Biosynthesis of oxygen and nitrogen-containing heterocycles in polyketides

• Franziska Hemmerling and
• Frank Hahn

Beilstein J. Org. Chem. 2016, 12, 1512–1550, doi:10.3762/bjoc.12.148

• presence of Lewis acids is a common synthetic strategy for making pyrans and furans. Hemiacetals are also biosynthesis intermediates where they are transformed into individually functionalised heterocycles or acetals. In many cases, these hemiacetals are also appropriately activated to react further

• 1.1.3. 1.3 Polycyclic systems During the biosynthesis of ionophoric terrestrial and marine polyethers, polyolefinic PKS products are first polyepoxidised and these epoxides are then opened in a so-called zipper mechanism that installs furan and/or pyran rings as well as cyclic acetals, if carbonyl

From steroids to aqueous supramolecular chemistry: an autobiographical career review

• Bruce C. Gibb

Beilstein J. Org. Chem. 2016, 12, 684–701, doi:10.3762/bjoc.12.69

• aromatic macrocycle [25]. Interestingly, as these hosts are tetra-acetals we initially tried removing the template under acidic conditions. However, even reflux in 1:1 EtOH/H2SO4 for a week failed to affect the starting material. Evidently, under reversible conditions, a broken acetal can easily reform

(Thio)urea-mediated synthesis of functionalized six-membered rings with multiple chiral centers

• Giorgos Koutoulogenis,
• Nikolaos Kaplaneris and
• Christoforos G. Kokotos

Beilstein J. Org. Chem. 2016, 12, 462–495, doi:10.3762/bjoc.12.48

Thermal and oxidative stability of Atlantic salmon oil (Salmo salar L.) and complexation with β-cyclodextrin

• Daniel I. Hădărugă,
• Mustafa Ünlüsayin,
• Alexandra T. Gruia,
• Cristina Birău (Mitroi),
• Gerlinde Rusu and
• Nicoleta G. Hădărugă

Beilstein J. Org. Chem. 2016, 12, 179–191, doi:10.3762/bjoc.12.20

• profile of nondegraded and degraded ASO was obtained by GC–MS analysis of the corresponding methyl esters (as well as acetals and ketals of FA degradation compounds). For derivatization, approximately 20 mg of ASO or the corresponding sample was dissolved in 3 mL of boron trifluoride–methanol solution in

Scope and limitations of the dual-gold-catalysed hydrophenoxylation of alkynes

• Adrián Gómez-Suárez,
• Yoshihiro Oonishi,
• Anthony R. Martin and
• Steven P. Nolan

Beilstein J. Org. Chem. 2016, 12, 172–178, doi:10.3762/bjoc.12.19

• (3ad), acetals (3ae) and naphthyls (3af) were well tolerated and the corresponding vinyl ethers were obtained in moderate to high yields (50–98%). On the contrary, the use of azide (3ag) or boronic ester (3ah) functionalities did not afford the desired products [29]. The latter case can be explained

• . 50 substrates, thus demonstrating the robustness and synthetic versatility of the methodology. The protocol tolerates a wide range of functional groups, such as nitriles, ketones, esters, aldehydes, acetals, naphthyls or allyls. Furthermore, we have successfully used polyphenols in a milder and more

Enantioselective additions of copper acetylides to cyclic iminium and oxocarbenium ions

• Jixin Liu,
• Srimoyee Dasgupta and
• Mary P. Watson

Beilstein J. Org. Chem. 2015, 11, 2696–2706, doi:10.3762/bjoc.11.290

• /BnBox catalyst, moderate to high yields and enantioselectivities were achieved in the addition of arylalkynes. Both yields and ee’s drop with vinyl- or alkylalkynes. Recently, we have demonstrated that enantioselective, copper-catalyzed alkynylations of oxocarbenium ions derived from chromene acetals 43

