Tandem β-Fragmentation−Hydrogen Abstraction Reaction of Alkoxy Radicals in Steroidal Systems

Boto, Alicia; Freire, Raimundo; Hernández, Rosendo; Suárez, Ernesto; Rodríguez, María S.

doi:10.1021/jo962252h

Cited by 26 publications

(15 citation statements)

References 29 publications

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“…The 13 C NMR data of ring A in 2 were similar to those in 1. Comparison of the 13 C NMR spectroscopic data in CDCl 3 of 2 with those of the moiety of ring A of β-hydroxy-5α,6α-epoxycholesta-7-one [9] suggested the trans-fused junction for rings A/B and the α-orientation for the epoxy unit at C-5/C-6. The ROESY correlation found between δ 1.38 (1H, m, H axial -1) and 3.58 (1H, m, H-3) indicated the β-orientation for the hydroxyl at C-3, while the ROESY correlation found between δ 0.83 (3H, s, H-18) and 2.11 (1H, m, H-20) indicated the β-orientation for the side chain at C-17.…”

Section: Resultsmentioning

confidence: 98%

“…When 10-CH 3 and 7-H were both in pseudo-axial positions of the boat, the distance between 19-H and 7-H was close enough to produce the ROESY correlation. Comparison of the 13 C NMR spectroscopic data in CDCl 3 of 1, with those of the moiety of ring A of 3β-hydroxy-5α,6α-epoxycholesta-7-one, [9] suggested the trans-fused junction for rings A/B and the α-orientation for the epoxy unit at C-5/C-6, which indicated that the boat of ring B of 1 was the endo-boat form. The deduction that 10-CH 3 and 7-H were both in pseudo-axial positions of the boat indicated that the methoxyl at C-7 was in α-orientation.…”

Section: Resultsmentioning

confidence: 99%

See 1 more Smart Citation

New oxidized sterols from Aspergillus awamori and the endo‐boat conformation adopted by the cyclohexene oxide system

Gao

Hong

Chen

et al. 2009

Magnetic Reson in Chemistry

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Two new oxidized sterols 1 and 2 were obtained from the active fraction of a mangrove fungus Aspergillus awamori isolated from the soils around the mangrove plant Acrostichum speciosum. Their structures were elucidated using spectroscopic methods as 22E-7alpha-methoxy-5alpha,6alpha-epoxyergosta-8(14),22-dien-3beta-ol (1) and 22E-3beta-hydroxy-5alpha,6alpha,8alpha,14alpha-diepoxyergosta-22-en-7-one (2). The NMR data and complete assignments for both DMSO-d(6) and CDCl(3) were given. Their cytotoxic activity against A549 cell line was evaluated. Furthermore, the detailed conformation analysis for ring B (cyclohexene oxide system) of sterol 1 was given on the basis of NOEs. The endo-boat conformation was considered as the preferred conformation for ring B rather than half-chair conformation.

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Section: Resultsmentioning

confidence: 98%

Section: Resultsmentioning

confidence: 99%

New oxidized sterols from Aspergillus awamori and the endo‐boat conformation adopted by the cyclohexene oxide system

Gao

Hong

Chen

et al. 2009

Magnetic Reson in Chemistry

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“…A distinctly different reaction cascade leading to tetrahydrofurans is illustrated in Scheme 40 [74]. It was proposed that these reactions proceed by iodine-mediated deoxygenation of the peroxyl radical to an slkoxyl radical, which undergoes intramolecular C-H abstraction to form a /?-keto radical [74].…”

Section: Oxygenation Of Cycloalkanols and Related Compoundsmentioning

confidence: 99%

“…It was proposed that these reactions proceed by iodine-mediated deoxygenation of the peroxyl radical to an slkoxyl radical, which undergoes intramolecular C-H abstraction to form a /?-keto radical [74]. Elimination of an Mhydrogen atom generates an enone, which is ultimately captured intramolecularly by conjugate hydroxyl addition to give a tetrahydrofuran (Scheme 40).…”

