Recent advances in the electrochemical construction of heterocycles

• Robert Francke

Beilstein J. Org. Chem. 2014, 10, 2858–2873, doi:10.3762/bjoc.10.303

•  24 [72]. In this example, 1,2-benzoquinone derivatives are generated in the presence of ketene N,O-acetals 70. Out of four possible regioisomers, 71a and 71b are exclusively formed (in ratios 71a/71b between 1:1 and 1:2). In a follow-up study, unreacted α-arylated ketene N,O-acetal intermediates (63

Synthesis of the pentasaccharide repeating unit of the O-antigen of E. coli O117:K98:H4

• Pintu Kumar Mandal

Beilstein J. Org. Chem. 2014, 10, 2724–2728, doi:10.3762/bjoc.10.287

• reductive opening of the benzylidene acetal on treatment with sodium cyanoborohydride in the presence of HCl/Et2O [25] furnished p-methoxyphenyl 3-O-acetyl-6-O-benzyl-2-deoxy-2-phthalimido-β-D-galactopyranoside (5) in 77% yield over two steps (Scheme 1). Trisaccharide acceptor 11 could be synthesized

• % yield. NMR spectroscopy confirmed the formation of compound 8 [δ 5.04 (d, J = 3.6 Hz, 1H, H-1B), 4.38 (d, J = 7.6 Hz, 1H, H-1A) in 1H NMR and at δ 103.9 (C-1A), 100.5 (C-1B) in 13C NMR spectra]. Following an earlier report [28], cleavage of the benzylidene acetal from compound 8 catalyzed by perchloric

Galactan synthesis in a single step via oligomerization of monosaccharides

• Marius Dräger and
• Amit Basu

Beilstein J. Org. Chem. 2014, 10, 2658–2663, doi:10.3762/bjoc.10.279

• assembly is the construction of the acetal or ketal glycosidic bond that links individual sugar residues together. The synthesis of an n-mer oligosaccharide generally requires at least n−1 separate glycosylation reactions, regardless of whether the molecule is assembled in a linear or convergent manner

• by HF followed by chromatographic separation afforded chito-oligosaccharides as long as hexasaccharides in reasonable yields [13]. In most of these oligomerizations the reducing end of the oligomer is a hemi-acetal, or a reduced or protected derivative thereof, and both anomers are present [14]. An

Synthesis and immunological evaluation of protein conjugates of Neisseria meningitidis X capsular polysaccharide fragments

• Laura Morelli,
• Damiano Cancogni,
• Marta Tontini,
• Alberto Nilo,
• Sara Filippini,
• Paolo Costantino,
• Maria Rosaria Romano,
• Francesco Berti,
• Roberto Adamo and
• Luigi Lay

Beilstein J. Org. Chem. 2014, 10, 2367–2376, doi:10.3762/bjoc.10.247

• derivative very similar to 5 but protected as a 4,6-O-benzylidene acetal also led to an anomeric mixture, suggesting that conformational factors might be also involved. In addition, the treatment of this mixture with salicylchlorophosphite produced a mixture of anomeric H-phosphonates, indicating that no

Indium-mediated allylation in carbohydrate synthesis: A short and efficient approach towards higher 2-acetamido-2-deoxy sugars

• Christopher Albler,
• Ralph Hollaus,
• Hanspeter Kählig and
• Walther Schmid

Beilstein J. Org. Chem. 2014, 10, 2230–2234, doi:10.3762/bjoc.10.231

• decomposition of these labile compounds. Conversion of the carbonyl moiety to an acetal group followed by treatment with azide nucleophiles also failed to yield any desired products. Therefore we decided to mask the aldehyde as an olefin. A Wittig reaction with methyl (triphenylphosphoranylidene)acetate (Ph3P

Application of cyclic phosphonamide reagents in the total synthesis of natural products and biologically active molecules

• Thilo Focken and
• Stephen Hanessian

Beilstein J. Org. Chem. 2014, 10, 1848–1877, doi:10.3762/bjoc.10.195

• ozonolysis, protection of the resulting aldehyde, reduction of the lactone ring to the lactol, and treatment with methylenetriphenylphosphorane delivered 89. Mild acidic hydrolysis of the acetal followed by oxidation then yielded 90 with the γ-lactone unit that constitutes ring A of acetoxycrenulide

• primary alcohol of the side chain was oxidized with Dess–Martin periodinane (DMP) to give aldehyde 183. The latter was subsequently reacted with trimethylsilyl ketene acetal 184 in the presence of oxazaborolidinone 185 to afford aldol product 186 and the C22-epimer as only isomers in a 1:1 mixture. Dess

