Barton Deoxygenation Research Articles

ChemInform Abstract: Reductive Deoxygenation of Alcohols: Catalytic Methods Beyond Barton—McCombie Deoxygenation

AbstractReview: 113 refs.

A concise approach for the synthesis of bitungolides: total syntheses of (-)-bitungolide B & E.

The first total synthesis of (-)-bitungolide B and a second-generation total synthesis of (-)-bitungolide E are described. The cornerstone of the approach comprises a convergent and flexible route involving Brown crotylation, highly diastereoselective substrate controlled Paterson anti-aldol reaction, hydroxyl-directed 1,3-syn/anti reduction, Barton-McCombie deoxygenation and RCM reactions. Via this route, a common intermediate 13 is readily accessible for the synthesis of the family of bitungolides A-E and franklinolides A-C.

β-Oxygen Effect in the Barton–McCombie Deoxygenation Reaction: Further Experimental and Theoretical Findings

The chemistry of (S)-methyl xanthates derived from xylo- and ribo-furanose derivatives in the presence of the stannyl radical is investigated. Xanthate derived from β-xylo-furanose affords exclusively a deoxygenated product; whereas, under the same reaction conditions, the α-ribo-furanose xanthate derivative produces quantitatively a hemithioacetal compound. We reasoned that in the case of the β-xylo-furanose derivative, a favorable β-oxygen effect in the Barton-McCombie deoxygenation reaction is operating where, according to theoretical calculations, unusual molecular orbital interactions (and not strain, as previously proposed) are present. These orbital interactions involve the SOMO (intermediary generated from the stannyl radical addition) with the σ* orbital of the bond undergoing cleavage and this with the two C-O antibonding orbitals anti oriented. Such molecular orbital interactions are not present in the α-ribo-furanose; therefore, the β-scission is highly delayed, and due to the reversibly nature of the stannyl radical addition, the ribo-furanose xanthate is forced to take an alternative route: the homolytic substitution (S(H)2) of the sulfide sulfur by stannyl radical. This radical addition gives the alkoxythiocarbonyl radical, which is trapped by Bu3SnH before the elimination of carbonyl sulfide; subsequently, radical stannyl addition followed by radical reduction produces the hemithioacetal.

Stereoselective Synthesis of (−)‐(1R,1′R,5′R,7′R)‐1‐Hydroxy‐exo‐brevicomin and (+)‐exo‐Brevicomin from 3,4,6‐Tri‐O‐acetyl‐D‐glucal

AbstractStereoselective syntheses of (−)‐(1R,1′R,5′R,7′R)‐1‐hydroxy‐exo‐brevicomin (1) and (+)‐exo‐brevicomin (2) were accomplished from 3,4,6‐tri‐O‐acetyl‐D‐glucal (5; Schemes 2 and 3). Chemoselective reduction, Grignard reaction, BartonMcCombie deoxygenation, and ketalization were used as key steps.

Reductive Deoxygenation of Alcohols: Catalytic Methods Beyond Barton–McCombie Deoxygenation

AbstractDeoxygenation is an extensively studied field in organic chemistry, and over the last decades many methods such as the Barton–McCombie deoxygenation reaction or deoxygenation with alkali metals in liquid ammonia have been established. Within this microreview, we discuss different strategies for the chemoselective catalytic deoxygenation of alcohols with special attention to their scope and limitations. The deoxygenation of derivatives of alcohols and their direct deoxygenation as step‐economic alternative are covered with current examples. Catalytic methods can serve as convenient and economic alternatives for established methods in the reduction of carbon–oxygen single bonds.

Stereocontrolled Total Synthesis of Polygalolide A

The total synthesis of polygalolide A, a secondary metabolite that was isolated from a Chinese medicinal plant, is reported. A key issue in this synthesis was construction of an oxabicyclo[3.2.1] skeleton, which was solved by the development of an intramolecular Ferrier-type C-glycosylation of a glucal with siloxyfuran as an internal nucleophile. The substrate was prepared from D-glucal by the introduction of trimethylsilylacetylene and siloxyfuran groups. Although C-glycosylation did not occur under the conditions found from model experiments, further examination revealed that the combination of trimethylsilyl trifluoromethanesulfonate (TMSOTf) and 2,4,6-collidine successfully afforded the desired product as a single diastereomer. The siloxy group at the C3 position played a crucial role in the stereocontrol of this reaction. The product was further transformed into a tetracyclic compound as follows: The vinyl ether and acetylenic moieties were reduced and the siloxy group was removed with a Barton-McCombie reaction. The construction of the six-membered ether and the γ-lactone provided the tetracyclic compound. Finally, a phenolic moiety was introduced by using a Mukaiyama aldol reaction to furnish polygalolide A.

