A comprehensive review of flow chemistry techniques tailored to the flavours and fragrances industries

• Guido Gambacorta,
• James S. Sharley and
• Ian R. Baxendale

Beilstein J. Org. Chem. 2021, 17, 1181–1312, doi:10.3762/bjoc.17.90

• quenched aldol outlet stream was directed into a cooled receiver vessel, to which trifluoroacetic anhydride (TFAA) was simultaneously added, creating a net semi-continuous process. Subsequent ammonia-mediated cyclisation of the resultant diene followed by alkylation with the chlorotriazolinone 48 yielded

• mixture is biphasic, and a preliminary separation is carried out before entering the reactor coil. After discontinuous purification procedures, the product 82 was yielded in 98% purity (72% yield). The second step, instead, is an acid-catalysed cyclisation which employs the same tubular reactor at lower

• temperature (26 °C) and reaction time (2 minutes). The cooled solution of 82 in hexane is mixed in flow with sulfuric acid and then heated at 29 °C. After the cyclisation, the stream was quenched with water whilst the temperature was maintained around 45 °C. The hexane was then removed using a countercurrent

Stereoselective synthesis and transformation of pinane-based 2-amino-1,3-diols

• Ákos Bajtel,
• Mounir Raji,
• Matti Haukka,
• Ferenc Fülöp and
• Zsolt Szakonyi

Beilstein J. Org. Chem. 2021, 17, 983–990, doi:10.3762/bjoc.17.80

• -1,3-diol [21][22]. For instance, deoxoprosophylline (5) as a cyclic 2-amino-1,3-diol target molecule was prepared by Kokatla et al. in an 8 step synthesis starting from Perlin aldehydes, via Pd(OH)2-catalyzed reductive azidoketon cyclisation [23]. Another synthetic pathway involves a stereoselective

Unexpected rearrangements and a novel synthesis of 1,1-dichloro-1-alkenones from 1,1,1-trifluoroalkanones with aluminium trichloride

• Beatrice Lansbergen,
• Catherine S. Meister and
• Michael C. McLeod

Beilstein J. Org. Chem. 2021, 17, 404–409, doi:10.3762/bjoc.17.36

• , followed by a favoured 6-endo-trig cyclisation driven by the 4-methoxy group to form the 6,6-spirocycle 11. The addition of water and subsequent ring-opening would then form the acid 12, which upon elimination of water would then provide the observed acid chloride 13. We wished to also determine whether

• form the 6,6-spirocycle 19. A 1,2-rearrangement would then produce the observed product 17. Presumably the analogous dichloroalkenone 6d does not undergo this cyclisation due to the deactivation of the ring by the ketone towards nucleophilic attack. Conclusion In conclusion, we have shown that 1,1,1

A sustainable strategy for the straightforward preparation of 2H-azirines and highly functionalized NH-aziridines from vinyl azides using a single solvent flow-batch approach

• Michael Andresini,
• Leonardo Degannaro and
• Renzo Luisi

Beilstein J. Org. Chem. 2021, 17, 203–209, doi:10.3762/bjoc.17.20

• cyclisation of 1-(1-azidovinyl)-4-methylbenzene (1a) in refluxing 2-MeTHF and cyclopentyl methyl ether (CPME) as green solvent candidates, and compared the results with the reaction conducted in toluene (Table 1). The reaction proceeded rapidly in CPME, while the use of 2-MeTHF resulted in longer reaction

Supramolecular polymers with reversed viscosity/temperature profile for application in motor oils

• Jan-Erik Ostwaldt,
• Christoph Hirschhäuser,
• Stefan K. Maier,
• Carsten Schmuck and
• Jochen Niemeyer

Beilstein J. Org. Chem. 2021, 17, 105–114, doi:10.3762/bjoc.17.11

• cyclisation. Thus, all compounds feature a central BINAM unit, which is connected to the BUs (GCP or ACP) via a propionamide linker [29]. The synthesis of compound 1 was carried out starting from BINAM by coupling with GCP derivate 5 [12][13] after activation with thionyl chloride (see Figure 3). Deprotection

