Enantioselective Synthesis of (−)‐(19R)‐Ibogamin‐19‐ol

The first total synthesis of the natural product (?)‐(19R)‐ibogamin‐19‐ol ((?)‐ 1 ) is reported (biogenetic atom numbering). Starting with L ‐glutamic acid from the chiral pool and (2S)‐but‐3‐en‐2‐ol, the crucial aliphatic isoquinuclidine (= 2‐azabicyclo[2.2.2]octane) core containing the entire configurational information of the final target was prepared in 15 steps (overall yield: 15%). The two key steps involved a highly effective, self‐immolating chirality transfer in an Ireland–Claisen rearrangement and an intramolecular nitrone‐olefin 1,3‐dipolar cycloaddition reaction (Scheme 3). Onto this aliphatic core was grafted the aromatic moiety in the form of N(1)‐protected 1H‐indole‐3‐acetic acid by application of the dicyclohexylcarbodiimide (DCC) method (Scheme 4). Four additional steps were required to adjust the substitution pattern at C(16) and to deprotect the indole subunit for the closure of the crucial 7‐membered ring present in the targeted alkaloid family (Schemes 4 and 5). The spectral and chiroptical properties of the final product (?)‐ 1 matched the ones reported for the naturally occurring alkaloid, which had been isolated from Tabernaemonatana quadrangularis in 1980. The overall yield of the entire synthesis involving a linear string of 20 steps amounted to 1.9% (average yield per step: 82%).     

A New Enantioselective Total Synthesis of Natural Vincamine via an Intramolecular Mannich Reaction of an Silyl Enol Ether

Natural Vincamine ( 1 ) has been synthesized in an enantioselective manner starting from the ethylpentenal 7 . In the key step a mixture of the diastereoisomeric racemates, 14 and 15 , was directly obtained from the silyl enol ether 11 and the dihydro-β-carboline 12 by the way of an intramolecular Mannich reaction of the intermediate 13 (Scheme 4). The undesired stereoisomers, 14 and 15b , were recycled to 15a using the related reversible Mannich reaction 18 ? 14 + 15 , followed by crystallization of the salt from 15a and (+)malic acid. 15a was converted to natural vincamine ( 1 ) in several steps including the known transformation 20→1 .     

Synthèses dans la série des bis-indéno-fluorènes V [1]. Dihydro-13, 15–11 H-bis-indéno[2, 1-b; 1′, 2′-h]fluorène et dihydro-13, 15–5H-bis-indéno[1, 2-a; 1′, 2′-h]fluorène

Starting from cyclohexene and 2, 2′, 4, 4′-tetramethylbiphenyl the linear bis-indenofluorene 13, 15-dihydro-11 H-diindeno [2, 1-b; 1′, 2′-h] fluorene (X) has been synthetised in 5 steps (overall yield 30%). As an intermediate product the 11, 13, 15-trioxo-derivative IX was obtained. By a side way the 13-oxo-derivative of X and the already known monoangular bis-indenofluorene 13, 15-dihydro-5 H-diindeno [1, 2-a; 1′, 2′-h] fluorene (XIX) were also obtained.     

Approaches to the synthesis of cytochalasans. Part 6. Synthesis of the C(14)–C(23) subunit of cytochalasins A,B. F and desoxaphomin

The synthesis of methyl (4R, 8R,)-10-bromo-8-methyl-4-(1,3,6-trioxaheptane)-2-deceneoate ( 5 ), a synthon for the construction of the macrocyclic moieties of the cytochalasins A ( 1), B. (2) F (3) and desoxaphomin ( 4 ) is described. (S)-Glutamic acid ( 6 ) was transformed to the C₅-epoxide 10 and 3-methylglutaric acid ( 11 ) to the C₅-bromide 15 . Coupling of both 10 and 15 by a CuI-catalyzed Grignard reaction gave the decanol 16 in very high yield. The latter was transformed by several steps to synthon 5 .     

