Stereoconvergent synthesis of C1-C17 and C18-C25 fragments of bafilomycin A1

The effective syntheses of the enantiomerically pure C1-C17 2 and C18-C25 3 fragments as promising synthetic intermediates of bafilomycin A₁, 1 have been achieved.     

A stereoselective synthesis of the C15-C25 subunit of (+)-lasonolide A

The C₁₅-C₂₅ upper THP segment 2 of (+)-lasonolide A has been synthesized efficiently via diastereomeric differentiation, iodocyclization and Julia-Kocieński's sulfone olefination to install its quaternary chiral center, cis-2,6-disubstituted THP and trans-olefin, respectively.     

Synthetic studies on bafilomycin A1: stereoselective synthesis of the C12-C17 fragment and its coupling with the C1-C11 subunit

Stereoselective addition of (E)-1-lithio-2-tributylstannylethylene on a chiral cyclic di-t-butylsilyleneketal C₁₄-C₁₇ aldehyde afforded the required Felkin-Anh adduct for the synthesis of the C₁₂-C₁₇ fragment of bafilomycin A₁, the configuration of which was assigned unambiguously. After appropriate coupling with the enantiopure C₁-C₁₁ fragment, the C₁₂-C₁₇ subunit obtained here can be used for the study of the 16-membered macrolide formation either by an acyl activation or an intramolecular Stille reaction. Intermolecular esterification of the 15-OH with an acyl activation of the carboxylic acid of the C₁-C₁₁ fragment, in modified Yamaguchi's conditions, affords here an intermediate for examining an intramolecular Stille coupling.     

Enantio-controlled synthesis of the crocyclic C14-C23 subunit of cytochalasin b

The synthesis of the C₁₄-C₂₃ macrocyclic subunit of cytochalasin B from pulegone is described. The key feature of this synthesis is the photo-induced rearrangement of epoxy diazomethyl ketone E to a γ-hydroxy alkenoic ester (Scheme 2). The intermediate epoxy alcohol F was prepared using the asymmetric Sharpless epoxidation. Considerable attention was given to the preparation of allylic alcohol 14, having a t-butyldimethylsilyloxy protecting function, as starting material for the asymmetric epoxidation, however, several unforeseen difficulties were encountered (Schemes 3 and 4). Satisfactory results were obtained using the methoxy group as protecting function (Scheme 5). The allylic alcohol 22 was prepared from bromide 20 by chain lengthening with propargyl alcohol. The Sharpless epoxidation of 22 took place with high induction. Conversion to epoxy ester 24, to diazo ketone 25 and photo-rearrangement to 26 and deprotection to give 28, completes the sequence.     

Identification of a series of C25-C40 acyclic isoprenoid hydrocarbons in crude oils

The identification of acyclic isoprenoid hydrocarbons in organic extracts of geological samples and particularly in petroleum deserves a great interest from the biological origin of the sedimentary organic matter and the understanding of geochemical processes.     

An expedient synthesis of spiroketals: model studies for the calyculin C16-C25 fragment

A new short strategy to prepare the spiroketal fragment of calyculins is presented. A novel Seyferth-Gilbert type homologation of hindered lactols to the corresponding alkynes has been achieved for the first time. The spirocyclization was achieved efficiently via a DIHMA (double intramolecular hetero-Michael addition) process of this hindered ynone. The spirocyclization rate is not dependent on the stereochemistry of the alkoxy substituent in the oxolane ring.     

Towards the total synthesis of Calyculin C: preparation of the C13-C25 spirocyclic core

A stereoselective synthesis of the C₁₃-C₂₅ of Calyculin C is described. Key steps involve the coupling of a terminal acetylene with a thiol ester and subsequent spirocyclisation using a double intramolecular hetero-Michael addition.     

The deceptive rearrangement of a C8H9 cation of the cage type

We produce evidence to show that the acetolysis of exo-seco-cubyl mesylate gives 7-tetracyclo[3.3.0.0^2,8,0^3,6]octyl acetate as sole product and not 5-tetracyclo[4.2.0.0^2,40^3,8]-octyl acetate.     