Section: Oxygenation Of Cycloalkanols and Related Compoundsmentioning

confidence: 99%

Peroxyl Radicals in Synthesis

Boukouvalas¹,

Haynes²

2001

Radicals in Organic Synthesis

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This chapter covers organic compounds prepared via intermediates which are alkyl peroxyl radicals (ROO') generated either by addition of ground state (triplet) oxygen to carbon-centered radicals, or by hydrogen atom abstraction from the corresponding hydroperoxides (ROOH). While some earlier work is briefly mentioned, the focus is primarily on new developments, published over the past decade or so. The emphasis is heavily on synthetic applications rather than physical, biological and environmental aspects of peroxyl radical chemistry. A book exclusively devoted to the latter topics has recently been published [l]. Some preparatively relevant pathways involving peroxyl radicals have been previously discussed in the context of reviews on the oxidation of organic molecules [2-41 and the synthesis of organic peroxides [5][6][7][8]. The present survey is clearly distinguished in scope; it is intended to cover a more diverse array of peroxyl radical pathways and to highlight recent applications in the field of natural product synthesis. The aim is to alert not only the peroxide specialist, but also the synthetic chemist in general, since the first-formed peroxidic products can be used as relay intermediates in the synthesis of nonperoxidic compounds. Autoxidation of HydrocarbonsThe oxidation of hydrocarbons and other organic molecules with oxygen or air is usually referred to as autoxidation [4, 7a, 91. This entirely atom-economical process (Eq. 1, Scheme 1) is ca. 18-24 kcal/mol exothermic and irreversible at temperatures below 250°C. Although the initiation step (Eq. 2) is not well understood [2, lo], it is clear that a chain radical process is involved, with addition of molecular oxygen to R as the initial propagation step (Eq. 3 ) . At oxygen pressures above 100 mm Hg, the reaction of R' with 0 2 proceeds with rate constants approaching the diffusioncontrol limit [7a, 9a]. The second propagation step involves hydrogen atom abstrac- Radicals in Organic Synthesis Edited

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“…We have used it in a one-step synthesis of A and A' rings of the tetranortriterpene limonene and related compounds (Eq. 19, Scheme 6) [48].…”

Section: Fragmentation Of Hemiacetalsmentioning

confidence: 99%

β‐Fragmentation of Alkoxyl Radicals: Synthetic Applications

Suárez¹,

Rodríguez²

2001

Radicals in Organic Synthesis

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The /?-fragmentation reaction is the reversible homolytic cleavage of a C-C bond a,P to an alkoxyl radical (I) which gives rise to a carbon radical and a C-0 double bond (11) (Scheme 1) [ 11. The equilibrium is strongly displaced to the fragmented Cradical (11). This is especially more pronounced in cyclopentane or smaller ring systems, or in a-substituted systems (R = alkyl, allyl, aryl) where the radical could be stabilized. For the cyclopentyloxyl radical, the equilibrium constant k,-/k, is approximately 2000, while it is only 10 for the cyclohexanyloxyl radical [2]. The formation of the final products 111 and IV is strongly influenced not only by the equilibrium constant but also by kl and k2, and the reaction can be best interpreted by using the Curtin-Hammett kinetic principle (31. In some cases when the reagent used reacts much faster with alkoxyl radicals than with carbon radicals (kl >> kz), the reverse cyclization reaction is possible and alkyl radicals can add intramolecularly to aldehydes or ketones generating alkoxyl radicals despite the unfavorable equilibrium [4].The regioselectivity of the fragmentation is not always apparent. In most cases the more stable radical is formed, but the different constants of equilibrium must be taken into account. A study of the fragmentation of 9-decalinoxyl radicals realized by Beckwith et al. is a good example of this [5]. Rearrangement at 0°C of 9-decalinyl hypobromite gives 6-bromocyclodecanone, whereas 2-(4-bromobutyl)-cyclohexanone is obtained at 8 1 "C. The increase in temperature displaces the equilibrium to the formation of an a priori less stable primary radical. The effect of alkyl substituents and ring size on the regioselectivity of the alkoxyl radical fragmentation has recently been evaluated by computational methods [6]. The importance of the activation energy and the energy of the reaction has been examined for a variety of cyclic and acyclic alkoxyl radicals. Radicals in Organic Synthesis Edited

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Tandem β-Fragmentation−Hydrogen Abstraction Reaction of Alkoxy Radicals in Steroidal Systems

Cited by 26 publications

References 29 publications

New oxidized sterols from Aspergillus awamori and the endo‐boat conformation adopted by the cyclohexene oxide system

New oxidized sterols from Aspergillus awamori and the endo‐boat conformation adopted by the cyclohexene oxide system

Peroxyl Radicals in Synthesis

β‐Fragmentation of Alkoxyl Radicals: Synthetic Applications

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