First Total Synthesis of Prionoid E,A Bioactive Rearranged Secoabietane Diterpene Quinone from Salvia prionitis

The first total synthesis of prionoid E ( 1 ), a rearranged secoabietane diterpene quinone isolated from Salvia prionitis, was achieved efficiently by means of Wacker oxidation (Scheme 5) and aldol condensation (Scheme 7) as the key steps in the synthetic sequence. Thus 1 was prepared in 15 steps in 3.7% yield starting on one hand from anisole (=methoxybenzene) and methylsuccinic anhydride (=dihydro‐3‐methylfuran‐2,5‐dione) via 4 (Scheme 3 and 5), and on the other hand from 2‐hydroxy‐2‐methylpropanoic acid via 5 (Scheme 6).     

δ-Santalolanaloga I Synthesen in der Isocamphanreihe, 27. Mitt.

The synthesis of 4-(3,3-dimethyl-2-exo-norbornyl)-2-methyl-2-buten-1-ol (5), an analogue of -santalol (1) has been described. One route to5 starts with isocamphenilanyl propionic acid (8) which can be prepared in 4 steps from isocamphenilanic acid (9). Also 4 steps lead from8 to the target molecule5 with an overall yield of 24%. By a second and more convergent route, starting from the very easily obtainable bicyclic ketone19, the allylic alcohol5 could be obtained again in 4 steps, but this time with an overall yield of 1.3%. A new and easy synthesis of isocamphenilanyl acetic acid (15), a potential starting material to5, has been described, also the preparation of some new isocamphane derivatives.

26. Mitt:Buchbauer G,Pernold W,Ittner M,Ahmadi MF,Dobner R,Reidinger R (1985) Monatsh Chem 116: 1209     

Synthèses dans la série des bis-indéno-fluorènes I. Le dihydro-13, 15-5H-bis-indéno[1, 2-a;1′, 2′-h]fluorène

The synthesis of 13,15-dihydro-5H-diindeno[1,2-a;1′,2′-h] fluorene in six steps starting from flourenone 4-carboxylic acid and 2,4-dichlorotoluene is described. As an intermediate product the 5, 13, 15-trioxo-derivative is obtained.     

Synthesis of Aristotelia-Type Alkaloids. Part VIBiomimetic Synthesis of (+)-Aristofruticosine

(S)-Perilla alcohol ( 5 ) was transformed into (S)-7-(phenylthio)-p-menth-1-en-8-amine ( 11 ) in five steps. Condensation of this building block with 1-(4-methoxyphenylsulfonyl)-1H-indole-3-acetaldehyde ( 12 ) led to the expected imine 15 which cyclized in 54% yield to protected 20-(phenylthio)hobartine 16 upon exposure to anh. HCOOH. Treatment of this intermediate with an alkylating reagent led to (+)-aristofruticosine protected in the indole moiety via an intramolecular, allylic nucleophilic displacement reaction. Subsequent reductive removal of the protecting group completed the first synthesis of the Aristotelia alkaloid (+)-aristofruticosine ((+)- 4 ). This straightforward synthesis confirmed the tentative structure (+)- 4 , proposed by Bick and Hai, and established the hitherto unknown absolute configuration of this metabolite.     

Investigations Concerning an Unexpected Reaction between the Dimsyl Anion ( = (Methylsulfinyl)methanide) and a γ,δ-Epoxy-ketone

The reaction of 2,2-dimethyl-5-(1,2-epoxypropyl)cyclohexanone ( 7 ) with t-BuOK in DMSO furnished a small amount of 5-(1-hydroxyprop-2-enyl)-2,2-dimethylcyclohexanone ( 12 ) and the 4 unexpected products 13–16 which contain one to three additional C-atoms (Scheme 2). The relative configuration of the major product 1-(4′,4′-dimethyl-2′,3′-dimethylidenecyclohexyl)propane-1,2-diol ( 15 ) was shown to be 1RS, 2RS,1′SR via NOE measurements performed on a derivative thereof. A crossover experiment in DMSO/[¹³C₂]DMSO 1:1 as solvent showed that the two additional C-atoms of this product originate from a single molecule of DMSO (Scheme 5). A tentative mechanistic scheme, consistent with all experimental observations, is proposed which involves a [2,3]-sigmatropic rearrangement of an (allylsulfinyl)methanide to a sulfenic acid as one of the key steps ( V → 24 , Scheme 8). We corroborated part of this hypothetic scheme by taking recourse to a model compound (7-(methylsulfinyl)-p-mentha-1,8-diene ( 32/33 ), readily prepared in two steps from perilla alcohol ( 30 )), which reacted as predicted by the proposed mechanism (Schemes 9 and 10).     

