications of gold-catalyzed ring expansions

• David Garayalde and
• Cristina Nevado

Beilstein J. Org. Chem. 2011, 7, 767–780, doi:10.3762/bjoc.7.87

• ). Nitrones also reacted as dipolarophiles in the presence of AuCl3, even if in some cases copper catalysts were found to be more effective at triggering the corresponding annulations [52]. By contrast, when alkoxy vinyl ethers were employed as dipolarophiles, the cycloaddition takes place prior to the

Rh-Catalyzed rearrangement of vinylcyclopropane to 1,3-diene units attached to N-heterocycles

• Franca M. Cordero,
• Carolina Vurchio,
• Stefano Cicchi,
• Armin de Meijere and
• Alberto Brandi

Beilstein J. Org. Chem. 2011, 7, 298–303, doi:10.3762/bjoc.7.39

• Göttingen, Tammannstrasse 2, 37077 Göttingen, Germany 10.3762/bjoc.7.39 Abstract Dienes embedded in quinolizidine and indolizidine structures can be prepared in four steps from cyclic nitrones and bicyclopropylidene. The key intermediates α-spirocyclopropanated N-heterocyclic ketones, generated via a

• so that the various specific cyclopropane transformations can be used as a synthetic tool. In recent years we have shown that 1,3-dipolar cycloadditions of nitrones 1 to the highly strained alkene bicyclopropylidene (BCP, 2) [5][6][7] afford spirocyclopropanated isoxazolidines 3 [8][9] which, on

• (Scheme 6). Conclusion Azaheterocycles 14, 15 and 17 containing a spiro-annelated VCP moiety have been synthesized starting from cyclic nitrones and BCP by a three-step two-pot sequence consisting of a 1,3-dipolar cycloaddition, thermal rearrangement and Wittig methylenation. These compounds in the

Addition of lithiated enol ethers to nitrones and subsequent Lewis acid induced cyclizations to enantiopure 3,6-dihydro-2H-pyrans – an approach to carbohydrate mimetics

• Fabian Pfrengle and
• Hans-Ulrich Reissig

Beilstein J. Org. Chem. 2010, 6, No. 75, doi:10.3762/bjoc.6.75

• -derived nitrones afforded syn- or anti-configured hydroxylamine derivatives 4a–d that were cyclized under Lewis acidic conditions to yield functionalized dihydropyrans cis- or trans-5a–d containing an enol ether moiety. This functional group was employed for a variety of subsequent reactions such as

• to a tricarboxylic acid core. Keywords: aminopyrans; carbohydrate mimetics; Lewis acids; lithiated enol ethers; nitrones; oxidative cleavage; stereodivergent synthesis; Introduction The pyran structural motif can be found in numerous bioactive natural products. Possible strategies towards their

• enantiopure aminopyrans E (Scheme 1). The sequence of nucleophilic addition of lithiated enol ethers A to nitrones B and Lewis acid promoted cyclization of the resulting 1,3-dioxolanyl-substituted hydroxylamine derivatives C, delivered the enantiopure dihydropyrans D in a highly stereodivergent fashion [8

Synthesis of densely functionalized enantiopure indolizidines by ring- closing metathesis (RCM) of hydroxylamines from carbohydrate- derived nitrones

• Marco Bonanni,
• Marco Marradi,
• Francesca Cardona,
• Stefano Cicchi and
• Andrea Goti

Beilstein J. Org. Chem. 2007, 3, No. 44, doi:10.1186/1860-5397-3-44

• yields from enantiopure hydroxylamines obtained straightforwardly from carbohydrate-derived nitrones. Background Indolizidine alkaloids have widespread occurrence in nature. They can be found in widely different organisms such as bacteria, fungi, higher plants, invertebrates and vertebrates.[1] For

• . [5][6][7][8][9][10][11][12][13][14][15][16][17][18] We accomplished the total syntheses of some indolizidine alkaloids and of several non-natural analogues employing chiral nitrones as key intermediates, either as dipolarophiles in 1,3-dipolar cycloaddition chemistry [19][20] or as electrophiles in

• nitrones. Experimental Sections. Experimental procedures, characterization of new compounds. Acknowledgements We thank Cassa di Risparmio di Firenze for granting a 400 MHz spectrometer. Maurizio Passaponti and Brunella Innocenti (Università di Firenze) are acknowledged for technical assistance. We thank