Selective cleavage alcohols

Acidic ether cleavages are typical nucleophilic substitution reactions, either SN1 or Sn2 depending on the structure of the substrate. Ethers with only primary and secondary alkyl groups react by an S 2 mechanism, in which or Br attacks the protonated ether at the less hindered site. This usually results in a selective cleavage into a single alcohol and a single alkyl halide. For example, ethyl isopropyl ether yields exclusively isopropyl alcohol and iodoethane on cleavage by HI because nucleophilic attack by iodide ion occurs at the less hindered primary site rather than at the more hindered secondary site. 

Potential-selective cleavage is a powerful method for potential-selective deprotection. Benzyl ethers can be cleaved to benzaldehyde and alcohol via oxidation of the benzyl group to a radical cation (Fig. 30) [148]. With the radical cation of... 

Finally, Inanaga s contribution to the development of chiral 4-dialkylaminopyrid-ine based catalysts for enantioselective acyl transfer relied on the use of C -symmetric 4-PPY derivative 36 (Fig. 7) [130]. This compound was obtained in an enantiopure form by selective cleavage of a carbamate intermediate using Sml, and allowed the KR of various. yec-alcohols with selectivity factors ranging from y = 2.1 to 14. 

An alcohol is oxidized with Fetizon s reagent in the presence of a very oxidation-sensitive dialkoxy alkene that, for instance, suffers selective cleavage with no reaction on the alcohol moiety on contact with PCC. 

Fuchigami and coworkers and Yoshida and coworkers independently found that anodic oxidation of benzylsilanes in the presence of nucleophiles such as alcohols and carboxylic acids resulted in a selective cleavage of the C—Si bond and the oxygen nucleophiles were introduced exclusively into the benzylic position (equation 6)11-13. In the absence of nucleophiles, the benzylsilane itself plays a role of a nucleophile and benzyl(trimethylsilyl-methyl)benzene is formed (equation 7)11,12. 

The selective cleavage of the IV-t-butoxycarbonyl group in the presence of TBDMS (t-butyldimethylsilyl) (selective only when the protected alcohol was phenolic) or TBDPS (ferf-butyldiphenylsilyl) (completely selective) ethers can be achieved68 using a saturated solution of HC1 in ethyl acetate. 

Selective cleavage of epoxides. The a- and /f-benzyloxy epoxides 1 and 3 (either cis or trans) undergo almost exclusive /(-addition with trimethylaluminum catalyzed by n-butyllithium or lithium methoxide to give the alcohols 2 and 4, respectively. The regioisomeric alcohols are formed only in traces. CHjMgBr, CH3Li, or (CH3)2Mg are much less selective.2... 

In contrast to the reaction of y-stannyl alcohol 50, the y-stannyl benzyl ether 53 results in selective cleavage of the butyl-tin bond by reaction with (PhIO)n 18/DCC/BF3 and, after quenching of the reaction mixture with aqueous NH4C1, afforded the chlorostannane in high yield. Interestingly, the chlorostannane both in solution and in the solid state adopts a 1,3-diaxial conformation through Sn-O hypervalent interaction [82]. 

Step 5 Selective cleavage of the sterically less hindered TBS ether. Step 6 Conversion of the 1° alcohol to the corresponding iodide. 

Selective cleavage of ROSHCH3)(Cf/s)2. These ethers are cleaved to the alcohol by NaN3 in DMF at 40° without effect on r-butyldimethyl- or t-butyldi-phenylsilyl ethers. 

The same authors have recently shown that in the presence of liOTf as an additive (20 mol %), the same precatalyst [RuTp(PPh3)(MeCN)2]PF6 (10 mol %) provided the selective cleavage of the carbon-carbon triple bond of terminal propargylic alcohols to form an olefin and CO (Eq. 14) [90],... 

UButylmethoxyphenylsilyl ethers (r-BMPSi ethers). In DMF in the presence of NfCjHj), this bromosilane reacts with primary, secondary, and tertiary alcohols to form silyl ethers in good yield, and also with some enolizable ketones to form enol silyl acetals. Selective silylation of primary alcohols is possible by use of CHjClj as solvent. The hydrolytic stability of these ethers is intermediate between that of t-butyldimethylsilyl ethers and that of t-butyidiphenylsilyl ethers. The most useful feature of this new protecting group is the selective cleavage by fluoride ion in the presence of other silyl ethers. 

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. 

The preparative electrochemical oxidation of allylsilanes proceeds smoothly and the C-Si bond is cleaved selectively without affecting other allylic C-H bonds [110-113]. This selectivity is ascribed to the selective cleavage of the C-Si bond in the cation radical intermediate. The resulting allyl radical intermediate is further oxidized to give the allyl cation intermediate, which is trapped by nucleophiles such as alcohols, water, carbamates, and tosylamides to give the corresponding allylic substitution products as shown in Eq. (25). Usually, the nucleophiles are introduced to both ends of the allyl cation, and therefore a mixture of two regioisomeric products is formed. 

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