Additive-assisted regioselective 1,3-dipolar cycloaddition of azomethine ylides with benzylideneacetone

• Chuqin Peng,
• Jiwei Ren,
• Jun-An Xiao,
• Honggang Zhang,
• Hua Yang and
• Yiming Luo

Beilstein J. Org. Chem. 2014, 10, 352–360, doi:10.3762/bjoc.10.33

• state B suffers from severe steric repulsion [45][46][47][48]. Presumably, the addition of 4-nitrobenzoic acid might facilitate the formation of dipole 6b. Similarly, the less sterically hindered transition state C leads to 5a as the major product. Further research work on the elaboration of the

Synthesis of the B-seco limonoid core scaffold

• Hanna Bruss,
• Hannah Schuster,
• Rémi Martinez,
• Markus Kaiser,
• Andrey P. Antonchick and
• Herbert Waldmann

Beilstein J. Org. Chem. 2014, 10, 194–208, doi:10.3762/bjoc.10.15

• on the diastereoselectivity of the Ireland–Claisen rearrangement (Scheme 10). However, the allyl ester 72 seems to be too sterically hindered to allow the [3,3]-sigmatropic rearrangement to proceed. After 2 days reaction time, only traces of rearrangement product 73 could be observed. Moreover, in

Flow microreactor synthesis in organo-fluorine chemistry

• Hideki Amii,
• Aiichiro Nagaki and
• Jun-ichi Yoshida

Beilstein J. Org. Chem. 2013, 9, 2793–2802, doi:10.3762/bjoc.9.314

• , electron-donating, and sterically hindered functional groups were obtained in good yields. This is an important feature of 'late-stage fluorination methodology' for efficient drug screening. [18F] is a fluorine radioisotope which acts as an important source of positrons. In the medical imaging modality

3-Pyridinols and 5-pyrimidinols: Tailor-made for use in synergistic radical-trapping co-antioxidant systems

• Luca Valgimigli,
• Daniele Bartolomei,
• Riccardo Amorati,
• Evan Haidasz,
• Jason J. Hanthorn,
• Susheel J. Nara,
• Johan Brinkhorst and
• Derek A. Pratt

Beilstein J. Org. Chem. 2013, 9, 2781–2792, doi:10.3762/bjoc.9.313

• expected to increase on going from chlorobenzene to acetonitrile due to the stronger H-bonding of the pyridinols and pyrimidinols ( ~ 0.5–0.7) to acetonitrile (vide supra) as compared to the sterically hindered phenols such as 10 and 11 ( ~ 0.2) [37]. The drop in α can only be explained by considering

Bis(benzylamine) monomers: One-pot preparation and application in dendrimer scaffolds for removing pyrene from aqueous environments

• Olivia N. Monaco,
• Sarah C. Tomas,
• Meghan K. Kirrane and
• Amy M. Balija

Beilstein J. Org. Chem. 2013, 9, 2320–2327, doi:10.3762/bjoc.9.266

• sterically hindered benzaldehydes 2b–g could also be utilized to prepare the corresponding bisimine products (Table 1, compounds 3b–g). Minimal impact on the overall yield of the reaction was observed upon varying the benzaldehyde concentration or substituting absolute ethanol for methanol; however, elevated

• . Bisamine products were obtained with electron-deficient, electron-rich, and sterically hindered benzaldehydes (Table 2, Compounds 4b–h). The formation of 4f was difficult, resulting in lower overall yields and longer reaction times, possibly due to the electron-withdrawing properties of the nitro group [13

Elucidation of the regio- and chemoselectivity of enzymatic allylic oxidations with Pleurotus sapidus – conversion of selected spirocyclic terpenoids and computational analysis

• Verena Weidmann,
• Mathias Schaffrath,
• Holger Zorn,
• Julia Rehbein and
• Wolfgang Maison

Beilstein J. Org. Chem. 2013, 9, 2233–2241, doi:10.3762/bjoc.9.262

• biocatalytic oxidation of vitispirane as a terpenoid with a conjugated double bond and a sterically hindered allylic position (Figure 2). Vitispirane is a flavor compound of vanilla and quince fruit and was identified in grape juice and wine [41][42]. Various syntheses of vitispirane have been reported in the

• position 9 in vitispirane (23) is sterically hindered (Scheme 5). Treatment of vitispirane (23) with PSA was indeed not a clean conversion giving a number of different oxidation products. From this mixture, however, three main products 24, 26a and 26b were identified unambiguously. So far, we have never

The chemistry of amine radical cations produced by visible light photoredox catalysis

