Sulfonate esters to protect alcohols

The sulfone moiety was reductively removed and the TBS ether was cleaved chemoselectively in the presence of a TPS ether to afford a primary alcohol (Scheme 13). The alcohol was transformed into the corresponding bromide that served as alkylating agent for the deprotonated ethyl 2-(di-ethylphosphono)propionate. Bromination and phosphonate alkylation were performed in a one-pot procedure [33]. The TPS protecting group was removed and the alcohol was then oxidized to afford the aldehyde 68 [42]. An intramolecular HWE reaction under Masamune-Roush conditions provided a macrocycle as a mixture of double bond isomers [43]. The ElZ isomers were separated after the reduction of the a, -unsaturated ester to the allylic alcohol 84. Deprotection of the tertiary alcohol and protection of the prima-... 

Protection of Alcohols. In addition to being used as a halide equivalent, methanesulfonates have also been used as protecting groups for alcohols and phenols. The use of methanesulfonate as a protecting group for alcohols is mainly limited to carbohydrate synthesis due to the lability of the methanesulfonate group toward nucleophilic attack. The sulfonate ester is stable to acidic conditions and can be cleaved by Sodium Amalgam. ... 

Sulfonates with a sulfur-bound C,H group can also eliminate alkoxide when treated with a strong base [82]. Phenylmethanesulfonic esters, for instance, are sufficiently base-labile to be useful protective groups for alcohols (Scheme 4.17). 

The hydroboration of acetylenes (3) with diisopinocampheylborane (IpC)2BH in THF led after refunctionalisation and transesterification to the olefins (4a, b, c) isolated in good yields. Monooxidation with mCpBA led to the sulfoxide (4d) whereas the sulfone (4e) was obtained with two equivalents of mCpBA. The same sulfone (4e) could also be obtained in an excellent overall yield by radical addition of phenylsulfonyl iodide to the pinacol ester of vinylboronic acid followed by a dehydroiodination in the presence of Et2N (87 % overall yield). The carboxylic ester (4a) could be transformed into the corresponding carboxylic acid (4f) (79 % yield) 11 which led to the acid chloride (4g) by treatment with freshly distilled thionyl chloride at 0°C (91 % yield), p-keto vinylboronates are easily accessible by oxidation of the corresponding protected allylic alcohol according to the following scheme ... 

Conversion of D to (+)-testudinariol A (149) is summarized in Figure 6.8. After reduction of D with DIBAL-H, the resulting aldehyde E was treated with dimethylaluminum chloride in dichloromethane to give the cyclized product F after TBS protection. Ketone G, prepared from F, was then treated with a chiral phosphonoacetate H in the presence of NaHMDS to give the desired (Z)-ester as the major product. Reduction of the (Z)-ester with DIBAL-H furnished I. Alcohol I was converted to bromide J and sulfone K, respectively. Alkylation of K with J afforded L. Its reductive desulfonization and silyl deprotection yielded (+)-testudinariol A (149) in 4.4% overall yield based on A (19 steps). The spectroscopic... 

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