2- tetrahydropyran 1-alkyne

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. 

Sodium 5,8,11,14-eicosatetraenoate, 3803 Sodium ethoxyacetylide, 1474 1,1,2,3-Tetrachloro-1,3-butadiene, 1385 f Tetrahydrofuran, 1607 Tetrahydronaphthalene, 3294 f Tetrahydropyran, 1959 Tridecanal, 3613 f Vinyl acetate, 1527 f 4-Vinylcyclohexene, 2999 See Indane-2-aldehyde ACETYLENIC COMPOUNDS ALKALI METALS ALKENES ALKYNES... 

Treatment of epoxy-alkyne derivatives 314 with dicobalthexacarbonyl [Co2(CO)6] and a catalytic quantity of Lewis acid, followed by oxidative demetallation of the intermediate cobalt complex 315 affords 2-alkynyl-3-acetoxy tetrahydropyrans 316 in excellent yield. Syn- or tf /7-tetrahydropyrans are obtained by selection of either the anti- or syn- starting epoxides 314 respectively (Scheme 79, Table 10) <1998T823, 2000JOC6761>. 

Spirocyclic 4-substituted tetrahydropyrans are readily obtained through the Prins reaction involving cyclic ketones, homoallylic alcohols and MeS03H <02H(58)659>. The cationic species generated when alkyne-Co complexes derived from 8-valerolactone are treated with SnCl4 undergo a double cyclisation to yield the oxaspiro[5.5]undecane <02T2755>. 

A related transformation was reported by the gronp of Floreancig for the synthesis of substituted tetrahydropyrans (equation 11). This transformation initially involves a hydration of the alkyne and then an elimination of a leaving group at the P position, followed by a conjngate addition. 

In the photoaddition of a saturated hydrocarbon to ethyl propiolate (equation 19) it is likely that the excited state of the acetylenic ester initiates reaction by abstracting a hydrogen atom from the hydrocarbon. The addition of cyclic ethers to an alkyne seems similar (equation 20), although a ketone sensitizer is required for addition of tetrahydropyran or dioxan . When reactions of this type involve a conjugated acetylenic ester, the first-formed a,p-unsaturated ester can normally undergo further photochemical reaction to produce the p,y isomer (see equations 19 and 20). 

The ring-opening addition of 2-dialkyIamino tetrahydropyran [59] or 1,3-oxazoIidine [60], leading to formation of acetylenic alkylamino alcohols has been reported. In the same way, substituted aldose, which has the structure of cyclohemiacetals, yields dihydroxy alkynes (see Table 4, entries 7-9) [61]. 

This type of reaction can be extended to alkynes bearing various types of polar substituents. Thus, silyl and tetrahydropyran-2-yl ethers can be converted into the corresponding 4-methyl-enecyclopent-l-enes 3. ... 

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