2.4- DMPM-ether

The additional presence of a 3-methoxy substituent on the benzyl group confers greater stability on the intermediate cation, and consequently oxidation of DMPM ethers by DDQ is even more facile. Yonemitsu and cowoikers have used this differential reactivity of substituted benzyl ethers to great effect in the total synthesis of the macrolide antibiotics methynolide, tylonolide, (95)-9-dihydroerythro-nolide and pikronolide. The pikronolide synthesis provides an excellent example of the selective, sequential deprotection of DMPM, MPM and benzyl ether protecting groups (Scheme 7). 

Many examples of the selective deprotection of benzyl, 4-methoxy-benzyl (MPM), and 3,4-dlmethoxybenzyl (DMPM) protected hydroxy groups have been detailed. Benzyl ethers but not MPM or DMPM ethers are cleaved on hydrogenolysls (H2- Raney Ni), while DMPM... 

Cleavage of MPM, DMPM, and TMPM ethers with DDQ in CHzCIs/HzO at 20 ... 

This ether has properties similar to the p-methoxybenzyl (MPM) ether except that it can be repioved from an alcohol with DDQ in the presence of an MPM group with 98% selectivity The selectivity is attributed to the lower oxidation potential of the DMPM group 1.45 V for the DMPM versus 1.78 V for the MPM. 

Cleavage of MPM, DMPM, andTMPM Ethers with DDQ in CH2CI2/H2O at 20°... 

Nomially, dry solvents are employed in the oxidation of unsaturated alcohols with DDQ. This is done because DDQ is decomposed by water.94c On the other hand, the use of wet solvents may not be deleterious, as a mixture of CH2CI2 and water is routinely employed for the deprotection of p-methoxybenzyl (PMB)10S and 3, 4-dimethoxybenzyl (DMPM)106 ethers with DDQ, and, when this deprotection leads to an unsaturated alcohol, a prolonged reaction allows a successful oxidation of the alcohol to a ketone.107... 

Introduction of substituents into the aromatic ring makes the benzylic system prone to further deblocking methods. Thus, o-nitrobenzyl ethers, available from o-nitrobenzyl bromide, are cleaved photolyti-caiiyi38 and the mono- and di-methoxybenzyl groups (Mpm and Dmpm) are removable by oxidation... 

Cleavage of MPM, DMPM and TMPM Ethers with DDQ in CHjClj/HjO at 20°C... 

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