On the mass spectrometric fragmentations of the bacterial sesterterpenes sestermobaraenes A–C

• Anwei Hou and
• Jeroen S. Dickschat

Beilstein J. Org. Chem. 2020, 16, 2807–2819, doi:10.3762/bjoc.16.231

• initiate a cationic cyclisation cascade, leading to structurally highly complex and usually polycyclic terpenes in just one enzymatic transformation. The initially formed products are non-functionalised terpene hydrocarbons or, if the terminal cationic intermediate of the cyclisation cascade is trapped by

• rearrangement to a2•+ and a hydride shift to b2•+ (Scheme 3A). This hydride migration is in reverse order compared to a similar step along the cationic cyclisation cascade during the biosynthesis of 2 (Scheme S1 in Supporting Information File 1). The subsequent inductive ring opening to c2•+ and α-cleavage of

• carbons are marked by purple dots. Supporting Information Supporting Information File 428: Mass spectra of the unlabelled and 13C-labelled compounds 1–3, and the cyclisation mechanism from GFPP to 1–3 by SmTS1. Funding This work was funded by the DFG (DI1536/7-2).

Recent developments in enantioselective photocatalysis

• Callum Prentice,
• James Morrisson,
• Andrew D. Smith and
• Eli Zysman-Colman

Beilstein J. Org. Chem. 2020, 16, 2363–2441, doi:10.3762/bjoc.16.197

• Bach’s recent review on the subject [13]. Photocatalyst 174 was first used in a cyclisation reaction where the putative mechanism involves a hydrogen bonding complex 175 between the catalyst and quinolone substrate 176 (Scheme 26) [2]. Subsequent photoexcitation promotes a photoinduced electron transfer

• to generate diradical 177 that then adds to the alkene to form diradical 178. A SET between the ketyl radical and the α-carbonyl radical generates enolate intermediate 179, which after proton transfer regenerates the catalyst and releases the desired cyclisation product 180 in a moderate yield and

• :4. Recently, Knowles et al. used a similar tricatalytic system for the enantioselective cyclisation of sulfonamides 238 (Scheme 37) [98]. In this case, the proposed mechanism involves a PCET step to give an N-centred radical that then cyclises enantioselectively to give the alkyl radical

Lipophilicity trends upon fluorination of isopropyl, cyclopropyl and 3-oxetanyl groups

• Benjamin Jeffries,
• Zhong Wang,
• Robert I. Troup,
• Anaïs Goupille,
• Jean-Yves Le Questel,
• Charlene Fallan,
• James S. Scott,
• Elisabetta Chiarparin,
• Jérôme Graton and
• Bruno Linclau

Beilstein J. Org. Chem. 2020, 16, 2141–2150, doi:10.3762/bjoc.16.182

• when the “cyclisation” operation is achieved from geminal motifs (Figure 7B). Significant logP reductions are observed, especially from the internal geminal fluorination motif as in G9 to E2 (0.72 logP units). Starting from a trifluoromethyl group (Figure 7C), similar logP reductions are achieved (e.g

• ., D5→E5 and G10→E5) which interestingly, are of the same magnitude as for the nonfluorinated derivatives (compare D1, G1 to E1, Figure 6). It is worth noting that “cyclisation” to give cyclopropanol derivatives substituted with a fluorinated methyl group (not shown) can also be considered. Marketed

• leads to a lipophilicity decrease, which remains the case when fluorination is present. For a C–F/C–H→C–C ‘cyclisation’, only minimal lipophilicity differences are observed (in either direction) when the C–F moiety was part on a monofluorinated motif. However, when the C–F moiety was part of a geminal

Access to highly substituted oxazoles by the reaction of α-azidochalcone with potassium thiocyanate