Synthesis of rigid p-terphenyl-linked carbohydrate mimetics

• Maja Kandziora and
• Hans-Ulrich Reissig

Beilstein J. Org. Chem. 2014, 10, 1749–1758, doi:10.3762/bjoc.10.182

• °C; b) 1. THF, 2 h, −78 °C; 2. H2O, 1 h, −78 °C → rt. Synthesis of 1,2-oxazine 4 by acetal formation from 10. Conditions: a) 1-bromo-4-(dimethoxymethyl)benzene (10 equiv.), CAN, CH2Cl2, 3 d, rt. Synthesis of bicyclic ketone 11 by Lewis acid-induced rearrangement and reduction to alcohols 12a and 12b

Hindered aryl bromides for regioselective palladium-catalysed direct arylation at less favourable C5-carbon of 3-substituted thiophenes

• Rongwei Jin,
• Charles Beromeo Bheeter and
• Henri Doucet

Beilstein J. Org. Chem. 2014, 10, 1239–1245, doi:10.3762/bjoc.10.123

• than with 2-bromonitrobenzene was obtained, as 4a, 4b and 4c were formed in a 24:31:45 ratio. The use of 2-bromobenzaldehyde was not successful, as the desired product 5b was only obtained with 10% selectivity. From more hindered 2-bromobenzaldehyde diethyl acetal, the selectivity in favour of desired

4-Hydroxy-6-alkyl-2-pyrones as nucleophilic coupling partners in Mitsunobu reactions and oxa-Michael additions

• Michael J. Burns,
• Thomas O. Ronson,
• Richard J. K. Taylor and
• Ian J. S. Fairlamb

Beilstein J. Org. Chem. 2014, 10, 1159–1165, doi:10.3762/bjoc.10.116

• , entry 2), a tosylate leaving group (Table 1, entry 4), and a halide (Table 1, entry 5). Somewhat more exotic functional groups such as a phosphonate ester (Table 1, entry 6) or a dimethyl acetal (Table 1, entry 7) were still tolerated in the reaction, albeit in more modest yields. A variety of alkylated

Amino acid motifs in natural products: synthesis of O-acylated derivatives of (2S,3S)-3-hydroxyleucine

• Oliver Ries,
• Martin Büschleb,
• Markus Granitzka,
• Dietmar Stalke and
• Christian Ducho

Beilstein J. Org. Chem. 2014, 10, 1135–1142, doi:10.3762/bjoc.10.113

• to introduce an N,O-acetal moiety by transformation of 7 into the dimethyloxazolidine 8, a reaction which required careful optimization though (Scheme 2, Table 1). Using boron trifluoride and 2,2-dimethoxypropane (2,2-DiMP), as known from the synthesis of the structurally related Garner's aldehyde

• [33][34], led to an unwanted cleavage of the silyl ether and furnished a mixture of the desired product 8, the TBDMS cleavage product 9 and the corresponding O,O-acetal 10 (Table 1, entry 1). Hence, further reaction conditions using different acid catalysts and solvents were investigated. Changing the

• catalyst to pyridinium p-toluenesulfonate (PPTS) only led to small amounts of the product after prolonged reaction times and could not suppress the formation of the O,O-acetal (Table 1, entries 2 and 3). The use of racemic camphorsulfonic acid (CSA) in toluene and acetone, respectively, resulted in the

Olefin cross metathesis based de novo synthesis of a partially protected L-amicetose and a fully protected L-cinerulose derivative

• Bernd Schmidt and
• Sylvia Hauke

Beilstein J. Org. Chem. 2014, 10, 1023–1031, doi:10.3762/bjoc.10.102

• ]. With a commercial sample of Pd on charcoal (10 wt %) a plethora of products was detected, four of which could be isolated and identified as the hydrogenated acetal 11, the methyl ether 13, and the two desilylated products 14 and 15 (Table 3, entry 1). Literature precedence exists for the Pd/C-induced

• of the acetal remained a problem and the combined yield of the desired products 11 and 12 was still unsatisfactory (Table 3, entry 2). This situation changed when we used Pd(OH)2 on charcoal (10 wt %) as hydrogenation catalyst, which resulted in the exclusive formation of acetal 11 and aldehyde 12 in

• 83% combined yield (Table 3, entry 3). For the deprotection of 11 and 12 a desilylation initiated by TBAF, followed by acidic hydrolysis was investigated first. With isolated acetal 11, these conditions induced a scrambling of the benzoate and led to a mixture of the desired pyranose 16 and the

Molecular architecture with carbohydrate functionalized β-peptides adopting 3₁₄-helical conformation

• Nitin J. Pawar,
• Navdeep S. Sidhu,
• George M. Sheldrick,
• Dilip D. Dhavale and
• Ulf Diederichsen

Beilstein J. Org. Chem. 2014, 10, 948–955, doi:10.3762/bjoc.10.93

• , galactose and xylose derivatives were incorporated as sugar units in rigid peptide templates. Keeping the proper β3-configuration for getting a β-peptide 314-helix (2–6) the isopropylidene protecting group and an anomeric acetal are structurally well tolerated. An additional perspective emerged from the

