A survey of protecting groups

A dioxolane can be used in this way to protect aldehydes and ketones from powerful, basic nucleophiles, and makes the first entry in the tour of important protecting groups we shall conduct you through in the next few pages. 

Protecting group Structure Protects From To protect To deprotect 

By protecting sensitive functional groups like ketones it becomes possible to make reagents that would otherwise be unstable. In a synthesis of the natural product porantherine, a compound based on the structure in the margin was needed. As it s a symmetrical secondary alcohol (see p. 216), a good way to make it is to add a Grignard reagent twice to ethyl formate. 

of course, a ketone-containing Grignard is an impossibility as it would self-destruct, so an acetal-protected compound was used. Acid-catalysed hydrolysis of the two dioxolanes, 

A synthesis could start with the same bromoketone as the one above reduction gives an alcohol, but alkylation of an alkynyl anion with this compound is not possible because the anion will just deprotonate the hydroxyl group. 

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The book Protective Groups in Organic Synthesis, which is listed in the general references, provides a thorough survey of protective groups and the conditions for introduction and removal. 

IEC and EN Standards ask for marking the type of protection as an abbreviation completed by the classification scheme (temperature class, explosion group) as described in the previous chapter. Table 5.1 gives a survey of the types of explosion protection and their abbreviations. Table 5.2 summarizes the IEC and EN marking code. 

Another method for achieving C7-lithiation involves blocking the C2 position with a removable group such as trimethylsilyl. After surveying several protecting groups, Snieckus achieved an acceptable yield with the C7-lithiation of A-(diethylcarbamoyl)-2-trimethylsilylindole 89 some of the electrophiles explored are shown below [19] (Table 16). 

A survey of coupling partners shows this to be a very general method that tolerates several functionaHties and proceeds readily at room temjjerature. Unlike most TBAF-promoted reactions, however, the Z-isomer of the silanol reacts at a much lower rate than the -isomer. In general, electron-deficient aryl iodides couple faster than their electron-rich counterparts. The synthetic potential of this new method of activation is clearly demonstrated in the synthesis of ( )-and (Z)-64. The coupHng reaction occurs cleanly in the presence of a TBS (t-butyldimethylsilyl)-protected alcohol, without any observable deprotection. Thus, not only is the compatibihty with silyl protective groups estabhshed, the concept of employing the silanol moiety as a prosthetic group for controlled carbon-carbon... 

Dixon, T.R., Dixon, T.J., A Report on a Survey of Packaged Dangerous Goods, Munitions and Pyrotechnics Recovered on the Beaches and in the Nearshore Waters of the British Isles (1 September 1982-31 August 1983). Stage 6. Marine Litter Research Programme. Advisory Committee on Protection of the Sea/Keep Britain Tidy Group, 1985. 

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