Unactivated terpenes as 2π components in intermolecular hetero diels-alder reactions. A short stereocontrolled approach to the robustadial and euglobal skeleton.

Krause, Michael; Martin, Harry J.; Hoffmann, Roald

doi:10.1016/s0040-4039(00)97132-6

Cited by 20 publications

(2 citation statements)

References 15 publications

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“…The aldehyde 11 was prepared from (1R)-(+)-nopinone (8) (Scheme 2). Homologation by a Wadsworth-Emmons reaction with triethyl phosphonoacetate and sodium hydride afforded 9,' which in turn was reduced with DIBAL-H at -78°C to give 1 0 .~ The alcohol 10 was oxidized with tetra-n-propylarnmonium permthenate and N-methyl morpholine N-oxide (9), giving the desired 1 1 .~ Using the reaction conditions described above, with 3,5-dimethoxyphenol (12) and 11, we obtained a 3:2 diastereoisomer ratio of 13a and 13b in 66% combined yield.3 With the spiropinene 2H-1-benzopyran ring system in hand, the next transformation towards robustadials was the replacement of the chromene C = C bond with a carbony1 at C4. To explore the desired transformation, the model compound 14 (Scheme 3) was subjected to oxymercuration (10) and bromohydrin formation reactions without success.…”

Section: Resultsmentioning

confidence: 99%

One-pot synthesis of precocene I and II and a formal synthesis of robustadial A and B via 2-phenyl-4H-1,3,2-benzodioxaborin

Bissada¹,

Lau²,

Bernstein³

et al. 1994

Can. J. Chem.

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Section: Resultsmentioning

confidence: 99%

One-pot synthesis of precocene I and II and a formal synthesis of robustadial A and B via 2-phenyl-4H-1,3,2-benzodioxaborin

Bissada¹,

Lau²,

Bernstein³

et al. 1994

Can. J. Chem.

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“…The organic layer was extracted with CH 2 Cl 2 (3 × 50 mL), dried (MgSO 4 ) and concentrated in vacuo to give an orange solid, which was purified by flash chromatography (CH 2 Cl 2 ) yielding 7 (1.08 g, 80%) as an orange solid; mp 81-82°C (cyclohexane). 1 (2) K on a Stoe AED2 4-circle diffractometer using MoKα graphite monochromated radiation, using 2θ/ω scans in the range 4-51° in 2θ. The structure was solved by direct methods using the programme SHELXS-97.…”

Section: -Hydroxymethyl-356-trimethyl-14-benzoquinone (7)mentioning

confidence: 99%

Diels-Alder Trapping of ortho-Quinone Methides. A New Entry to Substituted Xanthene-1,4-diones

Giraud¹,

Giraud²

1998

Synthesis

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Highly regioselective Diels-Alder reactions of a non-protected β -hydroxy quinone have been achieved after formation of chelated lithium alkoxides. In this report, we demonstrate on a model system, that the selectivity of reactions based on 1,3-dioxy-substituted quinones can be efficiently controlled by the addition of Lewis acid (AlMe 3 ), which chelates the substrate by the two oxygens.Recently, synthetic methods for the construction of chromane and spirochromane skeletons have been investigated as a route to robustadial and their important synthetic intermediates. 1-3 Interesting natural products of potential significance in treating malaria or as inhibitors of Epstein-Barr virus activation, 4,5 robustadials (e.g. robustadial A and B) and euglobals (e.g. euglobal 1a 1 and 1a 2 ), were isolated ten years ago from the leaves of Eucalyptus robusta by Nakanishi's group, 6 along with a variety of cycloadduct molecules whose structure suggested that their biosynthesis was the result of a biogenetic cycloaddition of corresponding terpenes and ortho -quinone methides ( orthoquinomethanes). The first total synthesis of robustadial dimethyl ethers, which was reported by Salomon and al ., 3,7 also proved their structure. It was followed by a few other studies describing partial or total syntheses. [8][9][10][11][12] ortho -Quinone methide reactive dienes, readily available from quinoid compounds, have found numerous applications in organic synthesis. 13 For example, they are ideally suited for the generation of a variety of annulated ring systems via intramolecular hetero-Diels-Alder cycloadditions. 14,15 This potentially general method could conceivably be used to prepare a large number of chromanes as euglobal analogs. 16 In the present study, different substituted quinones were converted to the corresponding o -quinone methides by an anionic way, 17 before being successfully trapped in situ either by another quinone to give chromanes or by unactivated alkenes to form euglobal skeletons. 18 The starting substituted quinones 7-9 were easily prepared on a multigram scale from 2,5-dimethoxy-3,4,6-trimethylbenzaldehyde (1) 19 in several steps (Scheme 1). Reduction of the aldehydic function of 1 with NaBH 4 in methanol furnished the corresponding alcohol 2 , which was converted either into the chloride 3 in over 85% yield by thionyl chloride in anhydrous dichloromethane, or into the benzyl ether derivative 6 in 88% yield by O -benzylation of 2 with benzyl bromide. Treatment of the chloride 3 with 1.1 equivalents of lithium methoxide (and respectively potassium tert -butoxide) gave the corresponding ether 4 (respectively 5 ) in one step with good yields (>85%). Subsequent oxidation of 2 , 4 , 5 and 6 with cerium(IV) ammonium nitrate (CAN) restored the quinone function and afforded the quinones 7-10 in good to excellent yields (80-96%).Among numerous reported methods for the formation of unstable o -quinone methides 20 we have chosen alkali treatment of the starting quinones. 17 This procedure has allowed us to generate selec...