Partialsynthesen und Reaktionen von Abietanderivaten (Lanugonen) aus Plectranthus lanuginosus und verwandten Verbindungen

Partial Syntheses and Reactions of Abietanoid Derivatives (Lanugones) from Plectranthus lanuginosus and of Related Compounds Interconversions by partial syntheses of several lanugones establish their absolute configuration at C(15). Unexpected reactions exemplify the unique reactivity of these abietanoic diterpenes, - Lanugone O ( 4 ) was prepared in several steps from (15S)-coleon C ( 8a ; Scheme 2) thus establishing its (15S)-configuration. One of the intermediates, the 12-O-acetyl-6-oxoroyleanone 12 , through acetyl-migration sets up an equilibrium with the vinylogous quinone 13 (Scheme 3). - The chirality at C(15) in the dihydrofuran moiety of lanugone Q ( 16 ) was proven by acid-catalyzed conversion of lanugone O ( 4 ) to 16 . - Instead of the usual nucleophilic attack shown by quinomethanes, lanugone L (1 ) is electrophilically substituted at C(7) by acetic anhydride/pyridine (Scheme 1). - In a homosigmatropic [1,5]-H-shift, lanugone G ( 17 ) in solution is converted to the corresponding allyl substituted royleanone 18 (Scheme 4). - Methanolysis of lanugone J ( 19 ) leads to the expected royleanone 20 having the 2-methoxypropyl side chain ( Scheme 5 ). Similar reactions were found in acetolytic reactions. However, treatment-of spirocoleons with SOCl₂/DMF produces mainly 12-deoxyroyleanones with allyl- and 2-chloropropyl groups, i. e. 19 → 26 and 27 ; 28 → 29 . The possible natural occurrence of these compounds is emphasized.     

Synthèses dans la série des bis-indéno-fluorènes,VI [1] Dihydro-12, 15–6H-bis-indéno[1. 2-b; 2′.1′-h]fluorène et dihydro-14, 15–8H-bis-indéno[2. 1-a; 2′. 1′-h]fluoréne

Starting from cyclohexene and 2, 2′, 5, 5′-tetramethylbiphenyl the linear bisindenofluorene 12, 15-dihydro-6H-diindeno [1.2-b; 2′.1′-h] fluorene (XX) has been synthesized in 5 steps (overall yield 27%). As an intermediate product the 6, 12, 15-trioxo-derivative XIX (greyish green crystals, blue alcaline vat) was obtained. By a side way, the 6-oxo-derivative of XX and the already known monoangular bis-indenofluorene 14, 15-dihydro-8H-diindeno [2.1-a; 2′.1′ -h] fluorene (XXVII) were also obtained. XX can also be prepared in several steps starting from 3-methyl-fluorene or fluorene.     

Electrofugal Fragmentation of Alkylcobalamin Derivatives Using Cob(I)Alamin and Heptamethyl Cob (I)yrinate as Catalysts