• Jie Hu,
• Jiang Wang,
• Theresa H. Nguyen and
• Nan Zheng

Beilstein J. Org. Chem. 2013, 9, 1977–2001, doi:10.3762/bjoc.9.234

• suppressed [65][66]. The reactions were also sensitive to the steric and electronic nature of phosphites. Phosphites are quite acidic; their pKas are similar to alcohols. Less sterically hindered phosphites reacted faster as did more acidic phosphites (e.g., diphenylphosphite). The König group applied the

Organocatalyzed enantioselective desymmetrization of aziridines and epoxides

• Ping-An Wang

Beilstein J. Org. Chem. 2013, 9, 1677–1695, doi:10.3762/bjoc.9.192

• to 4 days). The desymmetrization of meso-N-acylaziridines by the less sterically hindered selenosilane Me3SiSePh under the same conditions can be completed in several hours with good yields but moderate ee’s. According to the proposed mechanism, the formation of a small amount of PhSeH was involved

• enantioselectivities. The more sterically hindered groups such as iPr and cyclohexyl in catalysts OC-54 and OC-55 demonstrated better enantioselective inductions than the phenyl in OC-53. By using 10 mol % of Me3SiOH as additive, both high yields (up to 99%) and enantioselectivities (up to 95% ee) of the corresponding

• enantioselectivities were obtained under the reverse ratio of the reagents. Della Sala’s group [47] has demonstrated the first example of a meso-aziridine desymmetrization with selenium nucleophiles to produce chiral β-aminoselenides in high enantioselectivities (up to 99% ee). Surprisingly, when the sterically

Structures of the reaction products of the AZADO radical with TCNQF₄ or thiourea

• Hideto Suzuki,
• Yuta Kawahara,
• Hiroki Akutsu,
• Jun-ichi Yamada and
• Shin’ichi Nakatsuji

Beilstein J. Org. Chem. 2013, 9, 1487–1491, doi:10.3762/bjoc.9.169

• including oxygen [1]. Sometimes, however, the merit turns out to be a drawback by limiting its properties and reactivity. For example, 1 is inefficient in the oxidation of sterically hindered secondary alcohols. The AZADO radical (2) [2] is an intriguing nitroxide radical with adamantane-like structure and

Diastereoselective radical addition to γ-alkyl-α-methylene-γ-butyrolactams and the synthesis of a chiral pyroglutamic acid derivative

• Tomoko Yajima,
• Eriko Yoshida and
• Masako Hamano

Beilstein J. Org. Chem. 2013, 9, 1432–1436, doi:10.3762/bjoc.9.161

• -selectivities were not observed for substrates with pivaloyl groups, and it appears that steric hindrance between the pivaloyl group and the lactam γ-alkyl disturbs chelation. Using the acetyl group, which is less sterically hindered than the pivaloyl group, improved the chelate formation of γ-isobutyl or γ

Thiourea-catalyzed Diels–Alder reaction of a naphthoquinone monoketal dienophile

• Carsten S. Kramer and
• Stefan Bräse

Beilstein J. Org. Chem. 2013, 9, 1414–1418, doi:10.3762/bjoc.9.158

• cycloaddition of the sterically hindered dienophile 3 with diene 4 in up to 63% yield in high selectivity. Although long reaction times were a drawback, this represents the first time and the sole method for the generation of neat exo-product 5a, bearing a newly formed quaternary center. Additionally, tricycle

• -Leopoldshafen, Germany 10.3762/bjoc.9.158 Abstract A variety of organocatalysts were screened for the catalysis of the naphthoquinone monoketal Diels–Alder reaction. In this study we found that Schreiner's thiourea catalyst 10 and Jacobson's thiourea catalyst 12 facilitate the cycloaddition of the sterically

• hindered naphthoquinone monoketal dienophile 3 with diene 4. The use of thiourea catalysis allowed for the first time the highly selective synthesis of the exo-product 2a in up to 63% yield. In this reaction a new quaternary center was built. The so formed cycloaddition product 2a represents the ABC

Synthesis of the calcilytic ligand NPS 2143

• Henrik Johansson,
• Thomas Cailly,
• Alex Rojas Bie Thomsen,
• Hans Bräuner-Osborne and
• Daniel Sejer Pedersen

Beilstein J. Org. Chem. 2013, 9, 1383–1387, doi:10.3762/bjoc.9.154

• . 1H NMR analysis of the crude product showed the presence of approximately 5–10% of a structurally similar side-product that proved extremely difficult to remove by chromatography. The side product is likely the regioisomer formed by nucleophilic attack on the more sterically hindered epoxide carbon