Stereoselective Synthesis of the C31-C41 Spirolactam Fragment of Sanglifehrin A

A stereoselective synthesis of the spirolactam fragment (C31–C41) of a highly-potent immunosuppressant sanglifehrin A is described. The key steps involved in this synthesis are a desymmetrization protocol, Sharpless asymmetric epoxidation, Crimmins syn aldol reaction, Barton–McCombie deoxygenation, and acid-mediated spirolactamization.

ChemInform Abstract: Total Synthesis of α‐Elvucitabine.

Abstract Total synthesis of α-elvucitabine was achieved in 26% overall yield by a concise nine-step procedure starting from L-lyxose, with trimethylsilyl trifluoromethaneoulfonate (TMSOTf)–mediated stereocontrolled α-N-glycosidation and olefination through Barton–McCombie deoxygenation being the key steps, and the stereochemistry of the product was determined by nuclear Overhauser effect spectroscopy.

Photoinduced Isomerization of 23-Oxosapogenins: Conformational Analysis and Spectroscopic Characterization of 22-Isosapogenins

The first synthesis of 22-isospirostane derivatives is described. They were obtained by photochemical isomerization of 23-oxosapogenins. The structure of 23-oxo-22-isotigogenin acetate (12) was proved by a single crystal X-ray diffraction, while structures of 23-oxo-22-isodiosgenin acetate (13) and 23-oxo-22-isosarsasapogenin acetate (14) were elucidated by spectroscopic methods. 22-Isodiosgenin acetate (17) was obtained by NaBH(4) reduction of the 23-oxo derivative 13 followed by the two-step Barton-McCombie deoxygenation procedure. Conformational analysis of 22-iso compounds was carried out with CD and NMR, as well as DFT calculations.

Total Synthesis of α-Elvucitabine

Total synthesis of α-elvucitabine was achieved in 26% overall yield by a concise nine-step procedure starting from L-lyxose, with trimethylsilyl trifluoromethaneoulfonate (TMSOTf)–mediated stereocontrolled α-N-glycosidation and olefination through Barton–McCombie deoxygenation being the key steps, and the stereochemistry of the product was determined by nuclear Overhauser effect spectroscopy.

A Carbohydrate‐Based Synthesis of the C13–C22 Fragment of Amphidinolide X

AbstractA facile carbohydrate‐based route was developed for the synthesis of the tetrahydrofuran (C13–C22) fragment of amphidinolide X. Starting from L‐sorbose, the key reactions followed include the stereoselective synthesis of a quaternary center at C1, Barton–McCombie deoxygenation at C2, Mitsunobu inversion at C3, and chain elongation by a Wittig reaction at C5.

Total synthesis of epothilones using functionalised allylstannanes for remote stereocontrol

