Regioselectivity conjugated diene synthesis

The reaction has been applied successfully to the synthesis of a precursor of provitamin D. 520, which has a homoannular conjugated diene in the B ring[340]. Treatment of the 7 -carbonate 518 with Pd catalyst at 40 °C afforded the 5.7-diene 520 regioselectively in good yield. No heteroannular diene 521... 

The role of allylic sulfoxides as homoaldol equivalents in the synthesis of ( )-allylic alcohols was summarized earlier. A more recent finding is that allylic sulfoxides are precursors for conjugated dienes (equation 70). The elimination is regioselective but stereorandom. The 2,3-rearrangement of propar-gylic sulfenates gives allenyl sulfoxides. Allenyl sulfoxides are valuable synthetic intermediates. They can be converted into stereochemically homogeneous allenes, e.g. (205 equation 71). ... 

Allylic and dienyl sulfones have been prepared by conjugate addition to 1,3-dienes ". Phenylsulfonyhnercuration of conjugated dienes gives mercury adducts which can be treated with base to afford phenylsulfonyldienes. 2-(Phenylsulfonyl)-l,3-dienes can be stereo- and regioselectively functionalized via Michael addition of nucleophiles to give allylic sulfones. A key intermediate in the synthesis of a Monarch butterfly pheromone 4 was prepared by BackvaU and Juntunen by alkylation and subsequent palladium-catalyzed substitution of the allylic sulfone formed by Michael addition of dimethyl malonate to 2-(phenylsulfonyl)-l,3-butadiene (equation 10). 

We can conclude this section on Lewis acid catalysis with a striking example of its use in solving a problem in steroid synthesis. We saw, on p. 309, how 2,6-xyloquinone 6.230 added to the diene 6.229 with inappropriate regioselectivity for steroid synthesis. When boron trifluoride was added to the reaction mixture, it formed a salt 6.284 at the more basic carbonyl group, the one conjugated to the two methyl groups, which is also the less hindered. The result was that the polarisation of the LUMO of the C=C double... 

The stereo- and regioselective synthesis of conjugated alkadienes are of great importance in organic chemistry. A number of methods for the preparation of conjugated dienes and polyenes were developed uitilizing... 

Heck reactions on nonconjugated dienes create an electrophilic Jt-aUyl complex that is susceptible to nucleophilic attack. When the compound contains a conjugated diene, the regioselectivity becomes a very important issue, which must be considered. Cascade sequence involving a Heck reaction and a Jt-allyl reaction could be found in the formal total synthesis of Morphine by Overman [15]. Treatment of dienes with PdCOCOCFjj CPPhj) and NEtj in toluene led to formation of the backbone of Morphine in 56% yield (Scheme 7.6). 

Isolated double bonds are significantly less reactive than the conjugated diene moiety which is illustrated by the base-catalyzed regioselective hydroamination of myrcene to form diethylgeranylamine (24) [171]. This reaction is performed industrially on a multi ton scale as part of the Takasago menthol synthesis [175]. 

Backvall JE, Ericsson A (1994) Palladium-catalyzed regioselective addition of thiophenol to conjugated enynes. Efficient synthesis of 2-(phenylsulfinyl) and 2-(phenylsulfonyl) 1,3-dienes. J Org Chem 59 5850-5851... 

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