Protecting groups for dienes

PTAD has been used as a protecting group for dienes in steroids since it can be readily removed under mild conditions by treatment with base (e.g., K2CO3/DMSO) <85BSF849,95JOCl828>. 

Fe(CO)3 forms stable tf complexes of conjugated dienes, and acts as a useful protecting group for dienes, preventing reactions normally associated with carbon-... 

It is interesting that iron carbonyl can be used as a protecting group for dienes. Iron diene complex 491, for example, reacted with 492 in a Mukaiyama aldol reaction (sec. 9.4,C), giving an 81% yield of 493 and 494 in an 84 16 ratio.32 in a synthesis of heptitol derivatives,322 Pearson protected tropone as the iron tricarbonyl derivative (495), and prepared 496 using a multistep sequence before deprotecting the diene to give 497. 

Just as the tricarbonyliron group has been used as a protecting group for dienes, the tricarbonylchromium group has been used in a similar fashion to stabilize dihydropyridines which are normally difficult to handle and to isolate. The dihydropyridine is readily liberated from the complex by treatment with pyridine at room temperature (Kutney et ai., 1974). Dihydropyridines can also be generated from the corresponding tricarbonyliron complexes, recently synthesized by Alper (1975). 

Elliott and Shepherd (70) have investigated the effect of Os(II) coordination on dienes and report that the metal acts as a protecting group for electrophilic addition of bromine. Thus, the action of bromine on [Os(NH3)5 T 2-l,2-(l,3-butadiene) ]2+ is thought to result in the 3,4-dibromo-1-butene analog. 

The iron tricarbonyl unit has been described as a protecting group for a 1,3-diene, as in the acetylation of the complex of myrcene at low temperature (Scheme 20). The usual combinations of a,p- and p,"y-un-saturated ketones were formed. At higher temperatures, some acylation at the terminus of the diene complex was also observed. In an interesting extension, reaction of the complex with oxalyl chloride resulted in cyclization of the acid chloride initially formed by reaction at the alkene. ... 

Iron tricarbonyl complexes of 1,3-dienes (e.g. cyclohexa-1,3-diene) play an important role in organic synthesis the complexes are stable under a variety of reaction conditions, and iron carbonyls are inexpensive. The Fe(CO)3 group acts as a protecting group for the diene functionality (e.g. against additions to the C=C bonds), allowing reactions to be carried out on other parts of the organic molecule as illustrated by reaction 23.87. 

The utility of tricarbonyliron fragments as protecting groups for 1,3-dienes has been demonstrated first by Barton et al. They showed that the diene system in the B ring of ergosterol can be protected by formation of the tricarbonyliron adduct to allow selective catalytic hydrogenation of the side chain A -double bond (Scheme 4-157). ... 

The choice of the acyl substituent X for Diels-Alder reactions of l-N-acylamino-l,3-butadicnes depends on the particular synthetic problem. The acyl substituent has a moderate effect on the cycloaddition reactivity of these dienes, and also determines what amine unmasking procedures are required. As a result of their stability and the variety of amine deprotection procedures available, " the diene carbamates are the components of choice in most cases. A particularly attractive aspect of the diene synthesis detailed here is the ability to tailor the amino-protecting group... 

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