Reductive Oxa Ring Opening of 7-Oxabicyclo[2.2.1]heptan-2-ones. Synthesis of C-.alpha.-Galactosides of Carbapentopyranoses

A new approach to the asymmetric synthesis of pentadeca-1,3,5,7,9,11,13,15-octols and their derivatives is presented. It is based on the Sharpless asymmetric dihydroxylation (AD) of 4,4'-methylene[(1R,1'S,6R,6'S)-6-acetoxycyclohept-3-en-1-yl]bis(4-methoxybenzoate) (9), derived from a double [3+4] cycloaddition of the 1,1,3-trichloro-2-oxyallyl cation with 2,2'-methylenedifuran (1). The diol (-)-10, obtained in 98.4% ee from 9 with "AD-mix-beta(5x), was oxidised into (2R and 2S,4S,6R)-tetrahydro-2-hydroxy-6-((4S,6S)-(6-hydroxy-4-[(4-methoxybenzoyl)oxy]cyclohept-1-en-1-yl)-2-oxopropyl)-2H-pyran-4-yl 4-methoxybenzoates ((-)-18). By the combinations of Evans' anti and Nasaraka's syn reductions of aldol (-)-18 with the double Mitsunobu reaction, 16 diastereomeric pentadeca-1,3,5,7,9,11,13,15-octols and analogues can be obtained, in principle, with high enantio- and diastereoselectivities.

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“…[30]. All the 1 H NMR and 13 C NMR assignments were confirmed by 2 D NMR (COSY, NOESY, HMQC spectra).…”

Section: Methodsmentioning

confidence: 73%

A New, Non-Iterative Asymmetric Synthesis of Long-Chain 1,3-Polyols

Schwenter

Vogel

2000

Chem. Eur. J.

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“…Although the exact mechanism of the epoxide ring opening from intermediate 50 ' has not yet been determined, a PET process is involved. The C-0 bond cleavage is similar to the reductive cleavage of the same bond by metals in liquid ammonia The linear triquinane skeleton is easily accessible from methyl S-oxotricyclo-[5.4.0.0]2.6undecan-1 -carboxylate 67 [64]. Photoreduction of 67 affords ringexpanded intermediate 67" which cyclizes to the linearly fused triquinane 68 (Scheme 29, eq 1).…”

Section: Single-bond Fragmentationsmentioning

confidence: 96%

Photoinduced Electron Transfer in Radical Reactions

Cossy¹

2001

Radicals in Organic Synthesis

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Electron transfer reactions have been extensively studied in chemistry [ 11 and the photochemical approach to reactions of this type has become familiar to most organic chemists [2]. Photoinduced electron transfer (PET) processes involve two neutral molecules, a donor D, an acceptor A and an electronic excitation (Scheme 1).Production of radical-ions (D'+) and (A'-) depends on the oxidation potential E"'112 (D), the reduction potential FedlI2 (A) of the starting molecules and on the electronic excitation energy Eoo, according to the Rehm-Weller equation, where the Coulombic interaction term (AEc) in polar solvents such as acetonitrile can be neglected to a first approximation [3].When the PET is thermodynamically favorable (AC < 0), electron transfer proceeds at a rate close to diffusion controlled [4]. However, the fate of the radical-ion pair depends strongly on the nature of the solvent and on the rate of the reverse electron transfer to A and D in their ground state [ 5 ] . Use of polar solvents favors solvent-separated ion pairs and limits this reverse electron transfer process. Some compounds can be either donors or acceptors depending on their relative rcdox potentials such as is the case with alkenes and aromatic cyano compounds. For example, 1,4-dicyanonaphthalene (DCN) and 9,lO-dicyanoanthracene (DCA) are reducing agents in the presence of enones, ketones, alkylated arenes and alkenes, but they are oxidizing agents in the presence of amines. Furthermore, alkenes can be reducing agents in the presence of ketones and iminium salts, but oxidizing agents in the presence of DCN and DCA. We also note that amines are reducing agents in the presence of ketones, enones, esters, imides, halides and styrenes.In general, radical-anions generated by PET can be involved directly in radical reactions such as coupling reaction, reduction, intramolecular cyclization, single bond fragmentation or tandem reactions. However, for halides cleavage of the C-X bond takes place after the PET, a radical is generated and radical processes ensue. In contrast to radical-anions, radical-cations are generally not involved in radical Radicals inb Organic Synthesis Edited by Philippe-Renaud and Mukund P. Sibi

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“…The UV spectrum (l max 273.5 nm) and 13 C-NMR ( 21) and compound 4, 20) and the latter was also isolated from the same source. Comparison of the 1 H-and 13 C-NMR (Table 1) data of 1 with that of compound 4 20) suggests that 1 has the same skeletal structure.…”

mentioning

confidence: 99%