Preparation of isomeric 2-methyl-1-oxotriterpenoids of the 18α-oleanane series; Conformation of ring A

Klinot, J.; Světlý, Jarmil; Klinotová, Eva; Budêšínský, Miloś; Vystrčil, A.

doi:10.1135/cccc19862869

Cited by 4 publications

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“…Aldol condensation reactions of allobetulone ( 4 ) with benzaldehydes and heterocyclic aldehydes lead to the α,β-unsaturated ketones 26 [ 6 , 50 ]. Similarly, Claisen condensations of 4 with formate and oxalic esters have been used to prepare the synthetically useful 1,3-diketones 27 or their enol tautomers ( Figure 6 ) [ 16 , 51 , 52 , 53 ]. The formyl derivative 27 (R = H) was converted into the 2-fluoro-methylidene derivative by treatment with DAST [ 54 ].…”

Section: Simple Functionalisation Reactions Of Allobetulin Analogsmentioning

confidence: 99%

“…2,3-Epoxides may be formed by ring closure of the corresponding bromohydrins, available from reduction of 2-bromoallobetulones 28 (X = Br), by epoxidation of alkene 24 a [ 59 , 82 ] or by oxidation reactions of enol acetate 34 (R = Ac) as mentioned above [ 66 ]. The main feature of these epoxides is their propensity for ring opening reactions with nucleophiles [ 52 , 61 ]. Interestingly, 2α,3α-epoxide 44 on treatment with a methyl Grignard reagent underwent rearrangement (see also next part 3) before addition of the organometal, affording nor-A alcohol derivative 45 .…”

Section: Ring Fusion To the A-ring Of Allobetulinmentioning

confidence: 99%

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Allobetulin and Its Derivatives: Synthesis and Biological Activity

2011

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This review covers the chemistry of allobetulin analogs, including their formation by rearrangement from betulin derivatives, their further derivatisation, their fusion with heterocyclic rings, and any further rearrangements of allobetulin compounds including ring opening, ring contraction and ring expansion reactions. In the last part, the most important biological activities of allobetulin derivatives are listed. One hundred and fifteen references are cited and the relevant literature is covered, starting in 1922 up to the end of 2010.

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Section: Simple Functionalisation Reactions Of Allobetulin Analogsmentioning

confidence: 99%

Section: Ring Fusion To the A-ring Of Allobetulinmentioning

confidence: 99%

Allobetulin and Its Derivatives: Synthesis and Biological Activity

2011

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ChemInform Abstract: Triterpenes. Part 79. Preparation of Isomeric 2‐Methyl‐1‐oxotriterpenoids of the 18α‐Oleanane Series; Conformation of Ring A.

Klinot¹,

Světlý²,

Klinotová³

et al. 1987

ChemInform

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Studies directed toward the synthesis of the massileunicellins

Chai¹,

McIntosh²

2004

Tetrahedron Letters

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The fully substituted hydroisobenzofuran core of the massileunicellins containing 8 contiguous stereocenters was prepared in 12 steps from (S)-(+)-carvone. Noteworthy elements of the synthesis include a one-step oxidative rearrangement/epoxidation, a novel stereoselective directed reduction of a keto diol, and a directed hydrogenation of a congested tetrasubstituted alkene.In 1999 and 2000, Pietra et al reported the structures of a series of eleven highly oxidized eunicellin diterpenes, designated massileunicellins, from the gorgonian coral Eunicella cavolinii ( Figure 1).1 In 2009, Sheu et al disclosed the structures of a related set of natural products from Klyxum simplex that they named the simplexins.2 , 3 These compounds are the most densely functionalized and highly oxidized members of the eunicellin diterpene family. 4 All of the massileunicellins and most of the simplexins contain 9 contiguous stereocenters, while simplexins C-I possess two additional oxygen-containing stereocenters at C5 and C7. While there have been a number of total, formal and partial syntheses of various 2,11-cyclized cembranoids,5 , 6 there have been no reports outside our own concerning the synthesis of massileunicellins (or simplexins).7In our previous report, we described an aldol/cycloaldol approach to the massileunicellins that assembled hydroisobenzofuran 3 in 4 steps from (S)-(+)-carvone (Scheme 1). We also described an unusual oxidative rearrangement of 3° allylic alcohol 4 epoxy ketone 5. This oxidation, originally disclosed by Sundaraman and Herz, had not been employed in synthesis since their 1977 report.8The Herz oxidation has the advantage of installing the requisite β-C-O bond at C11 (4 to 5, Scheme 1). However, it would be necessary at some point to reduce the C10 C-O bond to the alkane oxidation state (cf. 1 and 2, Figure). In our earlier report, we elected to postpone addressing that issue in order to focus on installing the C3, C12 and C13 stereocenters. We describe herein the details of a modified approach that takes advantage of the Herz oxidation, but resolves the issue of the C10 oxidation state.In our modified synthesis, we retain the C9 ester rather than reducing it (cf. Scheme 1). In the course of optimizing the Herz oxidation of β-hydroxy ester 3, we found that Collins' reagent converted allylic alcohol 3 to epoxy ketone 7 in high yield (Scheme 2). Exposure of epoxide 7 to excess KHMDS/TMSCl simultaneously opened the epoxide and converted the Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. C12 ketone to an enol silane to give enol silane 8. Rubottom oxidatio...

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Preparation of isomeric 2-methyl-1-oxotriterpenoids of the 18α-oleanane series; Conformation of ring A

Cited by 4 publications

References 0 publications

Allobetulin and Its Derivatives: Synthesis and Biological Activity

Allobetulin and Its Derivatives: Synthesis and Biological Activity

ChemInform Abstract: Triterpenes. Part 79. Preparation of Isomeric 2‐Methyl‐1‐oxotriterpenoids of the 18α‐Oleanane Series; Conformation of Ring A.

Studies directed toward the synthesis of the massileunicellins

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