摘要:

AbstractBicyclic alcohol (±)‐2, an advanced intermediate for the synthesis of the methyl ester1of pentalenolactone E, has been synthesized in nine operations starting from aldehyde7. The key step6 → 9is a Pd‐catalyzed, tandem intramolecular alkyne allylation/carbonylation re

ISSN:0018-019X    

DOI:10.1002/hlca.19910740302

出版商:WILEY‐VCH Verlag GmbH    

年代:1991

数据来源: WILEY

摘要:

Synthesis of Enantiomerically Pure Mimulaxanthin and of Its (9Z,9′Z)‐ and (15Z)IsomersWe present the details of a synthesis of optically active, enantiomerically pure stereoisomers of mimulaxanthin (=(3s,5R,6R,3′S,5′R,6′R)‐6,7,6′,7′‐tetradehydro‐5,6,5′,6′‐tetrahydro‐β,β‐carotin‐3,5,3′,5′‐tetrol) either as free alcohols1aand24aor as their crystalline (t‐Bu)Me2Si ethers1band24b. Grasshopper ketone2a, a presumed synthon, unexpectedly showed a very sluggish reaction withWittig‐Hornerreagents. Upon heating with the ylide of ester phosphonates, an addition across the allenic bond occurred. On the contrary, a slow but normal 1,2‐addition took place with the ylide from (cyanomethyl)phosphonate but, unexpectedly, with concomitant inversion at the chiral axis. So a mixture of(6R,6S,9E,9Z)‐isomers6–9was produced {(Scheme 1). However, a fast and very clean 1,2‐addition occurred with the ethynyl ketone12to yield the esters13and14(Scheme 2). DIBAH reduction of the separated stereoisomers gave the allenic alcohols15and16in high yield. Mild oxidation to the aldehydes17and18followed by their condensation with the acetylenic C10‐bis‐ylide19led to the stereoisomeric 15,15′‐didehydromimulaxanthins20and22, respectively (Schemes 3and4). Mimulaxanthins1and24were prepared by partial hydrogenation of20and22followed by a thermal (Z/E)‐isomerization. As expected, the mimulaxanthins exhibit very weak CD curves, obviously caused by the allenic bond that insulates the chiral centers in the end group from the chromophor. On the contrary, some of the C15‐allenic synthons showed not only fairly strong CD effects but also a split CD curve which, in our interpretation, results from an exciton coupling between the allene and the C(9)C(10) bond. We postulate a rotation around the C(8)C(9) bond, presumably caused by an intramolecular H‐bond in16or by

ISSN:0018-019X    

DOI:10.1002/hlca.19910740303

出版商:WILEY‐VCH Verlag GmbH    

年代:1991

数据来源: WILEY

摘要:

AbstractThe spongeStellettasp. (Astrophorida, Stellettidae), collected at −700 m in the Coral Sea, is shown to contain sterones and sterols of the stigmastane type with a C(24)C(25) bond for which there is no precedent in the sea. Structure elucidation of the second abundant of these steroids, stigmasta‐4,24(25)‐dien‐3‐one ((+)‐1), is based on 1D and 2D NMR spectra and chemical transformation to acetate (−)‐5. Stigmasta‐4,24(25)‐diene‐3,6‐dione ((−)‐3), present in trace amounts in the sponge, was obtained in sufficient quantity for NMR study by oxidation of the also present, inseparable, and abundant 4:1 mixture of stigmasta‐5,24(25)‐dien‐3β‐ol (6) and its 5,6‐dihydro derivative7(Scheme 1). This oxidation also afforded the ketone analogues (+)‐8and (+)‐9, which could be separated, thus making structure elucidation possible. The 6β‐hydroxystigmasta‐4,24(25)‐dien‐3‐one ((+)‐4), also present in trace amounts in the sponge, was obtained in sufficient amount for NMR study, together with its C(6) epimer (+)‐11, by hydroperoxidation of (+)‐8followed by deoxygenation (Scheme 2). The last trace steroid of the sponge, stigmasta‐4,6,24(25)‐trien‐3‐one ((−)‐2), was structurally elucidated using limited NMR data and comparison with the other stigmastanes. These stigmastanes, as the only steroids of this sponge, are likely to function as stabilizers of its cell walls; their phytosteroid structure, for a sponge which lives in the dark of deep waters, suggest origin throug

ISSN:0018-019X    

DOI:10.1002/hlca.19910740304

出版商:WILEY‐VCH Verlag GmbH    

年代:1991

数据来源: WILEY

摘要:

AbstractFrom the gun resin Ammoniacum, an epimeric mixture2a/3aof highly functionalized prenylated chromandiones was isolated. Their structure was established by spectral data and single‐crystal X‐ray analysis. The compounds were isolated as acetates2b/3b, and these derivatives showedin vivohaemorrhagic activ

ISSN:0018-019X    

DOI:10.1002/hlca.19910740305

出版商:WILEY‐VCH Verlag GmbH    

年代:1991

数据来源: WILEY

摘要:

AbstractThe readily available alkyl dicyanoacetates1reacted with the 1,3‐dipolar reagents arenecarbonitrile oxides2′ and arenecarbonitrile imines5′ to afford 1,2,4‐oxadiazol and 1,2,4‐triazol derivatives. The arenecarbonitrile oxides2′ with electron‐donating groups on the arene ring gave products3a–dresulting from addition on both CN groups of1, and those with electron‐withdrawing groups provided mono‐adducts4a–e(Scheme 1). Arylnitrile imines5′ reacted with1to offer both bis‐ and mono‐addition products (Scheme 2); the bis‐adducts8a,bpossess an ester structure, whereas the mono‐adducts6a–dpres