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Preparation of α,β‐Unsaturated Carbonyl Compounds and Nitriles by Selenoxide Elimination

Reich

Wollowitz²

1993

Organic Reactions

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Hydrogenation and dehydrogenation reactions play a key role in synthetic organic chemistry. The discoveries that selenium and sulfur substituents could be easily introduced α to a variety of acidifying functional groups, and that these derivatives could be smoothly converted to olefins by selenoxide or sulfoxide syn elimination under mild conditions greatly broadened the range of α,β‐unsaturated carbonyl compounds that could be prepared from their saturated analogs. This chapter covers the preparation from enols or enolates of α‐RSe substituted carbonyl compounds and nitriles and their conversion to olefins by thermolysis of the derived selenoxides. Other procedures for preparing these compounds are discussed in a limited way, and they are included in the tables, but the two‐step dehydrogenation is by far the most frequently used. The rapid acceptance of the selenoxide elimination as a synthetic procedure was the result of two factors: the ease with which many organoselenium compounds could be prepared using readily available, powerful electrophilic and nucleophilic selenium reagents, and the mild conditions (−50° to 40°) under which selenoxides fragment to olefins. Recognized selenoxide eliminations to form olefins were reported in 1967 and 1970, well after the related sulfoxide eliminations had been studied extensively. The application of the sulfoxide and selenoxide eliminations were reported almost simultaneously by several groups in 1973. These reactions have been the subject of several reviews. The principal procedures and reagents for the preparation of the requisite selenium compounds were also identified at that point. Preparations discussed in this chapter are Method A: Reaction of enolates, enols, enol ethers, and enamines with benzeneselenenyl chloride or bromide, or diphenyl diselenide. Method B: Reaction of enols with seleninylating agents [C 6 H 5 Se(O)Cl, (C 6 H 5 SeO) 2 O]. Method C: Nucleophilic substitution of α‐halo carbonyl compounds with metal selenolates. Method D: Alkylation of α‐phenylseleno carbonyl compounds and acylation of α‐lithio selenides or selenoxides.

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Unactivated terpenes as 2π components in intermolecular hetero diels-alder reactions. A short stereocontrolled approach to the robustadial and euglobal skeleton.

Cited by 20 publications

References 15 publications

One-pot synthesis of precocene I and II and a formal synthesis of robustadial A and B via 2-phenyl-4H-1,3,2-benzodioxaborin

One-pot synthesis of precocene I and II and a formal synthesis of robustadial A and B via 2-phenyl-4H-1,3,2-benzodioxaborin

Diels-Alder Trapping of ortho-Quinone Methides. A New Entry to Substituted Xanthene-1,4-diones

Preparation of α,β‐Unsaturated Carbonyl Compounds and Nitriles by Selenoxide Elimination

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