PMB ether

From a PMB ether CH2(OMe)2, MOMBr, SnBt2, CICH2CH2CI, it, 57-81% yield. Phenolic PMB ethers were not converted efficiently. A BOM ether was prepared using this method. ... 

I2, MeOH, 65°, 12 h, 90% yield. PMB ethers are also cleaved, but benzyl ethers are stable. 

The synthesis of the key intermediate aldehyde 68 is outlined in Schemes 19-21. The two hydroxyls of butyne-l,4-diol (74, Scheme 19), a cheap intermediate in the industrial synthesis of THF, can be protected as 4-methoxybenzyl (PMB) ethers in 94% yield. The triple bond is then m-hydrostannylated with tri-n-butyl-tin hydride and a catalytic amount of Pd(PPh3)2Cl238 to give the vinylstannane 76 in 98 % yield. Note that the stereospecific nature of the m-hydrostannylation absolutely guarantees the correct relative stereochemistry of C-3 and C-4 in the natural product. The other partner for the Stille coupling, vinyl iodide 78, is prepared by... 

Interestingly, when a PMB ether is present at C2, (i-glycosides seem to predominate (see Scheme 3.3 and Ref. 11). Yan and Kahne attribute this to extreme steric requirements of the glycosyl acceptors in the examples reported.11 Boeckman suggests that the (i-sclcctivity is due to the high reactivity of the reactive intermediate, and perhaps participation by the PMB ether oxygen.9... 

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-... 

Application of the previously described reaction sequence allows an initial insertion reaction with dichloromethyllithium to give 20 Then 20 is enhanced by one C, residue using Grignard reagent 21 to produce 22. Another reaction with dichloromethyllithium results in a C chain extension to product 5. Since the insertion reaction with dichloromethyllithium was conducted twice, the distance between the boron atom and the PMB ether inemases by two carbon atoms. 

Oxidation-sensitive protecting groups, such as PMB ethers,106 resist the action of IBX under the experimental conditions used for the oxidation of alcohols. 

Step 2 Removal ofp-methoxybenzyl (PMB) ether with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) requires H20 (present in pH 7 buffer). 

A variant on iodine deprotections is a report on the cleavage of PMB ethers by a mixture of cerium(III) chloride heptahydrate and sodium iodide in refluxing acetonitrile.210 A proposed mechanism involves polarization of the ethereal link (158, Scheme 43), followed by generation of a quinone-like species, and attack by nucleophilic iodide to afford p-methoxybenzyl iodide. 

As shown in Scheme 29, the PSB group can be removed selectively in the presence of the PMB ether by peroxide oxidation, while the PMB group can be cleaved without affecting the PSB ether using 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQ). 

This resulted in a dramatic decrease of the reaction time to a mere 5 min with comparable yields. The sequence was accomplished upon full (H2/Pd- C) or partial and stereoselective reduction (Lindlar catalyst) of the double bond and cleavage of the PMB-ethers. 

PMB ethers can be cleaved oxidatively with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)11 in dichloromethane/water tor with cerium ammonium nitrate (CAN) in acetonitrile/water.12 Many other protecting groups such as esters, isopropylidene acetals, benzyl ethers, allyl ethers and f-butyldiphenyl silyl (TBDMS) ethers are stable to these conditions (Scheme 2.4). The cleavage reaction, with DDQ is initiated with a single-... 

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