Dihydropyran, purification

A2EPIN-2 -ONE, 44, 41 Dihydropyran, purification of, 41, 77 reaction with dichlorocarbene, 41, 76 Dihydroresorcinol, 41,56 methylation of, 41, 56 reaction with ethanol to yield 3-ethoxy-2-cyclohexenone, 40, 41 Dihydroresorcinoi. monoethyl ether, 40,41... 

A disadvantage of the THP group is the fact that a new stereogenic center is produced at C(2) of the tetrahydropyran ring. This presents no difficulties if the alcohol is achiral, since a racemic mixture results. However, if the alcohol is chiral, the reaction gives a mixture of diastereomers, which may complicate purification and/or characterization. One way of avoiding this problem is to use methyl 2-propenyl ether in place of dihydropyran (abbreviated MOP, for methoxypropyl). No new chiral center... 

Since better selectivity was obtained when the ligands were removed from the dihydropyran-derivatized polymer support, after synthesis they were cleaved from the polymer and used in catalysis without purification. Comparable selectivities were obtained with ligands that were used directly without purification and selected examples where the ligands were purified prior to use. For such an approach to be useful in catalyst development, it is critical that the material from the synthesis be of sufficient purity to be used without purification. 

A similar reaction with 2,3-dihydropyran instead of ethyl vinyl ether afforded the corresponding THP-ether, but the product was less stable under acidic conditions, partial regeneration of pristinamycin 11 being observed during purification on silica gel. 

Therefore, in order to obtain 1,5-PeD selectively, the multi-step method has been reported (4) [25]. This system is composed of three separated steps including dehydration of THFA to dihydropyran, hydration of dihydropyran to 5-hydroxyvaleraldehyde, and hydrogenation of 5-hydroxyvaleraldehyde to 1,5-PeD. The system requires the isolation and purification of the intermediates and the overall yield was 70%. Hydrogenolysis of THFA to 1,5-PeD also seems to be simple judging from the reaction formula (3), just like the case of the glycerol hydrogenolysis however, the selective synthesis of 1,5-PeD from THFA had not been realized. 

Tetrahydropyranylation of Alcohols. Protection of alcohol functionality as the THP ether is an often-utilized tool in organic synthesis. It must be noted that the reaction of a chiral alcohol with dihydropyran introduces an additional asymmetric center and hence a diastereomeric mixture is obtained (eq 1). This can lead to difficulties with purification, assignment of spectral features, etc., but does not prevent successful implementation. ... 

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