Methodologies for the synthesis of quaternary carbon centers via hydroalkylation of unactivated olefins: twenty years of advances

• Thiago S. Silva and
• Fernando Coelho

Beilstein J. Org. Chem. 2021, 17, 1565–1590, doi:10.3762/bjoc.17.112

• the Pd(II) intermediate, with retention of the configuration. When 1d-3,3-d2 was used in the presence of the Pd(II) catalyst, only the isotopomer 2d-6-d1 was obtained, indicating the migration of the Pd(II) alkyl complex from the site of the cyclization C(4) to the α-carbonyl position and pointing to

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

• are not included in this study, because in some cases no high quality mass spectra could be obtained. The mass spectra of the unlabelled compounds show several pronounced signals for fragment ions (m/z, mass-to-charge ratio). If a signal in a mass spectrum for a particular 13C-labelled isotopomer of a

Stereochemical outcomes of C–F activation reactions of benzyl fluoride

• Neil S. Keddie,
• Pier Alexandre Champagne,
• Justine Desroches,
• Jean-François Paquin and
• David O'Hagan

Beilstein J. Org. Chem. 2018, 14, 106–113, doi:10.3762/bjoc.14.6

• an enantiopure isotopomer of benzyl fluoride to identify whether the reaction conditions favour a dissociative (SN1) or associative (SN2) pathway. [2H]-Isotopomer ratios in the reactions were assayed using the Courtieu 2H NMR method in a chiral liquid crystal (poly-γ-benzyl-L-glutamate) matrix and

• stereochemistry of 12 was confirmed by X-ray crystallography of the 4-bromophenyl ester derivative 13. Alcohol 12 was activated as the tosyl ester at −20 °C, and then immediately displaced by LiAlD4 [17], inverting the stereocenter to afford the (S)-diarylmethane 10 isotopomer in 18% ee. 2H{1H} NMR in a PBLG

¹⁵N-Labelling and structure determination of adamantylated azolo-azines in solution

• Sergey L. Deev,
• Alexander S. Paramonov,
• Tatyana S. Shestakova,
• Igor A. Khalymbadzha,
• Oleg N. Chupakhin,
• Julia O. Subbotina,
• Oleg S. Eltsov,
• Pavel A. Slepukhin,
• Vladimir L. Rusinov,
• Alexander S. Arseniev and
• Zakhar O. Shenkarev

Beilstein J. Org. Chem. 2017, 13, 2535–2548, doi:10.3762/bjoc.13.250

• this approach for the syntheses of 15N-labelled tetrazolo[1,5-b][1,2,4]triazines and tetrazolo[1,5-a]pyrimidines [25] starting from 15N-labelled 5-aminotetrazole. However, due to proton tautomerism, the use of single-labelled [2-15N]-5-aminotetrazole led to the formation of isotopomer mixtures, which

The EIMS fragmentation mechanisms of the sesquiterpenes corvol ethers A and B, epi-cubebol and isodauc-8-en-11-ol

• Patrick Rabe and
• Jeroen S. Dickschat

Beilstein J. Org. Chem. 2016, 12, 1380–1394, doi:10.3762/bjoc.12.132

• , introduction of labelling into the various positions of the compound of interest may require a different synthetic strategy for each individual target isotopomer. Most of these studies made use of deuterium labellings that can frequently be introduced into reactive positions of a (functionalised) terpene

• requires a de novo synthesis for each isotopomer, or at least a partial degradation of a terpene and reconstruction with a 13C-labelled building block. Alternatively, a 13C-labelled terpene may be obtained by feeding of labelled precursors to the producing organism, but this strategy will require high

• isotopomers of farnesyl diphosphate. Incubation experiments were carried out using 1 mL of the enzyme fraction and 1 mL of a solution of the isotopomer of (13C1)FPP (0.2 mg/mL in H2O) for 3 h at 28 °C. The reaction mixtures were extracted with n-hexane (300 μL) and analyzed by GC–MS and GC–QTOF MS. Mass

Streptopyridines, volatile pyridine alkaloids produced by Streptomyces sp. FORM5

• Ulrike Groenhagen,
• Michael Maczka,
• Jeroen S. Dickschat and
• Stefan Schulz

Beilstein J. Org. Chem. 2014, 10, 1421–1432, doi:10.3762/bjoc.10.146

• incorporation of up to five acetate units by GC–MS, indicated by a mass shift of 10 amu for the molecular ion, from m/z 145 to m/z 155 (Figure 6a and 6b). Because of the dilution with unlabelled acetate the 13C10 isotopomer is of low abundance (Figure 6c). Nevertheless, the dose dependent incorporation of the

• acetate units clearly showed the presence of this isotopomer. Feeding of 2 mM sodium 13C2-acetate to agar plate cultures showed double incorporation compared to 1 mM sodium 13C2-acetate (Figure 6d). The results hint to a biosynthesis of the streptopyridines via the polyketide pathway. The proposed

Synthesis of enantiomerically enriched (R)-¹³C-labelled 2-aminoisobutyric acid (Aib) by conformational memory in the alkylation of a derivative of L-alanine

• Stephen P. Fletcher,
• Jordi Solà,
• Dean Holt,
• Robert A. Brown and
• Jonathan Clayden

Beilstein J. Org. Chem. 2011, 7, 1304–1309, doi:10.3762/bjoc.7.152

• product was established. Keywords: amino acid; asymmetric synthesis; conformational memory; isotopic label; isotopomer; quaternary stereogenic centre; Introduction In connection with our work on the control of conformation in helical foldamers [1][2][3] built from quaternary α-amino acids [4][5], we

• hydrogenation and cyclized by refluxing in methanol [56] to yield diketopiperazine 8. The major isotopomer displayed a 13C label in the more downfield of the two diastereotopic methyl groups (Figure 3) indicating that the more shielded methyl group, cis to the phenyl ring, is the principally unlabelled one

Isotopic labelling studies for a gold-catalysed skeletal rearrangement of alkynyl aziridines

• Paul W. Davies,
• Nicolas Martin and
• Neil Spencer

Beilstein J. Org. Chem. 2011, 7, 839–846, doi:10.3762/bjoc.7.96

• enhancements were therefore assigned to the unexpected formation of two separate isotopomers of 17. 13C-enrichment at C-5 (117.7 ppm) corresponds to the isotopomer 17a anticipated from the proposed 1,2-aryl shift mechanism. Isotopomer 17b was identified by enrichment at the quaternary C-4 position (127.0 ppm

• conditions: Only a single isotopomer of the 2,4-disubstituted pyrrole 19 was formed with 13C-enrichment at the C-5 position, alongside the 2,5-disubstituted isomer 18 (Scheme 9). The observed inconsistency between the reactions of 11 and 14 was investigated using alkyl substituted alkynyl aziridine 15

• expansion of metal coordinated alkynyl aziridines to the 5-membered cyclic cation B, which leads to isotopomer G (Path II) and 2,5-disubstituted pyrroles D (Path I), a competing regioisomeric ring-expansion could afford 4-membered ring intermediate I. Evolution of I by a ring-expanding 1,2-shift of the