• can also be achieved in high yields and ee’s (Scheme 16) [56]. In this case, the reaction concentration has a significant impact on the enantioselectivity. We also observed that the alkynylation of chromene acetals with electron-donating substituents proceeds in higher ee’s than less electron-rich

C–H bond halogenation catalyzed or mediated by copper: an overview

• Wenyan Hao and
• Yunyun Liu

Beilstein J. Org. Chem. 2015, 11, 2132–2144, doi:10.3762/bjoc.11.230

• alkenes via C–H cleavage is much less known in literature. In 2014, Yu and co-workers [66] reported the cascade synthesis of functionalized pyrrolones 66 via the dual C–H functionalization of α-alkenoylketene N,S-acetals 65. The construction of the products involved the oxidative alkene C–H amination and

An efficient synthesis of N-substituted 3-nitrothiophen-2-amines

• Sundaravel Vivek Kumar,
• Shanmugam Muthusubramanian,
• J. Carlos Menéndez and
• Subbu Perumal

Beilstein J. Org. Chem. 2015, 11, 1707–1712, doi:10.3762/bjoc.11.185

• transformation generates two C–C bonds in a single operation and presumably proceeds through a reaction sequence comprising 2-mercaptoacetaldehyde generation, nucleophilic carbonyl addition, annelation and elimination steps. Keywords: C–C bond generation; nitroarenes; nitroketene acetals; sulfur heterocycles

A new and efficient procedure for the synthesis of hexahydropyrimidine-fused 1,4-naphthoquinones

• Marcelo Isidoro P. Reis,
• Vinícius R. Campos,
• Jackson A. L. C. Resende,
• Fernando C. Silva and
• Vitor F. Ferreira

Beilstein J. Org. Chem. 2015, 11, 1235–1240, doi:10.3762/bjoc.11.137

• equivalents of alkyl- or aryl-formimines in situ. The latter compounds may serve as electrophilic agents for aminoalkylation reactions. Our research group also investigated the aminoalkylation of 2-amino-1,4-naphthoquinone with formaldehyde under microwave irradiation to produce two series of N,O-acetals and

• N,S-acetals. These compounds were obtained in good yields, and several of them showed promising antibacterial activity [42]. The structures of the synthesized 1,3,5-triazinanes were confirmed by spectroscopic techniques such as NMR, 1H and 13C-APT, infrared spectroscopy (FTIR) and high resolution

Synthesis of a hexasaccharide partial sequence of hyaluronan for click chemistry and more

• Marina Bantzi,
• Stephan Rigol and
• Athanassios Giannis

Beilstein J. Org. Chem. 2015, 11, 604–607, doi:10.3762/bjoc.11.67

• TBS group at O-4''' in 59% yield [22]. The excess amount of pyridine is necessary in order to avoid cleavage of the benzylidene acetals. Following the same concept, fully protected hexasaccharide 7 was synthesized. Therefore, thioglycoside 4 was activated with NIS and TfOH and subsequently combined

Cross-dehydrogenative coupling for the intermolecular C–O bond formation

• Igor B. Krylov,
• Vera A. Vil’ and
• Alexander O. Terent’ev

Beilstein J. Org. Chem. 2015, 11, 92–146, doi:10.3762/bjoc.11.13

• -pyridylsulfoximines 88. Using the N-(quinolin-8-yl)amide directing group Pd(OAc)2 catalyzed the double sp3 C–H alkoxylation of 96 for the synthesis of symmetric acetals 97 and unsymmetric acetals [84] (Scheme 20). The method demonstrates good functional group tolerance and the applicability in the synthesis of

Recent advances in the electrochemical construction of heterocycles

• Robert Francke

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

• coupling. In addition to enol ethers 1, vinyl sulfides and ketene acetals have successfully been cyclized according to Scheme 2 [34][35][36]. An interesting modification of this anodic coupling method was achieved by Yoshida, Nokami and co-workers using the “cation pool” concept [37][38][39]. In this

