C21 C22 Research Articles

Molecular fingerprints in shales from the Sanhu biogenic gas fields in eastern Qaidam Basin, NW China: Evidence of biodegradation of shale organic matter

This study provides evidence for hopane and sterane biodegradation in shale organic matter (OM) via geochemical characterization of shale extracts from the biogenic gas fields of the Sanhu Depression in eastern Qaidam Basin, NW China. Nineteen immature source rock cores of the upper Pliocene–Quaternary from the study area are divided into two groups based on distinct results of Rock-Eval analysis and molecular fingerprints. Organic-rich group A shales are dominated by the ααα 20R configurations of steranes, diagenetic hopanoids and are lean in the αββ 20S configurations of steranes, C27–C29 diasteranes, C21–C22 pregnanes and C19–C29 tricyclic terpanes. In contrast, organic-poor and sandy shales of group B have much higher hopane and sterane isomerization ratios, higher C21–C22 pregnanes, C27–C29 diasteranes and C19–C29 tricyclic terpanes but diminished diagenetic hopanoids. C29 5α-sterane and C31 αβ-hopane isomer concentrations and isomerization ratios reveal a depth-controlled diagenetic profile in group A samples whereas they are randomly or inversely distributed in group B. Moreover, the C27–C30 25-norhopane series has been identified in group B core extracts but are absent in group A. These differences are mainly ascribed to biodegradation of group B core OM. Regardless of microbial alteration of hopanes and steranes in the shale extracts, no significant biodegradation of n-alkanes has been observed in the extracts of both shale groups. However, instead of reflecting preferential degradation of biomarkers, this difference is caused by lower microbial availability of n-alkanes than the biomarkers in the special pore structure of fine-grained sediments and dilution of biodegraded n-alkane patterns by the newly generated n-alkanes from the shale OM. The rare occurrence of hopane and sterane biodegradation in fine sediments in the Sanhu Depression is ascribed to the low hydrocarbon yields from organic lean group B cores that failed to alter the biodegradation signature, since no similar biodegradation case has been reported in organic-rich biogenic gas source rock systems worldwide. Due to a self-sourced biogenic gas system consisting of frequently interbedded shales and silts/sands, the Sanhu biogenic gases form predominantly in the more porous and permeable rocks represented by group B but with most energy sources for microbial metabolism diffused from adjacent, organic-rich shales represented by group A.

Biodegradative production and destruction of norhopanes – An example from residual oil in a Paleogene paleomigration conduit on the Utsira High, Norwegian North Sea

Biodegradation of the more resistant hopanoids was examined using residual stains in a Paleogene paleomigration conduit on the Utsira High, Norwegian North Sea, together with related less degraded oils. Under currently anoxic conditions, the most severely biodegraded samples contain 25-nor, 28-nor and 25,28-dinorhopanes, in addition to regular hopanes, neohopanes, diahopanes and didiahopanes [9,15-dimethyl-25,27-dinorhopanes]. The C27 member of the 25,28-dinorhopanes comprises closely eluting 17α,21β and 17β,21α isomers. A pair of C31 compounds eluting just after the 17α-homohopanes were tentatively identified as the 22S/R isomers of homo-18α-neohopane. The 28,30-dinorhopane peak seems to comprise 17α,21β and 17β,21α isomers, with relatively more of the less thermodynamically stable 17α,21β contributing under the most severe biodegradation.The approximate order of increasing susceptibility to biodegradation was: gammacerane ≈ triaromatic 23,24-dimethylcholesteroids ≈ triaromatic dinosteroids < C21–C22 C-ring-monoaromatic steroids ≈ C21–C22 steranes < C21–C22 diasteranes < C31 methylhopanes < 22,29,30-trinorneohopane (27Ts) < C30 TPPs ≈ diahopane ≈ didiahopane ≈ 22,29,30-trinorhopane (27Tm) < 30-norneohopane (29Ts) ≈ C28+ 28-norhopanes/moretanes < C29+ hopanes ≈ C27+ diasteranes < C27+ steranes. This information can inform the selection of correlation parameters for heavily biodegraded oils, although the sequence should not be considered universally applicable. Regular hopanes declined steadily with increasing biodegradation. The 25-norhopane homologues may be initially produced, but are then also degraded, although the C26 component appears stable. There is a relative increase in 25,28-dinorhopane abundance with increasing biodegradation, which is likely to result from demethylation of 25- and 28-norhopanes; if so, there must also be some shortening of the E-ring side-chain. Concentration estimates suggest that there is an overall loss of the hopanoid skeleton with increasing severity of biodegradation.