The cob (I)alamin- ( 1(I) ) and the heptamethyl cob(I)ynnate- ( 2(I) ) catalyzed transformation of an epoxide to the corresponding saturated hydrocarbon 3→4→5 is examined (see Schemes 1 and 3–5). Under the reaction conditions, the epoxyalkyl acetate 3 is opened by the catalysts with formation of appropriate (b?-hydroxyalkyl)-corrinoid derivatives ( 13 , 14 , 17 , 18 , see Schemes 12 and 14). Triggered by a transfer of electrons to the Co-corrin-π system, the Co, C-bond of the intermediates is broken, generating the alkenyl acetate 4 (cf. Schemes 12 and 14) following an electrofugal fragmentation (cf. Schemes 2 and 12). The double bond of 4 is also attacked by the catalysts, leading to the corresponding alkylcorrinoids ( 15 , 19 , see Schemes 12 and 14) which in turn are reduced by electrons from metallic zinc, the electron source in the system, inducing a reductive cleavage of the Co, C-bond with production of the saturated monoacetate 5 (see Schemes 2, 5 and 12). In the cascade of steps involved, the transfer of electrons to the intermediate alkylcorrinoids ( 13–15 , 17–19 , see Schemes 12 and 14) is shown to be rate-limiting. Comparing the two catalytic species 1(I) and 2(I) , it is shown that the ribonucleotide loop protects intermediate alkylcobalamins to some extent from an attack by electrons. The protective function of the ribonucleotide side-chain is shown to be present in alkylcobalamins existing in the base-on form (cf. Chap. 4 and see Scheme 14).     

A Concise Synthesis of the HCV Protease Inhibitor BILN 2061 and Its P3 Modified Analogs

A concise synthesis of BILN 2061 was achieved through more efficient installation of P2 4‐quinoline moiety via S_N2 displacement of the β‐OBs group located on the 4‐hydroxyl proline intermediate, which was prepared from 4‐α‐hydroxyl proline analog via Mitsunobu reaction with inversion of stereochemistry. In addition, a short and practical synthesis for P3 unit is also described herein. Final assembly of four fragments for BILN 2061 was achieved with an overall yield of 58% in 4 steps from P1 to 15a . Furthermore several analogs of BILN 2061 ( WX‐1 – WX ‐ 5 ) containing modifications on P3 unit were synthesized successfully using the same synthetic route as described for the parent inhibitor BILN 2061 .     

Biosynthetic Production of [N2,1,3,7,9-15N]Guanosine and [1,3,7,9-15N]inosine and conversion into [N6,1,3,7,9-15N]adenosine for structure elucidation of RNA by heteronuclear NMR

A procedure was developed for the biosynthetic preparation of ¹⁵N-labelled guanosine and inosine through the action of a mutant Bacillus subtilis strain. Crude [N²,1,3,7,9-¹⁵N]guanosine and [1,3,7,9-¹⁵N]inosine were isolated from the culture filtrate by precipitation and anion-exchange chromatography (Scheme 1). No cell lysis and no enzymatic degradation was necessary. The per-isobutyrylated derivatives 1 and 2 were isolated from a complex mixture, purified by virtue of their different lipophilicity, and separated in three steps involving normal-and reversed-phase silica-gel chromatography. One litre of complex nutrient medium yielded 8.44 mmol of guanosine derivative and 2.84 mmol of inosine derivative with high average ¹⁵N enrichment (83.5 and 91.9 atom-%, resp.). [N⁶,1,3,7,9-¹⁵N]Adenosine ( 4 ) was obtained from 2′,3′,5′-tri-O-isobutyryl[1,3,7,9-¹⁵N]inosine ( 1 ) through the ammonolysis of its 1,2,4-triazolyl derivative with aqueous ¹⁵NH₃ (Scheme 2).     

A New Route for Preparation of 5‐Deoxy‐5‐(hydroxyphosphinyl)‐ D‐mannopyranose and ‐L‐gulopyranose Derivatives

Starting from methyl 2,3‐O‐isopropylidene‐α‐D ‐mannofuranoside ( 5 ), methyl 6‐O‐benzyl‐2,3‐O‐isopropylidene‐α‐D ‐lyxo‐hexofuranosid‐5‐ulose ( 12 ) was prepared in three steps. The addition reaction of dimethyl phosphonate to 12 , followed by deoxygenation of 5‐OH group, provided the 5‐deoxy‐5‐dimethoxyphosphinyl‐α‐D ‐mannofuranoside derivative 15a and the β‐L ‐gulofuranoside isomer 15b . Reduction of 15a and 15b with sodium dihydrobis(2‐methoxyethoxy)aluminate, followed by the action of HCl and then H₂O₂, afforded the D ‐mannopyranose ( 17 ) and L ‐gulopyranose analog 21 , each having a phosphinyl group in the hemiacetal ring. These were converted to the corresponding 1,2,3,4,6‐penta‐O‐acetyl‐5‐methoxyphosphinyl derivatives 19 and 23 , respectively, structures and conformations (⁴C₁ or ¹C₄, resp.) of which were established by ¹H‐NMR spectroscopy.     

