Tetrahydropyran 5-alken

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. 

Dihydropyrans and tetrahydropyrans add on alkenes when irradiated but some of the products are unstable. 3,3-Dimethylpyran-2,4-dione (664) and an unsymmetrical diene such as 2-methylprop-l-ene (665) give regiospecifically a 1 1 mixture of cis- and frans-fused pyran-4-one (666) (73T1317). Tetrahydropyran (667) does not undergo photoaddition unless a sensitizer such as benzophenone is present, but then it reacts with diethyl maleate (668) (67T3193). 

The effect of the nature of the electrophile on the stereoselectivity of reactions with substrates containing a terminal alkene and an allylic substituent is dramatically illustrated by some recent results with palladium electrophiles.124 Cyclizations of 3-methyl- or 3-phenyl-5-hydroxyalkenes with palladium catalysts proceed with high selectivity (>9 1) for the 2,3-trans isomer (equation 41).50-124 It is suggested that the steric interactions of the palladium-alkene complex affects the stereochemistry of these cyclizations. In some related cyclizations to form tetrahydropyran products (equation 42 and Table 10), reaction with iodine in the presence of sodium bicarbonate gives a different major diastereomer from cyclization with mercury(II) trifluoroacetate or palladium chloride.123... 

Steric effects similar to those shown in equations (39) and (40) are found when the substitution pattern leads to tetrahydropyran systems through 6-endo cyclization. Cyclizations of systems with an allylic oxygen and a syn alkene substituent give products rationalized by cyclization through H-in-plane conformations as shown earlier in equations (31) and (32).105 128 Examples with allylic methyl substitution have been reported also.1040... 

Catalytic oxy-palladation is an extremely useful method for the synthesis of functionalized THF and tetrahydropyran moieties. This reaction is brought about simply by treating a 1,4- or 1,5-hydroxy alkene with 0.1 mol-eq of Pd(II) salts and copper(I) chloride in DMF, with oxygen (equation 181)655. If this reaction is carried out in the presence of carbon monoxide in methanol, then an ester moiety is introduced into the product molecule (equation 182)656-658. If an alkene is introduced in place of the CO, then a tandem vinylation reaction also takes place (equation 183)659. 

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... 

Cydizations proceeding by intramolecular Sn2 reactions are usually irreversible, and will, therefore, not necessarily yield the thermodynamically most stable product but that which is formed most quickly. Scheme 9.13 depicts a cyclization in which the outcome depends on the configuration of the starting alkene. Interestingly, the cis isomer gives mainly rise to a strained, eight-membered ether even though a path to the less strained tetrahydropyran is, in principle, accessible. 

Prins cyclization of the acetal 327 can be conducted in the presence of a Lewis acid surfactant catalyst in water to afford 2,4,6-trisubstituted tetrahydropyrans. The reaction proceeds via ionization of the crji-iinsatuiated acetal and subsequent reaction with the tethered electron-rich alkene, proving that the interior of micelles are sufficiently anhydrous to protect Prins cyclization intermediates (Equation 138) <20030L4521>. 

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