Synthesis of (2S,3R)-3-amino-2-hydroxydecanoic acid and its enantiomer: a non-proteinogenic amino acid segment of the linear pentapeptide microginin

• Rajendra S. Rohokale and
• Dilip D. Dhavale

Beilstein J. Org. Chem. 2014, 10, 667–671, doi:10.3762/bjoc.10.59

• multicomponent condensation reactions of aldehyde, an amine and ketene silyl acetal derivatives to get the vicinal hydroxylamino acids [17]. In addition, a few strategies employ a chiral pool approach. For example, Wee et al. utilized the zinc-silver-mediated reductive elimination of α-D-lyxofuranosyl

Synthesis of chiral N-phosphinyl α-imino esters and their application in asymmetric synthesis of α-amino esters by reduction

• Yiwen Xiong,
• Haibo Mei,
• Lingmin Wu,
• Jianlin Han,
• Yi Pan and
• Guigen Li

Beilstein J. Org. Chem. 2014, 10, 653–659, doi:10.3762/bjoc.10.57

• esters [6][7][8][9][10]. Up to now, a series of nucleophilic substrates have been reported to react with α-imino esters, such as enamine [11][12][13][14], carbamate ammonium ylide [15], 1,3-dipolar cycle [16], boronic acid [17], acetylide [18], proparygylic anion [19] and ketene silyl acetal [20]. These

One-pot three-component synthesis and photophysical characteristics of novel triene merocyanines

• Christian Muschelknautz,
• Robin Visse,
• Jan Nordmann and
• Thomas J. J. Müller

Beilstein J. Org. Chem. 2014, 10, 599–612, doi:10.3762/bjoc.10.51

• the outcome of the sequence. The latter hypothesis of the iminium-ion stabilization obviously influences the lifetime of the zwitterion 13. Furthermore the enamine formed by deprotonation of 9 is a S,N-ketene acetal, which is significantly more nucleophilic than Fischer’s base (7). The higher

• the dipolar intermediate, which is responsible for the bifurcation as supported by computational studies, enamine nucleophiles favorably lead to 1-styryleth-2-enylideneindolones diastereomers for N-methyl-substituted anilides. The S,N-ketene acetal derived from dimethyl benzothiazolium favors the

Isocyanide-based multicomponent reactions towards cyclic constrained peptidomimetics

• Gijs Koopmanschap,
• Eelco Ruijter and
• Romano V.A. Orru

Beilstein J. Org. Chem. 2014, 10, 544–598, doi:10.3762/bjoc.10.50

• yields and in high diastereomeric excess (de 70–96%) [60]. It is worth noting that deprotection of the acetal-group allows modulation of rigidity and polarity of the final molecules. Our group reported a highly diastereoselective Ugi-MCR towards 3,4-alkyl-substituted prolyl mimics by reacting several

• orthogonally protected carbohydrate-derived azido-acetal and trimethylphosphine yielded the necessary cyclic imine, which then was exposed to benzoic acid and different isocyanides. The resulting Ugi-bisamides 163 were obtained in moderate to good yields (22–78%), in which the more sterically demanding

Silver and gold-catalyzed multicomponent reactions

• Giorgio Abbiati and
• Elisabetta Rossi

Beilstein J. Org. Chem. 2014, 10, 481–513, doi:10.3762/bjoc.10.46

Synthesis of new enantiopure poly(hydroxy)aminooxepanes as building blocks for multivalent carbohydrate mimetics

• Léa Bouché,
• Maja Kandziora and
• Hans-Ulrich Reissig

Beilstein J. Org. Chem. 2014, 10, 213–223, doi:10.3762/bjoc.10.17

• in natural products, in particular as toxins in marine organisms or plants [6][7]. A few reports in the literature deal with seven-membered polyhydroxylated ethers like septanosides (containing an anomeric center) [8] as well as compounds without an acetal moiety. Known methods for the construction

• single diastereomer) which may be due to the stabilized carbenium ion formed at the benzylic position. This rearrangement presents a relatively rare example of an intramolecular aldol-like reaction of an enol ether with an activated acetal (which may also be regarded as a special case of a Prins reaction

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

• pseudo-axial to avoid allylic A1,2-strain, the sigmatropic rearrangement occurred via a pseudo-axial attack of the silyl ketene acetal on the double bond in the cyclohexene ring. These results are in accordance with the observations of Ireland et al. [49], who examined the propensity for axial versus

• , entry 1). Intermediary, the silyl enol ether and the silyl ketene acetal are formed. However, after the rearrangement, the keto-functionality can be set free again during an acidic work-up. In terms of yield and diastereoselectivity there was no difference observed between rearrangements with

