PGroup_HO - Reisman, Ch 242 Acetals as Protective Groups: R...

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Reisman, Ch 242 O OMe R O OBn R methoxymethyl ether (MOM) benzyloxymethyl ether (BOM) 2,2,2-trichloroethoxymethyl ether O O R O O R OMe 2-methoxyethoxymethyl ether (MEM) O O R TMS 2-(trimethylsilyl)ethoxymethyl ether (SEM) O O R tetrahydropyranyl ether (THP) Acetals as Protective Groups: The above acetals can be cleaved under Bronsted or Lewis acidic conditions; conditions for selective cleavage are noted. – Generally stable to base. – Acetal protecting groups generally permit chelation. – Disubstituted ethers (i.e. THP) are typically less stable to acid than mono-substituted ethers (i.e. MOM). – Disubstituted ethers result in formation of a stereogenic center; this can sometimes complicate product analysis (if a diastereomeric mixture). cleaved with F Lipshutz TL 1980 , p. 3345 will form mixtures of diastereomers Grieco JOC 1977 , p. 3772 Boeckman TL 1985 , p.1411 can be cleaved in the presence of a MOM group Corey TL 1976 , p. 809 cleaved by hydrogenolysis cleaved with Zn/AcOH General Conditions: OH DIPEA, CH 2 Cl 2 Cl OR' O OR' – or – NaH, THF + aq. HCl, THF – or – p -TsOH, MeOH – or – BCl 3 , CH 2 Cl 2 , –78 °C R R OH R Ethers as Protective Groups: O R O R O R O R OMe allyl ether (allyl) trityl ether (Tr) benzyl ether (Bn) p- methoxybenzyl ether (PMB) O R NO 2 o -nitrobenzyl ether (NB) cleaved using DDQ or CAN Yonemitsu TL 1988 p. 4139; Yonemitsu Tetrahedron 1986 , p. 3021 phenolic allyl ethers cleaved w/Pd 0 Guibe Tetrahedron 1998 , p. 2967 cleaved under mildly acidic conditions cleaved by hydrogenolysis cleaved under photolytic conditions Bn, PMB, NB, and allyl ethers are robust; typically stable to acidic or basic conditions. – Bn, PMB, and NB groups can be cleaved under orthogonal conditions.
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PGroup_HO - Reisman, Ch 242 Acetals as Protective Groups: R...

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