• Mysore Bhyrappa Harisha,
• Pandi Dhanalakshmi,
• Rajendran Suresh,
• Raju Ranjith Kumar and
• Shanmugam Muthusubramanian

Beilstein J. Org. Chem. 2020, 16, 2108–2118, doi:10.3762/bjoc.16.178

• give the intermediate A which undergoes homolytic cleavage yielding B. Subsequent cyclisation results in the oxazole ring. During these optimization trials, it was interesting to note the formation of 2-aminothiazole, when ferric chloride was employed along with thiocyanate (Table 1, entry 3). There is

The biomimetic synthesis of balsaminone A and ellagic acid via oxidative dimerization

• Sharna-kay Daley and
• Nadale Downer-Riley

Beilstein J. Org. Chem. 2020, 16, 2026–2031, doi:10.3762/bjoc.16.169

• previous synthesis of balsaminone A (4) [22] (Scheme 3), silver oxide was used as the oxidative dimerizing agent for the formation of the binaphthoquinone intermediate 13. This was followed by photolytic cyclisation and ortho-formylation to give carbaldehyde 14. Conversion to balsaminone A (4) could then

Azidophosphonium salt-directed chemoselective synthesis of (E)/(Z)-cinnamyl-1H-triazoles and regiospecific access to bromomethylcoumarins from Morita–Baylis–Hillman adducts

• Soundararajan Karthikeyan,
• Radha Krishnan Shobana,
• Kamarajapurathu Raju Subimol,
• J. Helen Ratna Monica and
• Ayyanoth Karthik Krishna Kumar

Beilstein J. Org. Chem. 2020, 16, 1579–1587, doi:10.3762/bjoc.16.130

• hydroxy moiety at the carbonyl carbon of IIIb further drove the cyclisation to afford the bromomethylcoumarin 4a. Conclusion In summary, we reported the first protocol on the quaternary phosphonium salt-mediated direct synthesis of cinnamyltriazoles and 3-(bromomethyl)coumarins from Morita–Baylis–Hillman

Heterogeneous photocatalysis in flow chemical reactors

• Christopher G. Thomson,
• Ai-Lan Lee and
• Filipe Vilela

Beilstein J. Org. Chem. 2020, 16, 1495–1549, doi:10.3762/bjoc.16.125

• to fuse samarium oxide nanoparticles to TiO2 and ceria (CeO2) as a bifunctional heterogeneous photoredox Lewis acid catalyst for reductive cyclisation reactions, previously reported with ruthenium transition metal complex photocatalysts [157]. Both electrochemical and photochemical deposition

Potent hemithioindigo-based antimitotics photocontrol the microtubule cytoskeleton in cellulo

• Alexander Sailer,
• Franziska Ermer,
• Yvonne Kraus,
• Rebekkah Bingham,
• Ferdinand H. Lutter,
• Julia Ahlfeld and
• Oliver Thorn-Seshold

Beilstein J. Org. Chem. 2020, 16, 125–134, doi:10.3762/bjoc.16.14

• , lithium diisopropylamide (LDA)-mediated cyclisation of 2-(methylthio)benzamides, which were obtained by directed ortho-metalation of the respective benzamides followed by quenching with dimethyl disulfide [27] (Supporting Information File 1, Scheme S1). In general, we found the LDA-mediated cyclisation

Terpenes

• Jeroen S. Dickschat

Beilstein J. Org. Chem. 2019, 15, 2966–2967, doi:10.3762/bjoc.15.292

• generate a highly reactive cationic intermediate that can be subject to a cascade reaction through typical carbocation chemistry, including cyclisation reactions, hydride migrations and Wagner–Meerwein rearrangements [1][2]. The cascade is usually terminated by deprotonation or attack of water. The

Nanangenines: drimane sesquiterpenoids as the dominant metabolite cohort of a novel Australian fungus, Aspergillus nanangensis

