Tetrahydropyranyl group

The tetrahydropyranyl group, commonly used in synthetic procedures to protect hydroxyl groups, appears not to be safe when peroxidising reagents are used with tetrahydropyranyl ether derivatives, because explosive peroxides, not destroyed by the usual reagents, are produced. 

Treatment of the monochloro diethylene glycol 9 with dihydropyran yields the protected alcohol 10. Reaction of 2,2, 2"-triaminotriethylamine (tren) 11 with toluenesulfonyl chloride gives the tritoluenesulfonyl derivative 12. The pyranyl ether 10 may be condensed with the tris(sodium) salt of 12 leading to 13. Removal of the tetrahydropyranyl group is achieved in high yield under... 

As reported in Figure 2.5, nitroolefins (26), easily obtained by nitroaldol condensation between 5-nitro ketones (24) and aldehydes (25), are converted directly into the spiroketals (29) by reduction with sodium boronhydride in methanol. The one-pot reduction-spiroketalization of nitroalkenes (26) probably proceeds via the nitronate (27) that by acidification is converted into carbonyl derivatives, which spontaneously cyclize to emiketals (28). Removal of the tetrahydropyranyl group, by heating the acidic mixture during the workup, affords, in a one-pot reaction from (26), the desired spiroketals in 64-66% overall yields. The spiroketalization of (26)-(29b) proceeds in high ( )-diastereoselectivity. 

Recent methods for the cleavage of allyl ethers that have that have yet to be tested on the anvil of complex target synthesis include (a) diborane generated in situ by reaction of sodium borohydride with iodine in THF at 0 °C (cyanoT ester, nitro, acetonide and tetrahydropyranyl groups survive) 434 (b) cerium(Ill) chloride and sodium iodide in refluxing acetonitrile (benzyl. THP and Boc groups survive) 435 (c) iodotrimethylsilane in acetonitrile at room temperature 436 and (d) DDO in wet dichloromethane (secondary allyl ethers, benzyl, acetate and TBS groups survive).437... 

Potassium carbonate in methanol or HOAc-H20-THF (3 1 1) at room temperature is sufficient to cleave even terr-butyldimethylsilyl esters. The esters cleave faster than the corresponding ethers [Scheme 6.105].235 Their high base lability allows removal of rerr-butyldimethylsilyl esters in the presence of tert-butyldimethylsilyl ethers [Scheme 6,106].236 The dwerf-butylmethylsilyl group is sufficiently stable to allow selective removal of a tetrahydropyranyl group using pyridinium p-toluenesulfonate in warm ethanol.237... 

The ethers are stable to aqueous or alcoholic base, to hydrogenolysis (H2 Pd), and to mild chemical reduction (Zn-CH OH). The dimethyl-/-butylsilyl group is an alternative to the tetrahydropyranyl group but has the advantage that it does not have a chiral... 

Deprotections. Expansive graphite serves as a catalyst for removal of tetrahydropyranyl group in methanol from such ethers. 1,1-Diacetates are converted to carbonyl compounds and ACjO. ... 

Oxidative cleavage of t-butyldimethylsilyl ethersThese ethers are usually cleaved with tetra-n-butylammonium fluoride in aprotic solvents. The cleavage is also possible with NCS (1.1 equivalent) in DMSO (75-90% yield). A tetrahydropyranyl group is unaffected by NCS under these conditions. 

This material could also be obtained by treating the sodium salt of 0-(2-tetrahydropyranyl)-thiamine with phosgene, and eliminating the tetrahydropyranyl group with aqueous hydrochloric acid solution [1462]. 

S-Hydroxytabersonine (108), in another series of transformations, has been used in a partial synthesis of baloxine (352), an alkaloid of Melodinus balansae. Protection of the hydroxyl group in 108 as its tetrahydropyranyl ether 353, followed by regiospecific hydroboration-oxidation, gave a mixture of epimeric C-14 alcohols (354) that, on oxidation and removal of the tetrahydropyranyl group, gave baloxine (352), whose structure as 19S-hydroxy-14-oxovincadifformine is thus confirmed (241) (Scheme 17). 

Acidic hydrogens can be protected by alkylation, preferentially by an alkyl group that can be removed after the reaction. Benzyl, t-Bu and tetrahydropyranyl groups are most frequently used for OH and NH groups. The trimethylsilyl group is both stable and easily introduced and removed it can even serve to protect C—H bonds, e.g. ° ... 

S.A.M. Hesp, N. Hayashi, andT. Ueno, Tetrahydropyranyl and furanyl protectedpolyhydroxystyr ene in chemical amphfication systems, J. Appl. Polym. Sci. 42, 877 (1991) N. Hayashi, L. Schlegel, T. Ueno, H. Shiraishi, and T. Iwayanagj, Polyvinylphenols protected with tetrahydropyranyl group in chemical amplification positive deep UV resist systems, Proc. SPIE 1466, 377 (1991). 

An alternative route was also developed for the synthesis of ( )-pisiferic acid (196) as described in "Fig (17)". The starting material for the present synthesis was the already described alcohol (15), which on tetrahydropyranylation yielded the derivative (197). Metal hydride reduction of (197) afforded a mixture of alcohols whose tosyl derivative on heating with lithium bromide and lithium carbonate in dimethylformamide afforded the oily olefin (198). These conditions not only provoked the dehydrosulphonation but also the hydrolysis of the tetrahydropyranyl group, thus shortening the reaction sequence by one step. The oily olefin (198) on oxidation yielded the ketone (199), which was formylated, and subjected to Robinson annelation with methyl vinyl ketone prepared in situ following the procedure of Howell and Taylor [74]. The resulting adduct without purification was heated by boiling with sodium methoxide in methanol to obtain the tricyclic ketone (200). It was treated with... 

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