Qualitative and quantitative analysis of the n-alkanes C9-C17 and pristane in clean air masses

Summary An analytical method was developed for measuring n-alkanes (C₉ to C₁₇) and other hydrocarbons in tropospheric air with mixing ratios of a few ppt (10^–12) and higher. The hydrocarbons are collected in situ in absorption tubes, carefully protected against contamination and analysed later in the laboratory by gas chromatography. First data are reported for Atlantic air masses at the west coast of Ireland.

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Qualitative und quantitative Analyse der n-Alkane C₉-C₁₇ und von Pristan in reiner Luft

Zusammenfassung Es wurde eine analytische Methode entwickelt zur Messung der n-Alkane (C₉ bis C₁₇) und anderer Kohlenwasserstoffe in reiner troposphärischer Luft mit Mischungsverhältnissen von einigen ppt (10^–12) und aufwärts. Die Kohlenwasserstoffe wurden am Beobachtungsort angereichert, sorgfältig gegen Verunreinigung geschützt und später im Laboratorium gas-chromatographisch analysiert. Erste Daten für atlantische Luftmassen an der Westküste Irlands werden mitgeteilt.

     

Stereoselective synthesis of a fully protected C13-C23 fragment of tedanolide

The combination of a high-yielding dienyllithium addition and a highly diastereoselective 1,2-reduction allows the preparation of the completely protected C₁₃-C₂₃ fragment 3 of the potent cytotoxic agent tedanolide 1. A convergent approach was used, namely a late stage coupling of the dienyllithium 16 with the selectively protected aldehyde 5 followed by oxidation-reduction and final epoxidation to give 3. The dienylstannane 4 was prepared from the dibromide 6 in five steps, the key step being the highly regio- and stereoselective stannylcupration of the alkyne 7. The commercially available hydroxy ester 10 was converted in 11 steps to the aldehyde 5. The compound 3 could potentially be a key intermediate for the synthesis of tedanolide 1.     

A Baylis-Hillman approach to the synthesis of C1-C11 fragment of caribenolide I

Stereoselective synthesis of C₁-C₁₁ fragment of caribenolide I, a potent antitumour macrolide isolated from a marine dinoflagellate Amphidinium sp. is described. The key steps rely on asymmetric aldol reactions, to control the absolute configurations of C₂, C₃ and C₁₀ stereogenic centres.     

Synthesis of macrolide antibiotics. 2. Synthesis of the C9-C13 segments of erythronolides A,B and oleandonolide

The C₉-C₁₃ segments of some 14-membered macrolide antibiotics have been synthesized from levoglucosan.     

Synthesis of the C1-C12 fragment of the tedanolides. Selective hydroboration-protonation of allylic alcohol approach

The combination of highly stereoselective vinyllithium addition and hydroboration-protonation of the resulting allylic alcohol permits the preparation of the completely protected C₁-C₁₂ fragment 2 of the novel macrocyclic cytotoxic agent tedanolide 1.     

Zur Identifizierung und Trennung der aliphatischen Amine C1-C5

Ohne Zusammenfassung     

Stereoselective synthesis of the C9-C19 lactone-dipropionate fragment of calyculin C

A highly diastereoselective synthesis of the title fragment of calyculin C has been developed based on an internal asymmetric induction between a chiral aldehyde and Z-crotyl trifluorosilane.     

Diastereoselective synthesis of the 19-epi-C18-C25 segment of (−)-lasonolide A and an unusual inversion at C19

Diastereoselective construction of the 19-epi-C₁₈-C₂₅ segment of (−)-lasonolide A was achieved using a 5-exo-trigonal mode of radical cyclization for the creation of the contiguous quaternary and tertiary stereogenic centers at C₂₂ and C₂₃ as the key reaction step. During the dehydration stage, it was found that an unusual inversion of configuration took place.     

The [2,3] Wittig rearrangement of tertiary allylic ethers. Application to the synthesis of the C12---C25 subunit of zincophorin

Ketone , representing the C₁₂---C₂₅ subunit of the ionophore antibiotic zincophorin, has been prepared by a sequence incorporating the [2,3] Wittig rearrangement of a tertiary allylic ether and subsequent lactonization via intramolecular, acid-catalyzed S_N′ addition to an allylic ether.     

Synthesis of macrolide antibiotics. 1. Synthesis of the C1-C6 segment of 14-membered macrolide antibiotics.

The C₁-C₆ segment of a number of 14-membered macrolide antibiotics have been synthesized started from levoglucosan.