• Heather J. Lacey,
• Cameron L. M. Gilchrist,
• Andrew Crombie,
• John A. Kalaitzis,
• Daniel Vuong,
• Peter J. Rutledge,
• Peter Turner,
• John I. Pitt,
• Ernest Lacey,
• Yit-Heng Chooi and
• Andrew M. Piggott

Beilstein J. Org. Chem. 2019, 15, 2631–2643, doi:10.3762/bjoc.15.256

• [33][34][35]. Specifically, AstC contains a DxDTT motif [21], which is a variant of the DxDD motif known to be involved in class II-type protonation-initiated terpene cyclisation [36]. Importantly, the DDxxD motif in AstC contains a substitution of the second Asp for Asn, leading to a loss of

• ionisation-activated cyclisation activity. Two other HAD-like hydrolases, AstI and AstK, with the class I (DDxxD/E) motif, are therefore required to catalyse other dephosphorylation steps. Despite overall low sequence homology, alignment with the amino acid sequences of AstC, AstI, AstK and related homologs

• FE257_006542 contains both class I (DDxxD/E) and class II (DxDD, QW) terpene synthases, we propose that FE257_006542 is responsible for both the cyclisation into drimanyl diphosphate and the hydrolysis of the diphosphate into drim-8-ene-11-ol. We propose the next step in the pathway is hydroxylation at C-6 or

Synthetic terpenoids in the world of fragrances: Iso E Super^® is the showcase

• Alexey Stepanyuk and
• Andreas Kirschning

Beilstein J. Org. Chem. 2019, 15, 2590–2602, doi:10.3762/bjoc.15.252

• ) (Scheme 3) [17][18][19][20][21]. To produce Ambrelux (32), myrcene (1) is reacted with dienophile (31) in a Diels–Alder cycloaddition promoted under Lewis-acidic conditions. In order to obtain Iso E Super® (33), Brønstedt acid-mediated cyclisation, similar to the one utilised for the first synthesis of

• ionone (30), proved feasible on large scale. As it turned out, not only the one depicted, but several other cyclisation products formed. The main constituent was Iso E Super® (33). A minor byproduct is now referred to as Iso E Super Plus® (34, Scheme 4). Small modifications of the reaction conditions

• conditions for the synthesis of all Iso E Super® related compounds vary slightly. The main difference lies in a prolonged isomerisation process of the Diels–Alder product 32 before and after the second cyclisation step. Georgywood® (35) named after Georg Fráter is industrially produced with, e.g., methanol

Effect of ring size on photoisomerization properties of stiff stilbene macrocycles

• Sandra Olsson,
• Óscar Benito Pérez,
• Magnus Blom and
• Adolf Gogoll

Beilstein J. Org. Chem. 2019, 15, 2408–2418, doi:10.3762/bjoc.15.233

• gaining popularity as a unit in molecular machines and photodynamic systems a clear understanding of the effect of cyclisation on the photoisomerization is of general interest. Experimental Starting materials, solvents and reagents were commercially available and used without further purification except

Harnessing enzyme plasticity for the synthesis of oxygenated sesquiterpenoids

• Melodi Demiray,
• David J. Miller and
• Rudolf K. Allemann

Beilstein J. Org. Chem. 2019, 15, 2184–2190, doi:10.3762/bjoc.15.215

• substrates, thereby opening up direct and efficient synthetic routes to oxidised sesquiterpenoids. This represents in essence a reversal of the biosynthetic pathways to oxidised sesquiterpenoids since cyclisation occurs after FDP ‘oxidation’. 8-Methoxy-FDP (11) and 12-methoxy-FDP (12) were both prepared in

• = 5.17, t, JH,H = 7.0 Hz) due to the presence of the methoxy group two carbon atoms away. The ADS-catalysed 1,11-cyclisation of diphosphate 11 suggests that the 8-methoxy group prevents the formation of a conformation conducive to isomerisation to NDP (4, Scheme 1) and hence 1,6-cyclisation to generate a

