Ethers, allyl alcohol protection

The use of allyl ethers for the protection of alcohols is common in carbohydrate literature because allyl ethers are generally compatible with the various methods... 

Butyl ethers can be prepared from a variety of alcohols, including allylic alcohols. The ethers are stable to most reagents except strong acids. The /-butyl ether is probably one of the more underused alcohol protective groups, considering its stability, ease and efficiency of introduction, and ease of cleavage. 

The synthesis of the trisubstituted cyclohexane sector 160 commences with the preparation of optically active (/ )-2-cyclohexen-l-ol (199) (see Scheme 49). To accomplish this objective, the decision was made to utilize the powerful catalytic asymmetric reduction process developed by Corey and his colleagues at Harvard.83 Treatment of 2-bromocyclohexenone (196) with BH3 SMe2 in the presence of 5 mol % of oxazaborolidine 197 provides enantiomeri-cally enriched allylic alcohol 198 (99% yield, 96% ee). Reductive cleavage of the C-Br bond in 198 with lithium metal in terf-butyl alcohol and THF then provides optically active (/ )-2-cyclo-hexen-l-ol (199). When the latter substance is treated with wCPBA, a hydroxyl-directed Henbest epoxidation84 takes place to give an epoxy alcohol which can subsequently be protected in the form of a benzyl ether (see 175) under standard conditions. 

In two studies toward the total synthesis of natural products it could be shown that the a,jS-unsaturated esters derived from the vinylogous Mukaiyama aldol reactions can be further functionalized into advanced intermediates. The C1-C7 segment of oleandolide commences with the VMAR of aldehyde 68 derived from the Roche ester. The so-generated stereo-triad was protected as PMB ether and the ester 76 was reduced to the allylic alcohol. Sharpless asym-... 

The sulfone moiety was reductively removed and the TBS ether was cleaved chemoselectively in the presence of a TPS ether to afford a primary alcohol (Scheme 13). The alcohol was transformed into the corresponding bromide that served as alkylating agent for the deprotonated ethyl 2-(di-ethylphosphono)propionate. Bromination and phosphonate alkylation were performed in a one-pot procedure [33]. The TPS protecting group was removed and the alcohol was then oxidized to afford the aldehyde 68 [42]. An intramolecular HWE reaction under Masamune-Roush conditions provided a macrocycle as a mixture of double bond isomers [43]. The ElZ isomers were separated after the reduction of the a, -unsaturated ester to the allylic alcohol 84. Deprotection of the tertiary alcohol and protection of the prima-... 

Tetrahydropyranyl (TUP) and tetrahydrofuranyl ethers are important protecting groups for alcohols and phenols in organic synthesis, but they can also be converted to other useful functional groups [8, 118]. For example, allylation of a TUP ether should yield a highly functionalized molecule (Scheme 15). 

Type I (fast homodimerization) Terminal olefins, allylsilanes" Terminal olefins, allylsilanes," 1° allylic alcohols, ethers, and esters, " allyl boronate esters, allyl halides, alkyl-substituted allenes Terminal olefms, allyl boronate esters, 1° allylic alcohols, ethers, and esters,styrenes (no large ortho substit.), " " allyl allylsilanes, allyl sulfides, allyl phosphonates, " allyl phosphine oxides, protected allylamines ... 

Decarboxylation of 117 was effected by treatment of 117 with LiCl in hot, aqueous HMPA at 105 °C providing 118 as a mixture of diastereomers that were separated and carried forward individually. Protection of the secondary amide group as the corresponding methyl lactim ether was accomplished by treating 118 with trimethyloxonium tetrafluoroborate in dichloromethane that contained cesium carbonate. Next, the indole nitrogen atom was protected as the corresponding Boc derivative by treatment with dicarbonic acid bis(rm-butyl)ester in the presence of DMAP and the silyl ether was removed with tetrabutylammonium fluoride to provide diol 119 in 52-78% overall yield from 118. Selective conversion of the allylic alcohol to the corresponding... 

Z)-Trisubstituted aRylic alcohols. The conditions used by Bestmann et al. (7, 329) for preparation of (Z)-disubstitutcd alkcncs via the Wittig reaction also can provide a stereoselective route to (Z)-trisubstiluted allylic alcohols. An example is the reaction of ethylidenetriphenylphosphorane with the THP ether of hydroxy-acetone (equation 1). The stereoselectivity is decreased with other protecting... 

A 1,2-hydride shift has been invoked399 to account for the formation of p-methoxyphenylbutyraldehyde derivatives (337) during the treatment of />methoxy-benzyl-protected allylic alcohols (336) with zeolites. A similar C-glycosidation procedure involving Lewis acid-catalysed anomeric oxygen to carbon rearrangement of tetrahydropyranyl ether derivatives has been reported400 (see Scheme 82). It has been... 

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