• deprotection, esterification and MOM protection. The Ireland–Claisen rearrangement proceeded smoothly and gave a ca. 1.3:1 (78:79) mixture of diastereomers with the product resulting from the pseudo-axial attack of the silyl ketene acetal being the major diastereomer. In analogy to the results above the

Recent applications of the divinylcyclopropane–cycloheptadiene rearrangement in organic synthesis

• Sebastian Krüger and
• Tanja Gaich

Beilstein J. Org. Chem. 2014, 10, 163–193, doi:10.3762/bjoc.10.14

• vinylcarbenoid 120, which underwent cyclopropanation with the added olefin 118 to give bicycle 121 in decent yield. Double deprotection followed by selective acetal-protection of the less hindered alcohol and oxidation of the remaining unprotected alcohol moiety led to aldehyde 122. Wittig olefination resulted

• with a unique hexacyclic cage structure isolated from Gelsemium Sempervirens [121][122]. Starting from methyl acetoacetate (132), double deprotonation and addition of the more reactive anion to sorbal aldehyde furnished the corresponding alcohol, which was immediately protected as its acetal 133 using

Multigramme synthesis and asymmetric dihydroxylation of a 4-fluorobut-2E-enoate

• James A. B. Laurenson,
• John A. Parkinson,
• Jonathan M. Percy,
• Giuseppe Rinaudo and
• Ricard Roig

Beilstein J. Org. Chem. 2013, 9, 2660–2668, doi:10.3762/bjoc.9.301

• set the stage for mesylation, and conversion of 3 to fluoride 4 with an extremely economical reagent. Acetal cleavage and peracetylation released glycoside 5 which was converted to 6 via known methods. The main disadvantages of the approach are the extensive use which must be made of protection

• fluorination of silylketene acetal 18 with elemental fluorine. We decided to explore a halogen exchange approach from crotonic acid (20) which is commercially available cheaply, and in high diastereoisomeric purity (>98%). Diastereomeric purity is particularly important as the de novo syntheses must deliver

• -chromophoric syn-diol products. Educt elaboration was achieved via cyclic sulfate methodology, leading to the stereocomplementary anti-diols, and via acetal protection, ester reduction and one-pot oxidation/Wittig reaction, re-connecting this study to the published route to 6-deoxy-6-fluorohexoses

Garner’s aldehyde as a versatile intermediate in the synthesis of enantiopure natural products

• Mikko Passiniemi and
• Ari M.P. Koskinen

Beilstein J. Org. Chem. 2013, 9, 2641–2659, doi:10.3762/bjoc.9.300

• (formed in situ from the reaction of AcCl with MeOH) in MeOH [27]. The serine methyl ester hydrochloride salt was then protected with Boc anhydride. The N,O-acetal was formed using BF3·Et2O and DMP in acetone (Scheme 2). For the reduction they followed Garner’s procedure. This method allows an easy access

Investigating the continuous synthesis of a nicotinonitrile precursor to nevirapine

• Ashley R. Longstreet,
• Suzanne M. Opalka,
• Brian S. Campbell,
• B. Frank Gupton and
• D. Tyler McQuade

Beilstein J. Org. Chem. 2013, 9, 2570–2578, doi:10.3762/bjoc.9.292

• dimethyl acetal (DMF-DMA) [34]. Previous attempts to synthesize the nicotinonitriles via enamines resulted in poor yields due to dimerization of the starting alkylidene malononitrile [35][36][37]. A high yield enamine approach allows us to begin the synthesis from the commodity chemicals (acetone and

An overview of the synthetic routes to the best selling drugs containing 6-membered heterocycles

• Marcus Baumann and
• Ian R. Baxendale

Beilstein J. Org. Chem. 2013, 9, 2265–2319, doi:10.3762/bjoc.9.265

• and allows for good stereochemical control. The ribose component in these drugs is either derived directly from ribose itself as in zidovudine or prepared by total synthesis as in the case of lamivudine. The oxathiolane ring in lamivudine for instance can be prepared via an acetal exchange reaction

The chemistry of isoindole natural products

• Klaus Speck and
• Thomas Magauer

Beilstein J. Org. Chem. 2013, 9, 2048–2078, doi:10.3762/bjoc.9.243

• using an intramolecular Diels–Alder reaction. The synthetic endeavor began with the conversion of dihydropyran 79 to the acetal 80 (Scheme 9). Activation of the anomeric position with tin(IV) chloride presumably led to the formation of the oxonium ion 81. This intermediate was envisioned to undergo a

•  12) using a dimethylformamide acetal-mediated cyclodehydration of 105. The isoindole unit of 105 was prepared from the condensation of 103 and 104. Oxygen–sulfur exchange provided the thiophthalimide 105, which could be converted to magallanesine (97) via an intramolecular aldol condensation in one