• bisabolyl cation and the amorphane skeleton. Rather the active site conformations of 11 and cation 22 appear to enable a 1,11-cyclisation to 23. A subsequent [1,3]-hydride shift to cation 24 and deprotonation from C15 lead to 8-methoxy-γ-humulene (20, Scheme 3A). Alternatively, the nucleophilic 8-methoxy

1,2,3-Triazolium macrocycles in supramolecular chemistry

• Mastaneh Safarnejad Shad,
• Pulikkal Veettil Santhini and
• Wim Dehaen

Beilstein J. Org. Chem. 2019, 15, 2142–2155, doi:10.3762/bjoc.15.211

• -triazolium receptors A ferrocene-containing dicationic bis-triazolium macrocycle 4 (see Figure 4) has been designed and synthesized by utilising the intramolecular Eglinton cyclisation of an acyclic bis(triazolylalkyne)ferrocene precursor followed by alkylation. The anion sensing ability was investigated by

• aqueous/organic solvent mixtures [51]. A tetra-1,2,3-triazolium macrocycle 9 (Figure 9) was reported in 2012 by the Beer group. The synthesis was realized by using a CuAAC cyclisation of triazole bisazides and bisalkynes and subsequent alkylation. The charge-assisted C–H···anion interaction is used by

Genome mining in Trichoderma viride J1-030: discovery and identification of novel sesquiterpene synthase and its products

• Xiang Sun,
• You-Sheng Cai,
• Yujie Yuan,
• Guangkai Bian,
• Ziling Ye,
• Zixin Deng and
• Tiangang Liu

Beilstein J. Org. Chem. 2019, 15, 2052–2058, doi:10.3762/bjoc.15.202

• invertebrates [1][2]. More than 80,000 terpenoids have been identified and characterised [3][4][5]. These diverse and complex natural products are mostly derived from carbocation cyclisation with linear C5 isoprene precursors, which are catalysed by terpene synthases (TPSs) [6]. TPSs can be classified into

• three types based on their amino acid sequence. Type I TPSs are metal-dependent enzymes that initiate cyclisation by the elimination of diphosphate groups from precursors and carbocation formation, and type II TPSs initiate the catalytic process by the protonation of an olefinic double bond [7]. The

• universal linear precursor farnesyl pyrophosphate (FPP) and assembled by FPP synthases, using dimethylallyl diphosphate (DMAPP) and isopentenyl diphosphate (IPP) as substrates. The subsequent elimination of diphosphate from FPP is catalysed by sesquiterpene synthases, with further cyclisation steps to form

Synthesis of 1-azaspiro[4.4]nonan-1-oxyls via intramolecular 1,3-dipolar cycloaddition

• Yulia V. Khoroshunova,
• Denis A. Morozov,
• Andrey I. Taratayko,
• Polina D. Gladkikh,
• Yuri I. Glazachev and
• Igor A. Kirilyuk

Beilstein J. Org. Chem. 2019, 15, 2036–2042, doi:10.3762/bjoc.15.200

• moieties, and careful basification resulted in intramolecular cyclisation givnig 5b,c with a yield of 51% and 42%, respectively. Nitrones 5a–c readily react with 4-pentenylmagnesium bromide. Quenching of the reaction mixtures with water under aerobic conditions leads to partial oxidation of resultant N

Analysis of sesquiterpene hydrocarbons in grape berry exocarp (Vitis vinifera L.) using in vivo-labeling and comprehensive two-dimensional gas chromatography–mass spectrometry (GC×GC–MS)

• Philipp P. Könen and
• Matthias Wüst

Beilstein J. Org. Chem. 2019, 15, 1945–1961, doi:10.3762/bjoc.15.190

• deuterated compounds, multistage cyclisation reactions in the course of sesquiterpene biosynthesis could be substantiated for the first time for several structural classes in V. vinifera. Results and Discussion The HS-SPME–GC×GC–TOF–MS analysis of the grape variety Lemberger (Vitis vinifera subsp. vinifera