•  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

• , Scheme 24) have been identified as byproducts [73][74]. Furthermore, it was demonstrated that ketene N,S-acetals can also be employed for indole synthesis, although the use of these substrates is associated with lower yields compared to N,O-acetals [75]. 2.3 Acyl iminium ions and alkoxycarbenium ions in

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

• dealkoxylation of acetals [65] and for desilylation reactions of silyl ethers [66]. With in situ generated Pd/C (obtained from Pd(OAc)2 and charcoal according to Felpin’s protocol) [67] an improved selectivity was observed as the cleavage of the silyl groups could be suppressed. However, reductive dealkoxylation

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

• treatment did again lead to formation of enone 90. We assume that the additional substituents as well as the intermediary formed TMS ether cause excessive steric bulk and rigidity that prevent the silyl ketene acetals from adopting the required conformation. Hence, we considered enone 90 as an alternative

Synthesis of five- and six-membered cyclic organic peroxides: Key transformations into peroxide ring-retaining products

• Alexander O. Terent'ev,
• Dmitry A. Borisov,
• Vera A. Vil’ and
• Valery M. Dembitsky

Beilstein J. Org. Chem. 2014, 10, 34–114, doi:10.3762/bjoc.10.6

• hydroperoxy acetals 61a,b (Scheme 19) [250]. The ozonolysis of 9-methyleneheptadecane-7,11-diyl-bis(methanesulfonate) (63) gave 9-oxoheptadecane-7,11-diyl-bis(methanesulfonate) (64). The latter reacted with H2O2 in the presence of sulfuric acid (or iodine) as the catalyst to form 9,9-dihydroperoxyheptadecane

• the hydroperoxy acetals 229 under basic conditions. This method is suitable for the efficient synthesis of functionalized 1,2-dioxanes 230 in moderate to high yields (Scheme 65, Table 17). In early studies, scandium(III) triflate was used as the catalyst for the hydroperoxidation of ketones with the

New developments in gold-catalyzed manipulation of inactivated alkenes

• Michel Chiarucci and
• Marco Bandini

Beilstein J. Org. Chem. 2013, 9, 2586–2614, doi:10.3762/bjoc.9.294

• enabled the use of simple dimethyl acetals 6 in the carboalkoxylation of alkenes (Scheme 4) [32]. A 1:2 mixture of [picAuCl2] (7) and AgNTf2 efficiently catalyzed the double functionalization of aryl alkenes 4 in good yields and mild conditions. Dialkyl substituted olefins afforded the product only in

• moderate yields whereas monoalkyl olefins were completely unreactive under the optimized conditions. Although an activation of the acetals by the gold catalyst cannot be ruled out, a reaction pathway involving gold activation of the alkene, followed by addition of the alkyl gold intermediate 10 to the

• )]-catalyzed addition of aliphatic alcohols to alkenes. [Au(III)]-catalyzed carboalkoxylation of alkenes with dimethyl acetals 6. Isolation and reactivity of alkyl gold intermediates in the intramolecular hydroamination of alkenes. [Au(I)]-catalyzed intermolecular hydroamination of dienes. Intramolecular [Au(I

Synthesis of mucin-type O-glycan probes as aminopropyl glycosides

• David Benito-Alifonso,
• Rachel A. Jones,
• Anh-Tuan Tran,
• Hannah Woodward,
• Nichola Smith and
• M. Carmen Galan

Beilstein J. Org. Chem. 2013, 9, 1867–1872, doi:10.3762/bjoc.9.218

• cleavage of the 4,6-O-benzylidene acetals using p-TsOH in MeOH under sonication followed by removal of the N-Troc groups in the presence of LiOH and subsequent acetylation with pyridine and acetic anhydride formed the desired acetamido functionalities, removal of the ester groups under basic conditions was

