Oxacyclic ring systems

A number of reports have appeared on the use of carbonyl-alkene additions to construct oxacyclic ring systems efficiently with high diastereoselectivity. Nakata showed that even the formation of seven-membered cyclic ethers using the transformation can be remarkably efficient and utilised the strategy to form the E ring of the polycyclic ether marine natural product yessotoxin (Scheme 5.34).64... 

Flash-vacuum pyrolysis has been used in a general annulation protocol for the synthesis of fused carbocyclic and oxacyclic ring systems of type (Tables 1 and 2). The pre-... 

An MBH reaction was used to introduce key functionality to form a challenging oxacycle ring system. Aldehyde 85 was treated with ethyl acrylate in the presence of DABCO in a mixture of dioxane and water to deliver a diastereomeric mixture of alcohols 86a and 86b. After chromatographic separation of the diastereomers, each one was then converted to the corresponding oxacycle via a radical cyclization reaction. The thus afforded alcohols were acetylated to deliver acetate 87a and acetate 87b in 64% and 71%, respectively, over two steps. 

Prolonged reaction of the triols 25 with trimethyl orthoacetate and a weakly acidic catalyst affords a mixture of the 5- and 6-membered oxacycles from which the tetrahydropyran is obtained on equilibration (Scheme 13). By selection of the reaction conditions, it is possible to obtain either ring system exclusively <00CC1781>. 

A solid-phase synthesis of furo[3,2-3]pyran derivatives utilizing highly functionalized sugar templates has been reported <2003JOC9406>. After incorporation of alkenes within the sugar template, such as compound 95, the solid support is introduced via formation of the acid amide. This immobilized system then allows a ruthenium-catalyzed ring-closing metathesis that leads to the formation of the fused oxacycles. 

Eight-membered-ring oxacycles were available from an analogous route. Formation of the sodium enolate from oxacyloheptenone 85 and reaction with acyl silane 82 provided 86 in good yield and with high diastereoselectivity. The proper choice of enolate coimterion and solvent were crucial in these systems. ... 

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