• NPP are possible as intermediates of terpene biosynthesis, since the absolute configurations of their products from Vitis vinifera L. are unknown and the subsequent cyclisation reactions can be explained by the enantiomers of germacrene D [30][31]. In order to investigate whether the formation of δ

Formation of an unexpected 3,3-diphenyl-3H-indazole through a facile intramolecular [2 + 3] cycloaddition of the diazo intermediate

• Andrew T. King,
• Hugh G. Hiscocks,
• Lidia Matesic,
• Mohan Bhadbhade,
• Roger Bishop and
• Alison T. Ung

Beilstein J. Org. Chem. 2019, 15, 1347–1354, doi:10.3762/bjoc.15.134

• our compound 5. The formation of compound 8 can be explained by the rearrangement of 7 through a hydrogen-shift to form the diazo intermediate 9 which underwent a concerted [2 + 3] intramolecular cyclisation to 3,3-diphenyl-3H-indazole 8 (Scheme 2). The process was facilitated by the presence of a

• method a or b, respectively. The results confirm our initial hypothesis that the cyclisation occurred through the diazo species 9 (Scheme 2) and the Cu2+ plays no part in the reaction process. Conclusion The formation of the unexpected product 8 from 5 under anhydrous diazotisation conditions has led to

Phylogenomic analyses and distribution of terpene synthases among Streptomyces

• Lara Martín-Sánchez,
• Kumar Saurabh Singh,
• Mariana Avalos,
• Gilles P. van Wezel,
• Jeroen S. Dickschat and
• Paolina Garbeva

Beilstein J. Org. Chem. 2019, 15, 1181–1193, doi:10.3762/bjoc.15.115

• gray and dark gray circles). The 2-MIB synthases are present in members of all the three clades from the whole genome phylogenetic tree (Figure 1), but are most abundant in members of the clade depicted in red. These terpene synthases catalyse a unique cyclisation reaction utilizing the modified

• substrate 2-methyl-GPP to form 2-MIB (2) [25][26]. An S-adenosylmethionine (SAM) dependent methyl transferase is responsible for the methylation of GPP into 2-methyl-GPP (14, Scheme 2). Its isomerisation to 15 allows for a cyclisation via the cationic intermediates B and C to 2. Genes encoding for SAM

• enantiomers of the corresponding alcohols (R)- and (S)-albaflavenol (16ab) and the epoxide 4β,5β-epoxy-2-epi-zizaan-6β-ol (18) are known oxidation products that are all made by a cytochrome P450 monooxygenase [10][29] that is genetically clustered with the epi-isozizaene synthase for the cyclisation of FPP to

Mechanistic investigations on multiproduct β-himachalene synthase from Cryptosporangium arvum

• Jan Rinkel and
• Jeroen S. Dickschat

Beilstein J. Org. Chem. 2019, 15, 1008–1019, doi:10.3762/bjoc.15.99

• diphosphate. In-depth mechanistic investigations using isotopically labelled precursors regarding the stereochemical course of both 1,11-cyclisation and 1,3-hydride shift furnished a detailed catalytic model suggesting the molecular basis of the observed low product selectivity. The enzyme’s synthetic

• TSs is largely unknown [12]. In this study, we present the characterisation of a bacterial TS with a reduced selectivity both for substrates and for products together with the challenging investigation of its cyclisation mechanism by labelling experiments. Results and Discussion A bacterial β

• and 15 in the incubation with GPP and 18 in the experiment with GGPP, this TS from C. arvum is characterised as a multiproduct (+)-β-himachalene synthase (HcS) possessing additional mono- and diterpene cyclase activity. The structures of its minor products reveal the cyclisation mechanism of HcS Since