Acetonide selective cleavage

ClSi(CH,),/(C,H5)4NBr is highly effective for selective cleavage of MOM ethers in the presence of an acetonide. ... 

Mitsunobu reaction as well as by mesylation and subsequent base treatment failed, the secondary alcohol was inverted by oxidation with pyridinium dichromate and successive reduction with sodium borohydride. The inverted alcohol 454 was protected as an acetate and the acetonide was removed by acid treatment to enable conformational flexibility. Persilylation of triol 455 was succeeded by acetate cleavage with guanidine. Alcohol 456 was deprotonated to assist lactonization. Mild and short treatment with aqueous hydrogen fluoride allowed selective cleavage of the secondary silyl ether. Dehydration of the alcohol 457 was achieved by Tshugaejf vesLCtion. The final steps toward corianin (21) were deprotection of the tertiary alcohols of 458 and epoxidation with peracid. This alternative corianin synthesis needed 34 steps in 0.13% overall yield. 

Acetonide hydrolysis—cleavage. Terminal acetonides are selectively hydrolyzed and cleaved with HjIOs. Sometimes, a one-pot reaction can be accomplished. ... 

Selective cleavage of the acetonide using either PPTS in methanol or aqueous acetic acid furnishes 301 in 84—88% yield. Mesylation of 301 under standard conditions affords 508a... 

Cleavage rates for 1,3-dioxanes are greater than for 1,3-dioxolanes/ but hydrolysis of a trans-fused dioxolane is faster than that of the dioxane. In substrates having more than one acetonide, the least hindered and more electron-rich acetonide can be hydrolyzed selectively." In a classic example, 1,2-5,6-diace-toneglucofuranose is hydrolyzed selectively at the 5,6-acetonide. 

To avoid the retro-Diels-Alder reaction, 56 was dihydroxylated prior to the introduction of the bromine atom (57). Removal of the acetonide group followed by cleavage of the diol afforded a bis-hemiacetal. Selective reduction of the less-hindered hemiacetal group gave 58. The remaining hemiacetal was protected, and the ketone was converted to an enol triflate, thus concluding the synthesis of the electrophilic coupling component 51. 

Treatment of derivatives 198 with TFA followed by neutralization with aqueous ammonia allowed the /-butoxy-carbonyl (BOC) cleavage and subsequent A-alkylation leading to 199, which after hydrolysis of the acetonide afforded the corresponding tetrahydroxypyrrolizidine that showed selective, but moderate, inhibition of amylogluco-side from RJiizopus mold (Scheme 47) <2004TA323>. 

Cleavage of methoxyethoxymethyl ethers.2 The reagent selectively cleaves MEM ethers in the presence of benzyl, silyl, allyl, and methyl ethers as well as acetals, acetates, and benzoates. However, methoxymethyl ethers and acetonides are cleaved at about the same rate as MEM ethers. 

The required chain extension of 12 was accomplished via deprotonation with NaH and condensation with aldehyde 7 to afford the Diels-Alder precursor 13 in 50% yield. Thermolysis of triene 13 and lactam 3 in xylene at 170 C for four days resulted in the desired cycloaddition to 14. Chromatographic purification permitted isolation of pure 14 in addition to a small amount of an exo isomer (>4 1 ratio). Acid treatment induced cleavage of both the silyl ether and acetonide. Reprotection of the diol and selective epoxidation of the A olefin produced 15 in 64% yield from 12. Epoxide 12 was then transformed to the isomeric allylic alcohol 16 by conversion of the alcohol to the bromide followed by reductive elimination. Protecting-group manipulation and subsequent oxidation the gave aldehyde 17, which was homologated and hydrolyzed to give seco acid 18 in 32% overall yield from 16. 

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