Geochemical research of asphaltite formation in the Orenburg region

The results of the studies of the physicochemical properties and genesis of asphaltites from the deposits of the Orenburg region are considered. Based on the data of a study on the distribution of hydrocarbon biomarkers (n-alkanes, isoprenanes, steranes, and terpanes), it was found that deposit formation occurred in carbonate layers under reducing conditions. The distribution of steranes and terpanes confirmed a sufficient maturity of the asphaltite. The C28/C29 sterane ratio of 0.31 corresponds to Silurian age according to Grantham or to Silurian–Devonian age according to the pregnane coefficient (a ratio of the sum of C21–C22 pregnanes to the sum of C21–C22 pregnanes and C27–C29 regular steranes) of 9.0%. Thus, it is likely that asphaltites from the Ivanovk deposit in the Orenburg region were redeposited in Silurian/Devonian source rock formations.

Total Synthesis of Biselyngbyolide B and Its C21-C22 Z-Isomer.

Investigations toward the synthesis of the 18-membered macrolactone biselyngbyolide B (2) from a C1-C13 and a C14-C23 fragment are described. As a key reaction in the synthesis of the C1-C13 fragment, we used an asymmetric propargylation of chiral vinylketene silyl N, O-acetal 12. Access to a C14-C23 fragment featuring a skipped diene and a sensitive allyl alcohol function was initially attempted via reductive fragmentation of a pyran template. However, this ring opening on iodide 32 with t-BuLi led to dienynol 33 with a 21 Z double bond. With a silyl protecting group at 3-OH and by implementing an intramolecular Stille coupling for macrolactonization, the 21 Z-isomer of biselyngbyolide B (47) was obtained. For preparation of a C14-C23 fragment with the 21 E-configuration, a cross-coupling of vinylstannane 48 with 4-bromocrotonate (49) set the configuration of the two double bonds. Biselyngbyolide B (2) was then accessed by an intramolecular Heck coupling. In preliminary biological cytotoxicity assays, 2 turned out to be active, whereas the 21 Z-isomer 47 was much less active. The 3-OMEM analogue 40 was devoid of activity. These results support the notion that the side chain with the correct configuration is relevant for binding to the Ca2+-ATPase and the biological activity.

Physicochemical Properties and Genesis of the Asphaltites of Orenburg Oblast

In this article, the results of the studies of the physicochemical properties and genesis of asphaltites from the deposits of Orenburg oblast are considered. Based on the data of a study on the distribution of hydrocarbon biomarkers (n-alkanes, isoprenanes, steranes, and terpanes), it was found that deposit formation occurred in carbonate layers under reducing conditions. The distribution of steranes and terpanes confirmed a sufficient maturity of the asphaltite. The C28/C29 sterane ratio of 0.31 corresponds to Silurian age according to Grantham or to Silurian–Devon age according to the pregnane coefficient (a ratio of the sum of C21–C22 pregnanes to the sum of C21–C22 pregnanes and C27–C29 regular steranes) of 9.0%. Thus, it is likely that asphaltites from the Ivanovka deposit in Orenburg oblast were redeposited in Silurian/Devon source rock formations.

Biodegradation of tricyclic terpanes in crude oils from the Bohai Bay Basin

A suite of 33 biodegraded crude oils from the Miaoxi Depression and Liaodong Bay, Bohai Bay Basin, China were analyzed to investigate the effect of biodegradation on biomarkers. Among these samples, sequential biodegradation of C19–C26 tricyclic terpanes was observed in the most degraded crude oils, in which C27–C29 regular steranes and hopanes were almost completely removed. This suggests that alteration of tricyclic terpanes occurs when almost all hopanes and steranes have been removed. The C21–C22 steranes and diasteranes were unaffected in the crude oil samples, and so they can be used as conserved “internal standards” to evaluate the biodegradation of tricyclic terpanes. The tricyclic terpane distributions, absolute concentrations and their relative abundance compared to the biodegradation-resistant components show systematic changes with increasing biodegradation. The results illustrate that the relative susceptibility of the tricyclic terpanes to biodegradation decreases with increasing carbon number, with the exception of the C20 tricyclic terpane. No demethylated tricyclic terpanes were detected, indicating that tricyclic terpanes were biodegraded without microbial demethylation to generate demethylated counterparts (17-nor-tricyclic terpanes). Tentatively, we predict that the biodegradation pathways of tricyclic terpanes are: (1) microbial oxidation of the methyl group of the side chain to a carboxyl group with formation of tricyclic terpanoic acids, followed by (2) demethylation of the tricyclic terpanoic acids at C-10 and generation of demethylated tricyclic terpanoic acids. Verification of this predicted biodegradation pathway will require analytical data for the related biodegradation products.

ChemInform Abstract: Synthetic Studies of Antitumor Macrolide Laulimalide: A Stereoselective Synthesis of the C17-C28 Segment.