Catalytic asymmetric tandem Friedel–Crafts alkylation/Michael addition reaction for the synthesis of highly functionalized chromans

• Jiahuan Peng and
• Da-Ming Du

Beilstein J. Org. Chem. 2013, 9, 1210–1216, doi:10.3762/bjoc.9.137

• to 96 h. A similar result was observed in the case of product 3j (Table 4, entry 10). A steric effect was also observed in this reaction. When the substrate 1g bearing two sterically hindered bromine atoms on the phenyl ring was used, the yield and stereoselectivity of the desired product 3k

Enantioselective reduction of ketoimines promoted by easily available (S)-proline derivatives

• Martina Bonsignore,
• Maurizio Benaglia,
• Laura Raimondi,
• Manuel Orlandi and
• Giuseppe Celentano

Beilstein J. Org. Chem. 2013, 9, 633–640, doi:10.3762/bjoc.9.71

• -derived catalyst C developed by our group [7][8]. Also in other systems, such as the Matsumura-type catalysts developed by Zhang [9] of type D, the presence of a proper sterically hindered element seems to be much more decisive than the ability of the tertiary alcohol to make a possible, but not probable

Efficient Cu-catalyzed base-free C–S coupling under conventional and microwave heating. A simple access to S-heterocycles and sulfides

• Silvia M. Soria-Castro and
• Alicia B. Peñéñory

Beilstein J. Org. Chem. 2013, 9, 467–475, doi:10.3762/bjoc.9.50

• substitution to account for the obtained results. Furthermore, sterically hindered ortho-substituted aryl iodides afforded good yields of the coupling products 2 (Table 3, entries 8 and 9). For example, the ortho- and para-methoxy aryl iodides rendered comparable isolated yields of thioacetate derivatives 2c

Electron and hydrogen self-exchange of free radicals of sterically hindered tertiary aliphatic amines investigated by photo-CIDNP

• Martin Goez,
• Isabell Frisch and
• Ingo Sartorius

Beilstein J. Org. Chem. 2013, 9, 437–446, doi:10.3762/bjoc.9.46

• comparatively small geometry changes and is less influenced by sterical constraints than bond-forming reactions, because it can occur over longer distances than the contact distance [45]) should be so strongly decelerated in relation to the other chemical processes taking place in that sterically hindered

Asymmetric Diels–Alder reaction with >C=P– functionality of the 2-phosphaindolizine-η¹-P-aluminium(O-menthoxy) dichloride complex: experimental and theoretical results

• Rajendra K. Jangid,
• Nidhi Sogani,
• Neelima Gupta,
• Raj K. Bansal,
• Moritz von Hopffgarten and
• Gernot Frenking

Beilstein J. Org. Chem. 2013, 9, 392–400, doi:10.3762/bjoc.9.40

• (endo) and TS2 (exo) and the products 8a and 8a’, respectively. On the other hand, attack of the diene from the sterically hindered Re face leads to the transition structure TS3 and the product 10. Energetics Ab initio investigations of the DA reaction of phosphaethene with 2H-phosphole [49] and with

• -butadiene reveal that the Re face is sterically hindered, and consequently, attack of the diene occurs preferentially from the Si face. Thermochemical data also support a preferential endo attack of the diene from the Si face. However, the absolute values for the energy barrier calculated for this method

Polar reactions of acyclic conjugated bisallenes

• Reiner Stamm and
• Henning Hopf

Beilstein J. Org. Chem. 2013, 9, 36–48, doi:10.3762/bjoc.9.5

• value is about half way between the (E) and (Z) coupling constants of other oligoenes [35] an unambiguous decision is difficult on this basis alone. We prefer the (E)-configuration, though, since the alternative diastereomer and the path leading to it would be much more sterically hindered

Asymmetric synthesis of γ-chloro-α,β-diamino- and β,γ-aziridino-α-aminoacylpyrrolidines and -piperidines via stereoselective Mannich-type additions of N-(diphenylmethylene)glycinamides across α-chloro-N-sulfinylimines

• Gert Callebaut,
• Sven Mangelinckx,
• Pieter Van der Veken,
• Karl W. Törnroos,
• Koen Augustyns and
• Norbert De Kimpe

Beilstein J. Org. Chem. 2012, 8, 2124–2131, doi:10.3762/bjoc.8.239

• disfavored. The formation of the (SS,2S,3S)-γ-chloro-α,β-diaminocarboxylamides syn-5 can be explained by a boatlike transition-state model TS-6b involving the (E)-N-p-toluenesulfinylaldimines 3 [32][33][34][35]. This less sterically hindered transition state TS-6b, in which the haloalkyl group (–CClR2

