Cyclopentane, fragmentation reactions

The synthesis of new 11-deoxyprostaglandin analogs with a cyclopentane fragment in the oo-chain, prostanoid 418, has been accomplished by a reaction sequence involving nitrile oxide generation from the nitromethyl derivative of 2-(oo-carbomethoxyhexyl)-2-cyclopenten-l-one, its 1,3-cycloaddition to cyclopenten-l-one and reductive transformations of these cycloadducts (459). Diastereoisomers of a new prostanoid precursor 419 with a 4,5,6,6a-tetrahydro-3aH-cyclopent[d isoxazole fragment in the oo-chain have been synthesized. Reduction of 419 gives novel 11-deoxyprostanoids with modified a- and oo-chains (460). 

In practice, however, the annulation does not take place because the aldol condensation is essentially a reversible reaction and strain between the two cyclopentane rings shifts the equilibrium towards the reactants rather than towards the products. In order to overcome this difficulty, the synthetic scheme was substantially modified and 3-acetoxy-2-ethoxypropene (4) was used as the umpoled C3 fragment and the aldol condensation was substituted by a Wittig reaction, as... 

The cationic pathway allows the conversion of carboxylic acids into ethers and acetals. The decarbox-ylative elimination of vicinal diacids or f3-silylcarboxylic acids in combination with cycloaddition reactions leads efficiently to cyclobutenes or cyclohexadienes. Cationic rearrangements or fragmentations initiated by anodic decarboxylation allow the synthesis of specifically substituted cyclopentanes and ring extensions by one or four carbon atoms. 

Cycloaddition. Reaction of the 1,3-dipolar species derived from methyl 2-phenylthiocyclopropyl ketone with silyl vinyl ethers furnishes functionalized cyclopentanes. A related reaction is the trapping of a fragmented cyclobutane. ... 

Several examples of the stereoselective formation of cyclopentane rings promoted by samarium diiodide have been reported. Thus a pinacol-like coupling of a 1,5-dialdehyde derivative produced 75 in which the newly formed hydroxyl groups R, R were in a y/z-arrangement. (See Vol. 29, p.238, ref. 95 for a related reaction). In contrast, reaction of a 5-hexenal, produced by zinc-assisted Grob fragmentation of a methyl 6-deoxy-6-iodopyranoside, produced 76 and 77 in which the exocyclic groups R, R (or R, R ) were anti to each other. ... 

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