A stereoselective synthesis of the C17–C28 segment of the potent antitumor macrolide, laulimalide has been accomplished. The key steps are a ring-closing olefin metathesis to construct the dihydropyran unit, nucleophilic addition of an alkynyl anion to the Weinreb amide, stereoselective reduction of the resulting ketone to set the C20-hydroxyl stereochemistry, and elaboration of the C21–C22 trans-olefin geometry.

Enantioselective Synthesis of Oasomycin A, Part I: Synthesis of the C1–C12 and C13–C28 Subunits

Oasomycin A (I) is a member of the desertomycin family of macrolide natural products and was isolated in 1993 by Thiericke and co-workers during the screening of secondary metabolites of Streptoverticillium baldacii. The overall structure of oasomycin A was established through extensive NMR spectroscopic studies and by direct comparison to other members of the desertomycin family. In 2001, Kishi and coworkers issued a proposal for the relative and absolute stereochemistry of oasomycin A through the use of their “universal NMR database” (Scheme 1). To validate the structural assignment of oasomycin A, we decided to pursue the synthesis of the reported structure, I. Herein, and in the following Communications, we describe our efforts which culminated in the synthesis of oasomycin A (I) and confirm the stereochemical assignment by Kishi and co-workers. First, we describe the synthesis of the C1–C12 and C13–C28 subunits II and III, respectively, which will be united by using the Julia coupling reaction (Scheme 1). Our plan for the synthesis of the C13–C28 fragment III is outlined in Scheme 2. Aldehyde III could be derived from a chelate-controlled reduction of the a,b-unsaturated ketone IV, which in turn can be disconnected at the C21 C22 bond to afford fragments V and VI. We predict successful union of these two fragments because of the greater reactivity of the Weinreb amide VI, which is required to avoid self-condensation of V (M=Li, MgX). This prediction is based on the supposition that the inductive effect of the a-alkoxy substituent should elevate the amide reactivity of VI above that of its reaction partner V. Synthesis of subunit II commenced with the aldol addition of known b-ketoimide 1 to aldehyde 2, which afforded the desired anti,anti aldol adduct as an 84:16 mixture of Scheme 1. Retrosynthetic analysis of oasomycin A (I).

Stereoselective Synthesis of the Monomeric Unit of SCH 351448

AbstractThe monomeric unit of the macrodiolide SCH 351448 has been synthesized from three building blocks. Strategic disconnections were chosen between C21–C22 (Wittig) and C10–C11 (stereoselective aldol). The cis configuration of both 2,6‐disubstituted tetrahydropyran rings was established by a stereoselective cationic reduction. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2006)

3,3′-Benzylidenebis(4-hydroxy-6-methylcoumarin). Erratum

An error in the paper by Vijayalakshmi et al. [Acta Cryst. (2002), E58, o659–o660] is corrected. The last sentence of page o659 should read "The C21—C22 distance of 1.534 (2) Å is longer than an unstrained Csp3—Car bond, but is in the range characteristic of sterically crowded structures…".

On the Mechanism of Indeno[1,2,3-c,d]-pyrene Ozonation

Calculations on the strongly cancerogenic indeno[1,2,3-c,d]pyrene and all its possible primary and secondary ozonides by the MNDO method indicate the breaking of the C21–C22 bond of the pyrene fragment as the energetically most favourable first step of the oxidative degradation. Localization energies calculated within the simple Huckel model predict that the attack of the C21–C22 pair by ozone is most probably also kinetically the most preferred reaction path. The rather low localization energy of C20 in the pyrene fragment indicates the possibility of competitive reactions between ozonolysis and the formation of phenols and quinones. The theoretical results are supported by experimental findings on the ozonation of indeno[1,2,3-c,d]pyrene in aqueous medium.

Fragmentations and rearrangements of 22-hydroxyl substituted milbemycins - synthesis of a key lactone intermediate

Beckmann fragmentation of 22-oximino milbemycins resulted in the cleavage of the C21–C22 bond to produce a key lactone intermediate which can be used to synthesise new spiroacetals. Cleavage of the C21–O25 bond and recyclisation to produce a furyl derivative is also described.

The conformational analysis of leukotriene D4 in aqueous solution based on high-field nuclear magnetic resonance measurements

The conformational analysis of LTD4 in D2O was carried out based on the analysis of vicinal proton–proton coupling constants, nonselective proton longitudinal relaxation times, and nuclear Overhauser effect data. The H6—C6 to C12 region of the molecule is transoid, the predominant rotamer about C5—C6 is gauche, and two rotamers can be used to describe the rotation about C12—C13. There are three rotamers that can be used to describe the rotation about the C21—C22 bond of cysteine. Finally, chelation between an Yb3+, shift reagents and LTD4 was not observed.