Ferf-Butyldimethylsilyl ether

G. D. Kishore Kumar and S. Baskaran, A facile, catalytic, and environmentally benign method for selective deprotection of ferf-butyldimethylsilyl ether mediated by phosphomolybdic acid supported on silica gel, /. Org. Chem., 70 (2005) 4520-4523. 

Addition of allylic zinc bromides to nitrones, generated in situ from allylbro-mides and zinc powder in THF (670), allyltributylstannane (671) and lithiated allyl ferf-butyldimethylsilyl ether (672), proceeds regioselectively in good yields and is used to synthesize homoallyl hydroxylamines (Scheme 2.189). The latter were subjected to an iodo cyclization reaction (see Scheme 2.186). 

The unusual system franx-RulOl lTMPl/N OllO atm)/fluorobenzene epoxidised linear alkenes, cholesteryl acetate and the ferf-butyldimethylsilyl ether of citronellol [592] cholest-5-ene-3-one was oxidised to the 6a and 6p alcohols and the enedione by franx-RulOl lTMPl/OXCgH (Table 3.2) [593]. With ter-penes the 6,7-double bonds were selectively epoxidised by fran -Ru(0)2(TMP)/... 

Scheme 21). Scheme 22 illustrates an example of kinetic resolution of a racemic allylic alcohol with a 1,3-hydrogen shift. When racemic 4-hydroxy-2-cyclopentenone is exposed to a cationic (/ )-BINAP-Rh complex in THF, the S enantiomer is consumed five times faster than the R isomer (32). The slow-reacting stereoisomer purified as the crystalline ferf-butyldimethylsilyl ether is an intermediate in prostaglandin synthesis (33). These isomerizations may occur via initial Rh-olefinic bond interaction (34). 

In asymmetric syntheses, the bulky (i-Pr)3Si group can direct 1,2-addition of phosphite 92 to (S j-triisopropylsilyloxy lactaldehyde (93) to afford adduct 94 preferentially along with the by-product 95 (equation 42)157. The stereochemical outcome results from the bulk of the (i-Pr)3Si protecting group9. Furthermore, the steric congestion in allylic cyanohydrin trimethylsilyl and ferf-butyldimethylsilyl ethers influences their regioselective 1,2- and 1,4-additions to carbonyl compounds under basic conditions158. 

Selective deprotection of trialkylsilyl ethers can also be accomplished by Nafion-H. Trimethylsilyl ethers are cleaved to the corresponding alcohols under mild conditions680 [Eq. (5.241)]. Nafion-H with Nal (1 equiv.) in methanol was shown to readily cleave ferf-butyldimethylsilyl ethers (room temperature, 4—25 h,... 

Spirocycles can be obtained from intramolecular radical biaryl coupling reactions when suitable substituents are present for an alternative stabilization of the cyclohexadienyl intermediate (c.f. 69, Scheme 25). Otherwise, rearomatization can occur by cleavage of one substituent from the quaternary center of the spirocycle, such as the C-P bond in 69. First examples for an alternative reaction course have been reported in studies on the photochemically induced cyclization of iodoarenes [134]. Recently, ferf-butyldimethylsilyl ethers [135] and azides [136] were identified as well-suited substituents to lead the ipso substitution into the pathway towards spirocycles (Scheme 27). 

Cycloaddition of orf/io-quinone 162 with ferf-butyldimethylsilyl ether of sinapyl alcohol (163) yielded the 1,4-dioxane-linked adduct 164, as shown in Scheme 10b, but no regioisomer was detected. The ketal group in 164 was removed under acidic conditions and the derived ketone, a trans-cis mixmre in the ratio of 2 1, on treatment with alkali, yielded the frans-isomer, 165, in a good yield. The ketone 165 was converted to the key intermediate aldehyde 166 through the iodoform reaction, esterification, LAH reduction, and oxidation with Dess-Martin periodinane after protection of the hydroxy groups by MOM groups (Scheme 10b). 

In some cases it may be desirable to block a particular functional group so as to direct lanthanide association to another site. A comparison of the effect of blocking binding at hydroxyl groups by preparing trifiuoroacetate, ferf-butyldimethylsilyl ether and trimethyl silyl ether derivatives was reported. The ferf-butyldimethylsilyl ether derivative was most effective at blocking lanthanide association. 

Deprotecting a ferf-butyldimethylsilyl ether to re-form an alcohol... 

Of the possible silyl ethers, trimethylsilyl and ferf-butyldimethylsilyl ethers are used most frequently. The use of the more stable ferf-butyldimethylsilyl ethers is preferred in several cases [544-547]. The various methods for the activation of glycosyl silyl ethers in glycosylation reactions are summarized in Table 4.14. 

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