Two syntheses of the C(7)–C(16)-fragment 41 of epothilone D 2 were developed that were based on tin(IV) bromide mediated reactions of 5,6-difunctionalised hex-2-enylstannanes with aldehydes. In the first synthesis, (5S)-6-tert-butyldimethylsilyloxy-5-hydroxy-2-methylhex-2-enyl(tributyl)stannane 20 was reacted with (E)-but-2-enal to give (2S,7R,4Z,8E)-1-tert-butyldimethylsilyloxy-5-methyldeca-4,8-diene-2,7-diol 26 containing ca. 20% of its (7S)-epimer. Following desilylation, the crystalline (2S,7R)-triol 32 was protected as its acetonide 33 and esterified to give the (4-methoxybenzyloxy)acetate 34. An Ireland–Claisen rearrangement of this ester gave methyl (2R,3S,10S,4E,7Z)-3,7-dimethyl-10,11-(dimethylmethylene)dioxy-2-(4-methoxybenzyloxy)undeca-4,7-dienoate 35 that was converted into (2S,9S,6Z)-2,6-dimethyl-9,10-(dimethylmethylene)dioxydec-6-en-1-ol 41 by regioselective alkene manipulation, ester reduction and cleavage of the resulting terminal diol 40 with a reductive work-up. The second synthesis involved the tin(IV) bromide mediated reaction between the stannane 20 and (3S)-4-(4-methoxybenzyloxy)-3-methylbutanal 44 that gave (2S,7S,9S,4Z)-1-tert-butyldimethylsilyloxy-5,9-dimethyl-10-(4-methoxybenzyloxy)dec-4-ene-2,7-diol 45 containing ca. 20% of its (7R)-epimer. After desilylation and protection of the vicinal diol as its acetonide 46, a Barton–McCombie reductive removal of the remaining hydroxyl group gave the (2S,9S,6Z)-2,6-dimethyl-9,10-(dimethylmethylene)dioxydec-6-en-1-ol 41 after oxidative removal of the PMB-ether. The first of these syntheses uses just one chiral starting material, but the second is shorter and more convergent. It was therefore modified by the use of (5S)-6-tert-butyldimethylsilyloxy-5-(2-trimethylsilylethoxy)methoxy-2-methylhex-2-enyl(tributyl)stannane 49 that reacted with (3S)-4-(4-methoxybenzyloxy)-3-methylbutanal 44 to give a 50:50 mixture of the C(4)-epimers of (2S,9S,6Z)-10-tert-butyldimethylsilyloxy-1-(4-methoxybenzyloxy)-2,6-dimethyl-9-(2-trimethylsilylethoxy)methoxydec-6-en-4-ol 50 with high fidelity for formation of the (Z)-alkene. Following the Barton–McCombie deoxygenation, the product 52 was taken through to (2S,9S,6Z,10E)-2,6,10-trimethyl-11-(2-methyl-1,3-thiazol-4-yl)-9-(2-trimethylsilylethoxy)methoxyundeca-6,10-dienal 59 that corresponded to the fully functionalised C(7)–C(17) fragment of epothilone D 2. A precedented stereoselective aldol condensation followed by O-protection, selective deprotection, oxidation and macrocyclisation then gave the macrolide 71 that was deprotected to complete a synthesis of epothilone D 2. Finally regio- and stereo-selective epoxidation gave epothilone B 1.

7-Deazapurine (Pyrrolo[2,3-d]pyrimidine) 2-Deoxyribonucleosides: Syntheses and Transformations

This review reports on the synthesis of 7-deazapurine (pyrrolo[2,3-d]pyrimidine) 2-deoxyribonucleosides, including β-D- and β-L-enantiomers, fluoro derivatives, and 2,3-dideoxyribonucleosides. It covers the various aspects of convergent nucleoside synthesis. Stereochemically defined α-D and α-L 2-deoxyribonucleosides as well as sugar derivatives were prepared by nucleobase anion glycosylation. This glycosylation reaction is regioselective for the pyrrole nitrogen and stereoselective for β-nucleoside formation. Common glycosylation protocols lead to 7-deazapurine 2-deoxyribonucleosides with unusual glycosylation sites. 7-Deazapurine 2,3- dideoxyribonucleosides were also obtained from 2-deoxy- or 3-deoxyribonucleosides by Barton-McCombie deoxygenation, by elimination of sugar hydroxyl groups or by anion glycosylation. Another aspect of the review is the functionalization of pyrrolo[2,3-d]pyrimidine nucleosides. A broad range of reporter groups were introduced by the Sonogashira cross coupling or the copper(I)-catalyzed Huisgen- Meldal-Sharpless “click” reaction. The application of 7-deazapurine nucleosides as antiviral or anticancer agents, and the use of 7- deazapurine nucleoside triphosphates in the Sanger dideoxy DNA-sequencing are also reported.