ISSN:0018-019X    

DOI:10.1002/hlca.19910740306

出版商:WILEY‐VCH Verlag GmbH    

年代:1991

数据来源: WILEY

摘要:

AbstractA new approach to the total, asymmetric synthesis ofD‐threo‐L‐talo‐octose ((−)‐1) and its derivatives is presented. It is based on the chemoselectiveWittig‐Hornermonoolefination of a 5‐deoxy‐D‐ribo‐hexodialdose derivative4obtained by selective reduction of (−)‐5‐deoxy‐2.3‐O‐isopropylidene‐/β‐D‐ribo‐hexofuranurono‐6,1‐lactone ((−)‐3). Allylic bromination of the resulting methyl (E)‐oct‐6‐enofuranuronate (+)‐5followed by intramolecular nucleophilic displacement of the so‐obtained bromides gave a 13.3:1 mixture of (−)‐methyl (E)‐l,4‐anhydro‐6,7‐dideoxy‐2,3‐O‐isopropylidene‐β‐L‐talo‐oct‐6‐enopyranuronate ((−)‐8) and methyl (E)‐l,4‐anhydro‐6,7‐dideoxy‐2,3‐O‐isopropylidene‐α‐D‐allo‐oct‐6‐enopyranuronate (9). The double hydroxylation of the enoate (−)‐8followedKishi'srule and gave the correspondingD‐threo‐β‐L

ISSN:0018-019X    

DOI:10.1002/hlca.19910740307

出版商:WILEY‐VCH Verlag GmbH    

年代:1991

数据来源: WILEY

摘要:

AbstractPalladium‐catalyzed ene‐type cyclizations of 1,6‐enynes2yield methylidene‐cyclopentane derivatives3. Removal of protecting groups from3furnishes the diols4and5, whose configurations were established by a combination of X‐ray and1H‐NMR techniques. The predominant formation of isomers4is discussed in the context of two alternative hypothetical mechanisms previously proposed for this type of

ISSN:0018-019X    

DOI:10.1002/hlca.19910740308

出版商:WILEY‐VCH Verlag GmbH    

年代:1991

数据来源: WILEY

摘要:

AbstractSubstituedN‐(isoxazol‐4‐yl)thioureas1undergo a transformation in the presence of hexacarbonylmolybdenum and acid to yield functionalized thiazoles3in a one‐pot reaction. In a few cases, 1,4,5‐trisubstituted dihydroimidazolethiones4are also isolated as side products. Mechanistic considerations are outlined and scope and limitations of this new methodology

ISSN:0018-019X    

DOI:10.1002/hlca.19910740309

出版商:WILEY‐VCH Verlag GmbH    

年代:1991

数据来源: WILEY

摘要:

AbstractNaturally occurring (−)‐(R,R)‐α‐necrodol ((−)‐1) and its C(4)‐epimer (−)‐2are obtained in 84 and 44% yields, respectively, by lithium ethylenediamide (LEDA) treatment of the corresponding β‐necrodols (−)‐3and (−)‐4(Scheme 1, Table), both readily available from (−)‐campholenyl acetate ((−)‐i) by an efficient stereoselective synthesis. The thermodynamically preferred (−)‐(R)‐γ‐necrodol ((−)‐5) becomes the major product (≥ 80% yield) after either prolonged treatment with LEDA or exposure of α‐ and β‐necrodols to BF3·Et2O. In an alternative route, (+)‐5is prepared starting from (+)‐campholenal ((+)‐ii)viaPd‐catalysed decarbonylation to (−)‐(S)‐1,4,5,5‐tetramethylcyclopent‐l‐ene ((−)‐6) and subseque

ISSN:0018-019X    

DOI:10.1002/hlca.19910740310

出版商:WILEY‐VCH Verlag GmbH    

年代:1991

数据来源: WILEY

摘要:

Catalytic Carbonylation of Acetone Oxime at Bisoximatodiruthenium Complexes: A Simple Access to Anhydrous Acetonine. Isolation and Molecular Structure of [Ru2(CO)5(Me2CNO)2(Me2CNOH)]In the presence of catalytic amounts of [Ru2(CO)4(Me2CNO)2(Me2CNOH)2] (1), acetone oxime reacts with carbon monoxide to give 2,3,4,5‐tetrahydro‐2,2,4,4,6‐pentamethylpyrimidine (‘acetonine’). CO2, and NH3. The reaction proceeds presumablyviacarbonylation of Me2CNOH to the unstable intermediate Me2CNOCHO which undergoes decarboxylation to give the corresponding imine Me2CNH. The final product is assumed to result from the cyclotrimerisation of Me2CNH with elimination of NH3. Evidence for the intermediacy of the imine comes from the analogous reaction of acetophenone oxime, PhMeCNOH, which gives the corresponding imine PhMeCNH as a stable product. The isolation of the complex [Ru2(CO)5(Me2CNO)2(Me2CNOH)] (2) suggests the carbonylation to take place at the brid

ISSN:0018-019X    

DOI:10.1002/hlca.19910740311

出版商:WILEY‐VCH Verlag GmbH    

年代:1991

数据来源: WILEY