Tetrahydropyrans, stereospecific synthesis

A very efficient, stereospecific synthesis of DL-ribose was based26 on the use of l,l-diethoxy-5-(tetrahydropyran-2-yloxy)-2-pentyn-3-ol as the substrate. Catalytic hydrogenation of this alkyne to the cts-alkene was accompanied by cyclization, to give 2-ethoxy-2,5-dihydro-5-(tetra-hydropyran-2-yloxy)furan (35). cis-Hydroxylation of the double bond in 35 was effected with potassium permanganate, yielding the ethyl DL-ribofuranoside derivative 36, which was hydrolyzed to DL-ribose. 

Another stereospecific synthesis of2-deoxy-DL-en/tfiro-pentose was based10 on cis-l-(tetrahydropyran-2-yloxy)-2-penten-5-ol (156) as the substrate. cts-Hydroxylation of 156, followed by oxidation of the unprotected, primary hydroxyl group to an aldehyde group according to Barton s procedure, yielded the desired sugar 2 in low yield. 

Tetrahydropyran epoxides 12, the synthesis of which involves a RCM, undergo a base-induced rearrangement to 3,4-dihydro-2//-pyran-4-ols 13. These compounds are converted stereospecifically to 3,6-dihydropyrans 14 on treatment with allyltrimethylsilane (Scheme 6) <00EJO3145>. 

Davis J, Jones JB (1979) Enzymes in organic synthesis. 16. Heterocyclic ketones as substrates of horse liver alcohol dehydrogenase. Stereospecific reductions of 2-substituted tetrahydropyran-4-ones. J Am Chem Soc 101 5405-5410... 

A remarkable ring closure of polyenic acetals can be performed in a highly stereospecific manner with SnCl4 to form polycyclic products with a multiplicity of asym. centers. 3-Alkyl-4-halogeno-tetrahydropyrans can be obtained by a novel reaction of olefins with paraformaldehyde and hydrogen halides . 1,3-Diphenyl-naphtho[2,3-c]furan, the first known stable 2,3-naphthoquinonoid, is an extremely reactive diene component in Diels-Alder syntheses . A modified Pechmann coumarin synthesis uses enamine derivatives of yS-ketoacids without catalyst. ... 

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