An efficient access to the synthesis of novel 12-phenylbenzo[6,7]oxepino[3,4-b]quinolin-13(6H)-one derivatives

• Wentao Gao,
• Guihai Lin,
• Yang Li,
• Xiyue Tao,
• Rui Liu and
• Lianjie Sun

Beilstein J. Org. Chem. 2012, 8, 1849–1857, doi:10.3762/bjoc.8.213

• sterically hindered tert-butyl groups is not problematic although slightly lower yields were obtained when the aryl-ring was substituted in o-position by a tert-butyl group. The scope and generality of the newly synthesized compounds 3a–l during the present investigation are listed in Table 1 together with

• yields and melting points. As shown in Table 1, high yields of 3 were achieved irrespective of the electronic nature or positions of the substituents, except for in the cases of 3i and 3k (Table 1, entries 9 and 11). The relatively lower yields of 3i and 3k may be ascribed to the sterically hindered

Synthetic studies towards bottromycin

• Stefanie Ackermann,
• Hans-Georg Lerchen,
• Dieter Häbich,
• Angelika Ullrich and
• Uli Kazmaier

Beilstein J. Org. Chem. 2012, 8, 1652–1656, doi:10.3762/bjoc.8.189

• this approach failed (as did all other activations investigated) because of the formation of thiazolone side product, which could not be cyclized. In parallel, we tried to figure out if the amidine formation is possible with sterically hindered endothiopeptides (Scheme 2). As a model substrate we chose

• cyclic amidine was obtained in 51% yield. Conclusion In conclusion, we could show that thio-Ugi reactions are an excellent synthetic tool for the synthesis of highly sterically hindered endothiopeptides. S-Methylation and subsequent amidine formations can be carried out in an inter- as well as in an

Automated synthesis of sialylated oligosaccharides

• Davide Esposito,
• Mattan Hurevich,
• Bastien Castagner,
• Cheng-Chung Wang and
• Peter H. Seeberger

Beilstein J. Org. Chem. 2012, 8, 1601–1609, doi:10.3762/bjoc.8.183

• affect glycosylation yields. Working in a solid-phase environment could, in principle, additionally reduce the accessibility of a sterically hindered nucleophile. Thus, the possibility of accessing branched structures was explored on the solid support [5]. Sialyl LewisX tetrasaccharide 27 (Scheme 5), has

Synthesis of conformationally restricted glutamate and glutamine derivatives from carbonylation of orthopalladated phenylglycine derivatives

• Esteban P. Urriolabeitia,
• Eduardo Laga and
• Carlos Cativiela

Beilstein J. Org. Chem. 2012, 8, 1569–1575, doi:10.3762/bjoc.8.179

• starting materials, one of them containing a sterically hindered N atom, protected by two methyl groups, namely [C6H5C(H)(CO2Me)NMe2] [17][18], and the other one containing a less hindered, but strongly electron-withdrawing, triflate (Tf) group [C6H5C(H)(CO2Me)NHTf] [19]. The orthopalladation of [C6H5C(H

Reactions of nitroxides XIII: Synthesis of the Morita–Baylis–Hillman adducts bearing a nitroxyl moiety using 4-acryloyloxy-2,2,6,6-tetramethylpiperidine-1-oxyl as a starting compound, and DABCO and quinuclidine as catalysts

• Jerzy Zakrzewski

Beilstein J. Org. Chem. 2012, 8, 1515–1522, doi:10.3762/bjoc.8.171

• with increasing chain length of the alcohol moiety in an acrylate [20][45]. The alcohol moiety in 4-acryloyloxy-2,2,6,6-tetramethylpiperidine-1-oxyl is undoubtly sterically hindered, so the expected reaction times will be long; however, this nitroxide has been chosen based on the availability of such

Highly enantioselective access to cannabinoid-type tricyles by organocatalytic Diels–Alder reactions

• Stefan Bräse,
• Nicole Volz,
• Franziska Gläser and
• Martin Nieger

Beilstein J. Org. Chem. 2012, 8, 1385–1392, doi:10.3762/bjoc.8.160

• MacMillan on various model systems (Figure 3) [37]. The effect of these catalysts is based on the formation of iminium ions by condensation of the dienophile bearing a carbonyl group, with the sterically hindered imidazole catalyst. In this way, one side is shielded and only the other side can be attacked