Dihydropyran oxides rearrangement

Hydrolysis of 2,3-dihydropyran by dilute hydrochloric acid gives 5-hy-droxypentanal (7S>%), which is readily reduced to 1,5-pentanediol. The 2,3 dihydropyran is prepared by dehydration and rearrangement of tetra hydrofurfuryl alcohol over aluminum oxide. ... 

The control of the configuration of the side chain at C-8 in a synthesis of C-gly-coside antibiotic pseudonomic acid 212 described by DeShong [46] resulted from a Claisen-Johnson rearrangement Dihydropyrane derivatives 213-214 after treatment with triethyl orthoacetate afforded an inseparable mixture of diastereomers. The anomeric center was then reduced and the side chain secondary alcohol oxidized. Diastereomeric ketones 215 and 216 were isolated at this stage in a 2 1 ratio (Scheme 6.33). 

Interestingly, as shown in Scheme 8.103, if furfural (furan 2-carboxalehyde) is reduced with hydrogen (H2) in the presence of a Ni catalyst, the corresponding primary alcohol, tetrahydrofurfuryl alcohol, results. Then, if the that primary alcohol is heated to 350°C in the presence of an alumina (aluminum oxide, AI2O3) catalyst, rearrangement and dehydration occur to yield dihydropyran (oxahex-2-ene). 

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