Total Synthesis of (+)‐Machaeriols B and C and of Their Enantiomers with a Cannabinoid Structure

An efficient and concise synthesis of the biologically interesting (+)‐machaeriol B ( 2 ) and its enantiomer 5 was accomplished from O‐phenylhydroxylamine ( 7 ) in four steps (Scheme 2). In addition, the first total synthesis of natural (+)‐machaeriol C ( 3 ) and its enantiomer 6 was achieved from the readily available ester 15 in eight steps (Scheme 4). The key strategies in the syntheses of 2 and 5 involved benzofuran formation through a [3,3]‐sigmatropic rearrangement and trans‐hexahydrodibenzopyran formation by a domino aldol‐type/hetero‐Diels–Alder reaction. In the case of 3 and 6 , the key steps were stilbene formation by a Horner–Wadsworth–Emmons reaction and trans‐hexahydrodibenzopyran formation by domino reactions.     

Preparation of Optically Enriched Secondary Alkyllithium and Alkylcopper Reagents—Synthesis of (−)‐Lardolure and Siphonarienal

Optically enriched secondary alkyl iodides were converted into secondary alkyllithium and secondary alkylcopper compounds with very high retention of configuration. Quenching with various electrophiles, including chiral epoxides, provided a range of chiral molecules with high enantiomeric purity (>90 % ee). This method has been applied in an iterative fashion in the total synthesis of (?)‐lardolure in 13 steps and 5.4 % overall yield (>99 % ee, dr>99:1) and siphonarienal in 15 steps and 5.6 % overall yield (>99 % ee, dr>99:1) starting from commercially available ethyl (R)‐3‐hydroxybutyrate (>99 % ee).     

Controlled Allylic Transformations via the Meisenheimer Rearrangement

Described are synthetic sequences which effect allylic transformations I and II. Sequence I involves (1) conversion of a primary allyl alcohol into the corresponding N, N-dimethyl-amine oxide, (2) [2,3]-rearrangement to give an N, N-dimethylhydroxylamine and (3a) reduction to give the ‘rearranged’ secondary or tertiary allyl alcohol [e.g. 36 → 35 → 37 → 40 ]. Sequence II involves the same steps (1) and (2), followed by (3b) N-methylation of a secondary N, N-dimethyl-hydroxylamine and (4) Hofmann elimination to give a vinyl ketone [e.g. 11 → 12 → 13 → 14 → 15 ].     

Total Synthesis of (+)‐β‐Himachalene

An enantioselective synthesis of (+)‐β‐himachalene ( 2 ) was accomplished starting from (1S,2R)‐1,2‐epoxy‐p‐menth‐8‐ene ( 3 ) in 15 or 16 steps with an overall yield of ca. 6% (Schemes 3, 5, and 6). Key transformations include an Ireland–Claisen rearrangement, a Corey oxidative cyclization, and a ring expansion.     

A Novel Synthesis of the Macrocyclic Spermidine Alkaloid (+)‐(S)‐Dihydroperiphylline

A novel, short, and highly stereoselective synthesis of the macrocyclic spermidine alkaloid (+)‐(S)‐dihydroperiphylline ( 15 ) is described. The key synthetic steps were the stereoselective addition of the chiral amine 1 to the cinnamate 2 and cyclization of the bis[toluene‐4‐sulfonamide] precursor 12 in the presence of Cs₂CO₃ as a template. Unambiguous assignments of the signals in both the ¹H‐ and ¹³C‐NMR spectra of 15 were achieved by 2D NMR spectra.