Hydrogenation dihydropyrans

The 7, i5-unsaturated alcohol 99 is cyclized to 2-vinyl-5-phenyltetrahydro-furan (100) by exo cyclization in aqueous alcohol[124]. On the other hand, the dihydropyran 101 is formed by endo cyclization from a 7, (5-unsaturated alcohol substituted by two methyl groups at the i5-position. The direction of elimination of /3-hydrogen to give either enol ethers or allylic ethers can be controlled by using DMSO as a solvent and utilized in the synthesis of the tetronomycin precursor 102[125], The oxidation of the optically active 3-alkene-l,2-diol 103 affords the 2,5-dihydrofuran 104 in high ee. It should be noted that /3-OH is eliminated rather than /3-H at the end of the reac-tion[126]. 

Butane, 2,3-0-isopropylidene-2,3-dihydroxy-1,4-bis(diphenylphosphino)-catalyst in homogeneous asymmetric hydrogenation, 6, 781 Butane-1,4-dioic acid, 2,2-di(indolyl)-synthesis, 4, 226 Butanenitrile, 4-hydroxy-dihydropyran synthesis from, 3, 769 Butanoic acid, -y-aryl-y-amino-synthesis, 1, 433 1-Butanol... 

Preparation of the requisite side chain starts by alkylation of ethyl acetoacetate with 1,3-dibromopen-tane the initially formed bromoketone (shown as the enol 97) undergoes O-alkylation under the reaction conditions to give the dihydropyran 98. Reaction of that masked hydroxy ketone derivative with hydrogen... 

Reduction, by hydrogen and Raney nickel—Continued dihydropyrane to tetrahydropy-ranc, 23, 90... 

Tetrahydropyrane has been prepared by hydrogenation of dihydropyrane using a platinum black catalyst 1 by heating pen-tamethylene bromide with water 2 3 or with water and zinc oxide in a sealed tube 4 or by heating pentamethylene glycol with three volumes of 60 per cent sulfuric acid in a pressure tube.5... 

Under reasonable conditions tetrahydropyran ring is not affected. Catalytic hydrogenation of 1,2-dihydropyran over Raney nickel at room temperature and 2.7 atm saturated the double bond to give quantitative yield of tetrahydropyran [634. ... 

Af,N -Dichlorodiiminosuccinonitrile (12) (Section III) reacts with styrene, -methylstyrene, and 2,3-dihydropyran to give the pyrazines 126 directly with loss of 2 mol hydrogen chloride (74JOC3373) (Scheme 43). 

The indirect dehydrogenation to pyranones has received rather more attention. The allylic bromination of 5,6-dihydropyran-2-one by NBS, followed by the elimination of hydrogen bromide, is described in detail and offers the attractions of mild conditions and easy isolation of the product (770S(56)49). A similar approach was used to synthesize the steroidal pyran-2-ones from the fully saturated lactone (354), with a combination of dehydrogenation and dehydrobromination to achieve oxidation (64MI22400). 

Procedures for THP-ether formation. Use of hydrochloric acid. Concentrated hydrochloric acid (0.3 ml) was added to a mixture of 2,3-dihydropyran (0.75 mol) and the alcohol (0.5 mol). Reaction commenced immediately on shaking and was moderated by cooling in an ice-water bath. The mixture was shaken for a further 30 minutes, allowed to stand overnight, diluted with ether (150 ml), and the solution washed twice with aqueous sodium hydrogen carbonate solution. The ethereal solution was dried and evaporated, and the residue distilled under reduced pressure. 

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