Cationic domino reactions

In most of the hitherto known cationic domino processes another cationic process follows, representing the category of the so-called homo-domino reactions. In the last step, the final carbocation is stabilized either by the elimination of a proton or by the addition of another nucleophile, furnishing the desired product. Nonetheless, a few intriguing examples have been revealed in which a succession 

Domino Reactions in Organic Synthesis. Lutz F. Tietze, Gordon Brasche, and Kersten M. Gericke Copyright 2006 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 3-527-29060-5 

In the following sections, we detail another functionality which is of major value in the area of carbocationic domino processes, namely the epoxides. On the basis of their high tendency to be opened in the presence of Lewis or Bronsted acids, thereby furnishing carbocationic species, several challenging domino procedures have been elaborated relatively recently. 

Using this methodology, cyclic ethers of different ring-size can also be constructed. Due to the fact that many natural products of marine origin include fused polycyclic ether structural units, a convenient entry to this structural element is of continuing challenge. 

Reaction conditions 1 eq BF3OEt2, CH2CI2, -40 °C, 10-30 min, H20 quench then Ac20, Et2N, CH2CI2. 

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In working towards the synthesis of nonracemic 3-deoxyschweinfurthin B (1-42), an analogue of the biological active schweinfurthin B, Wiemer and coworkers developed an acid-catalyzed cationic domino reaction to afford the tricyclic diol (f ,f ,i )-l-41 from 1-40 in moderate yield (Scheme 1.11) [11]. 

A very impressive multiple cationic domino reaction was used in the enan-tioselective total synthesis of (-)-gilbertine (1-190), described by Blechert and coworkers [50]. When the tertiary alcohol 1-184 is treated with TFA, a carbocation is formed which undergoes a cascade of cyclizations to afford 1-190 in very good yield (61 %) (Scheme 1.44). The cations 1-185 to 1-189 can be assumed as intermediates. 

Another cationic-cationic domino reaction which follows the way of biosynthesis is used by Koert et al for the synthesis of the whole left part of the polyether etheromydn.1141 The starting step is an add catalyzed cyclization of a diepoxide to give a bistetrahydrofuran with creation of three new oxa-cycles in over 30 % yidd. 

Blechert et al. have proposed a new effective approach to the synthesis of different indole alkaloids using cation domino reactions (95S592, 97AGE1474). The total yield of uleine (7a), produced on the basis of this concept, is 32%, which is quite unique in the synthesis of polycyclic indole alkaloids. Note that this is the total yield, starting from cyclohexenone 46 the yield of the key step of the synthesis - the cyclization of carbazole 47 into 7a is 95% (Scheme 12). 

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