Platinum- and gold-catalyzed cycloisomerization reactions of hydroxylated enynes.
Exposure of enynes containing a hydroxyl group at one of the propargylic positions to catalytic amounts of either PtCl2 or (PPh3)AuCl/AgSbF6 results in a selective rearrangement with formation of bicyclo[3.1.0]hexan-3-one derivatives. The same products are obtained by a "one-pot" process on treatment of an alkynal with allylchlorodimethylsilane (4) and PtCl2 via a reaction cascade involving an initial platinum-catalyzed allylation followed by the cycloisomerization of the homoallylic alcohol formed in situ. This novel skeletal reorganization process was implemented into a concise total synthesis of the terpenes sabinone (18) and sabinol (19). Furthermore it is shown that conversion of the hydroxylated enynes into the corresponding acetates followed by reaction with a cationic gold catalyst formed from (PPh3)AuCl and AgSbF6 opens entry into isomeric products bearing the ketone function at the C-2 position of the bicyclo[3.1.0]hexane skeleton. The outcome of a deuterium labeling experiment and the analysis of the stereochemical course of the cycloisomerization reaction are consistent with the formation of cyclopropylmethyl platinum carbene species as reactive intermediates.
Mamane
Victor
V
Max-Planck-Institut für Kohlenforschung, D-45470 Mülheim/Ruhr, Germany.
Gress
Tobias
T
Krause
Helga
H
Fürstner
Alois
A
D000475:Alkenes / Q000138:chemical synthesis / Q000737:chemistry; D000480:Alkynes / Q000138:chemical synthesis / Q000737:chemistry*; D002384:Catalysis; D003500:Cyclization; D003521:Cyclopropanes / Q000138:chemical synthesis / Q000737:chemistry; D017612:Gold Compounds / Q000737:chemistry*; D007536:Isomerism; D015394:Molecular Structure; D010084:Oxidation-Reduction; D017671:Platinum Compounds / Q000737:chemistry*
D000475:Alkenes; D000480:Alkynes; D003521:Cyclopropanes; D017612:Gold Compounds; D017671:Platinum Compounds; C038016:gold chloride; C030173:platinum chloride