Synthesis of deuterium labeled NMDA receptor inhibitor – 20-Oxo-5β-[9,12,12- 2H 3]pregnan-3α-yl- l-glutamyl 1-ester

20-Oxo-5β-[9,12,12- 2H 3]pregnan-3α-yl- l-glutamyl 1-ester 11 was synthesized as an internal standard for quantification of a neuroprotective NMDA receptor ligand, 20-oxo-5β-pregnan-3α-yl- l-glutamyl 1-ester 18 and its metabolites, in plasma and tissue. 11α-Hydroxy-progesterone ( 1) was reduced under basic conditions to yield the corresponding 5β-steroid. Protection of the 3- and 20-oxo groups and oxidation of the 11α-hydroxy group was then followed by a deuterium exchange, conducted under basic conditions using deuterated methanol. Next, the carbonyl moiety at C-11 was reduced and the 11α-hydroxyl group removed through utilization of the Barton–McCombie reaction. Subsequent deprotection of the 3- and 20-acetals and stereoselective reduction of the 3-oxo group gave the desired trideuterated pregnanolone ( 8). This was coupled with protected glutamic acid, which was then deprotected to yield [9,12,12- 2H 3]-pregnanolone glutamate ( 11) with >99% isotopic purity.

A Carbohydrate-Based Total Syntheses of (+)-Pyrenolide D and (−)-4-epi-Pyrenolide D

Efficient total syntheses of (+)-pyrenolide D (5) and (-)-4-epi-pyrenolide D (6) have been achieved from a known 5-deoxy-d-xylose derivative 9 in ten steps with 19% overall yield either exclusively as 5 or as pure 5 and 6 in a 3:2 ratio. Key steps involved are one-pot epoxidation-cyclization by Corey-Chaykvosky reagent, reductive Barton-McCombie deoxygenation, and per-acid mediated oxidative spiroketalization.

Synthesis and antiviral evaluation of α- l-2′-deoxythreofuranosyl nucleosides

The synthesis of a series of α- l-2′-deoxythreofuranosyl nucleosides featuring the nucleobases A, T, C and U is described in seven steps from 1,2-O-isopropyledene-α- l-threose, involving a Vorbrüggen coupling and a Barton-McCombie deoxygenation protocol as the key steps. All analogues, including a phosphoramidate nucleoside phosphate prodrug of the T analogue, were evaluated against a broad panel of different viruses but found inactive, while also lacking notable cellular toxicity. The thymidine analogue showed inhibition to mitochondrial thymidine kinase-2 (TK-2), herpes simplex virus type 1 (HSV-1) TK, varicella-zoster virus (VZV) TK and Mycobacterium tuberculosis thymidylate kinase.

A New Synthesis of 2-chloro-2′-deoxyadenosine (Cladribine), CdA)

A new efficient route for the synthesis of 2-chloro-2′;-deoxyadenosine (Cladribine), CdA) has been developed. The key step of this method was selective deprotection of the acetyl group at the 2′ position; the 3′, 5′ acetyl groups were not affected. This can be accomplished efficiently with hydroxylamine hydrochloride and sodium acetate in pyridine. The 2′ hydroxyl group was removed by the Barton-McCombie reaction. Using this strategy, CdA was prepared in five steps and 31.0% yields.

ChemInform Abstract: Deoxygenation of Alcohols by the Reactions of Their Xanthate Esters with Triethylsilane: An Alternative to Tributyltin Hydride in the Barton-McCombie Reaction.

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ChemInform Abstract: Synthesis of 7-Deoxy- and 7,10-Dideoxytaxol via Radical Intermediates.

7-Deoxytaxol and 7,10-dideoxytaxol were prepared from baccatin III, employing the Barton deoxygenation procedure and the Holton acylation method at C-13

ChemInform Abstract: Syntheses of Novel Piperidin-4-ylphosphinic Acid, and Piperidin-4-ylphosphonic Acid Analogues of the Inhibitory Neurotransmitter 4-Aminobutyric Acid (GABA).

Piperidin-4-ylphosphinic acid, methyl(piperidin-4-yl)phosphinic acid and piperidin-4-ylphosphonic acid analogues of the GABAA agonist piperidin-4-ylcarboxylic acid (isonipecotic acid) were synthesised. The acid groups were introduced using a sequential Pudovik addition followed by a Barton deoxygenation procedure and finally followed by acidic hydrolysis. The mild and efficient procedure gave the target amino acids in good yields.