Gold-catalyzed cyclization of allenyl acetal derivatives

• Dhananjayan Vasu,
• Samir Kundlik Pawar and
• Rai-Shung Liu

Beilstein J. Org. Chem. 2013, 9, 1751–1756, doi:10.3762/bjoc.9.202

• Dhananjayan Vasu Samir Kundlik Pawar Rai-Shung Liu Department of Chemistry, National Tsing Hua University, Hsinchu, 30013, Taiwan (ROC) 10.3762/bjoc.9.202 Abstract The gold-catalyzed transformation of allenyl acetals into 5-alkylidenecyclopent-2-en-1-ones is described. The outcome of our

• of the acetate group at the allyl cation intermediate. Keywords: allenyl acetals; 5-alkylidenecyclopent-2-en-1-ones; cyclization; gold catalysis; Introduction Gold-catalyzed cyclization/cycloaddition reactions [1][2][3][4][5] are useful synthetic methods to construct complicated carbo- and

• oxacyclic frameworks. Such cascade reactions have been well studied on various difunctionalized molecules including oxoalkynes [6][7][8][9][10][11][12][13], oxoallenes [14], oxoalkenes [15] and allenyl acetals [16][17][18]. In this cascade sequence, two new rings and three chemical bonds are generated in a

Re₂O₇-catalyzed reaction of hemiacetals and aldehydes with O-, S-, and C-nucleophiles

• Wantanee Sittiwong,
• Michael W. Richardson,
• Charles E. Schiaffo,
• Thomas J. Fisher and
• Patrick H. Dussault

Beilstein J. Org. Chem. 2013, 9, 1526–1532, doi:10.3762/bjoc.9.174

• a Brønsted acid. Keywords: allylation; hemiacetal; O,O-acetal; O,S-thioacetal; peroxyacetal; Re2O7; S,S-acetal; Introduction The synthetically important conversions of hemiacetals to acetals, thioacetals, or homoallyl ethers are typically achieved through activation of the substrate with a strong

• , tetrahydropyranol 4a and the O-t-butyldimethylsilyl ether of the latter (4b). While good yields of acetals were obtained from the reaction with primary or secondary alcohols, or t-butyl hydroperoxide, acetalization with phenol proceeded in poor yield. Attempted acetalizations of 2,3,4,6-tetrabenzylglucose, the

• nucleophiles Perrhenate proved ineffective for catalyzing the formation of N,O-acetals (Table 6). Although these investigations were complicated by the limited solubility of some of the nucleophiles in dichloromethane, similar results were obtained in acetonitrile, where solubility was less of an issue. The

Diastereoselective synthesis of nitroso acetals from (S,E)-γ-aminated nitroalkenes via multicomponent [4 + 2]/[3 + 2] cycloadditions promoted by LiCl or LiClO₄

• Leandro Lara de Carvalho,
• Robert Alan Burrow and
• Vera Lúcia Patrocinio Pereira

Beilstein J. Org. Chem. 2013, 9, 838–845, doi:10.3762/bjoc.9.96

• Materiais Inorgânicos, Universidade Federal de Santa Maria, 97105-900, Santa Maria, Rio Grande do Sul, RS, Brazil 10.3762/bjoc.9.96 Abstract Chiral nonracemic aminated nitroso acetals were synthesized via diastereoselective multicomponent [4 + 2]/[3 + 2] cycloadditions employing new (S,E)-γ-nitrogenated

• nitroalkenes 5a–c as heterodienes, ethyl vinyl ether (EVE) as a dienophile, and selected electron-deficient alkenes as 1,3-dipolarophiles. The employment of different organic solutions of LiClO4 or LiCl as promoter systems provided the respective nitroso acetals with yields from 34–72% and good levels of

• -dipolarophiles to furnish nitroso acetals of type 4 in an inter- or intramolecular fashion [1][2][3][8][9]. These nitroso acetals can be transformed into functionalized pyrrolizidin-3-ones and in sequence into alkaloid nuclei [1][3][10][11]. The majority of the tandem nitroalkene cycloadditions require the

Spectroscopic characterization of photoaccumulated radical anions: a litmus test to evaluate the efficiency of photoinduced electron transfer (PET) processes

