Mercury(II)-Catalyzed Synthesis of Spiro[4.5]decatrienediones from Allenyl Ketones and Comparison with Silver(I)-, Palladium(II)- and Brønsted Acid-Catalyzed Reactions
✍ Scribed by A. Stephen K. Hashmi; Lothar Schwarz; J. W. Bats
- Publisher
- John Wiley and Sons
- Year
- 2000
- Tongue
- English
- Weight
- 204 KB
- Volume
- 342
- Category
- Article
- ISSN
- 1615-4150
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✦ Synopsis
The synthesis of the spiro[4.5]decane terpene skeleton from methoxy [1] and hydroxy substituted arenes has been stimulated by the interest in naturally occuring sesquiterpenes like spirovetivanes, acorones or alaskanes [1b,3] and their analogues. Among these methods is Nagao's approach [4] that utilizes easily available allenyl benzyl ketones and stoichiometric amounts of Lewis acids (LA) like BF 3 • Et 2 O, ZnI 2 or AlCl 3 . One major synthetic limitation of this method was the need of at least two methoxy substituents on the aromatic system. With only one methoxy group, the yield dropped from more than 80% to 8%.
We were interested in overcoming both limitations i.e. to use catalytical amounts of lewis acids and only one methoxy substituent. We also wanted to address the question of selectivity, since it was known from work of Marshall's and our groups [5, 6] that silver or palladium catalysts are able to cause a cycloisomerization of the allenyl ketone to a furan.
Recently, we published preliminary results concerning the spirocyclization . Here, we now want to report the mercury-catalyzed reactions in full detail and compare them with silver-, palladium-, and BrÝnsted acid-catalyzed reactions.
Synthesis of the Substrates
One class of substrates used in this investigation were the allenyl methoxybenzyl ketones 3a -b. They were prepared by the addition of allenylmagnesium bromide (2) to the esters 1a -b at -78 °C.
Mercury(II)-Catalyzed Synthesis of Spiro[4.