• Maurizio Fagnoni,
• Stefano Protti,
• Davide Ravelli and
• Angelo Albini

Beilstein J. Org. Chem. 2013, 9, 800–808, doi:10.3762/bjoc.9.91

• ], aliphatic acetals and ketals [16], ethers [17], organostannanes [18][19], organosilanes [20][21][22], aromatics [10][23][24], and even alkanes [25][26][27]. BET (path a') could ensue, thus leading to no chemical change. However, when the D–X•+ intermediate contained a good electrofugal group (such as a

Inter- and intramolecular enantioselective carbolithiation reactions

• Asier Gómez-SanJuan,
• Nuria Sotomayor and
• Esther Lete

Beilstein J. Org. Chem. 2013, 9, 313–322, doi:10.3762/bjoc.9.36

• stabilized by coordination with a Lewis-basic substituent, react with different electrophiles in a highly diastereoselective manner. On the other hand, when acetals derived from cinnamyl alcohols are used as substrates, cyclopropanes 6 can be obtained in high enantiomeric excess by warming the reaction

Removal of benzylidene acetal and benzyl ether in carbohydrate derivatives using triethylsilane and Pd/C

• Abhishek Santra,
• Tamashree Ghosh and
• Anup Kumar Misra

Beilstein J. Org. Chem. 2013, 9, 74–78, doi:10.3762/bjoc.9.9

• Abhishek Santra Tamashree Ghosh Anup Kumar Misra Bose Institute, Division of Molecular Medicine, P-1/12, C.I.T. Scheme VII-M, Kolkata-700054, India; FAX: 91-33-2355 3886 10.3762/bjoc.9.9 Abstract Clean deprotection of carbohydrate derivatives containing benzylidene acetals and benzyl ethers was

• reported for the removal of benzylidene acetals by using strong protic and Lewis acids [10][11][15] as well as some heterogeneous acidic catalysts [16][17]. Removal of benzylidene acetal under nonacidic conditions includes hydrogenolysis using hydrogen gas over Pd/C [18], or treatment with hydrazine [19

• development of a mild, neutral reaction condition for the removal of benzylidene acetals would be useful in the derivatization of a carbohydrate framework. Mandal et al. reported the removal of benzyl esters/ethers and the reduction of alkenes/alkynes by catalytic transfer hydrogenation using a combination

The chemistry of bisallenes

• Henning Hopf and
• Georgios Markopoulos

Beilstein J. Org. Chem. 2012, 8, 1936–1998, doi:10.3762/bjoc.8.225

A Wittig-olefination–Claisen-rearrangement approach to the 3-methylquinoline-4-carbaldehyde synthesis

• Mukund G. Kulkarni,
• Mayur P. Desai,
• Deekshaputra R. Birhade,
• Yunus B. Shaikh,
• Ajit N. Dhatrak and
• Ramesh Gannimani

Beilstein J. Org. Chem. 2012, 8, 1725–1729, doi:10.3762/bjoc.8.197

• underwent Claisen rearrangement to give 4-pentenals 3a–h. Protection of the aldehyde group of the 4-pentenals as acetals 4a–h and subsequent oxidative cleavage of the terminal olefin furnished nitroaldehydes 5a–h. Reductive cyclization of these nitroaldehydes yielded the required 3-methylquinoline-4

• with ethylene glycol furnished the corresponding acetals 4a–h in good yields (Table 1). From the NMR spectra of these acetals, it was clear that they were also a mixture of diastereomers, although they appeared to be homogeneous on TLC. All attempts to separate the diastereomers at this stage were also

• unsuccessful. Subjecting these acetals to oxidative cleavage in aq THF furnished the aldehydes 5a–h in good yields (Table 1). The NMR of these aldehydes revealed them again to be a mixture of diastereomers, although they appeared to be homogeneous on TLC. Reductive cyclization of these nitroaldehydes furnished