Gold(I)-catalyzed divergence in the reactivity of 3-silyloxy 1,6-enynes: Pinacol-terminated vs Claisen-terminated cyclization cascades

Organic Letters

Volumn 10, Issue 12, 2008, Pages 2605-2607

  Baskar, Baburaj ^a   Bae, Hyo J ^a   An, Sang E ^a   Cheong, Jae Y ^a   Rhee, Young H ^a   Duschek, Alexander ^b   Kirsch, Stefan F ^b  

^a Pohang University of Science and Technology (POSTECH)   (South Korea)

^b TECHNICAL UNIVERSITY OF MUNICH   (Germany)

Author keywords

[No Author keywords available]

Indexed keywords

EID: 57849109788     PISSN: 15237060     EISSN: None     Source Type: Journal    
DOI: 10.1021/ol8008733     Document Type: Article

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45. Although not illustrated in this paper, cyclization-pinacol reaction of acyclic substrates 1 is generally low yielding (< 50%).
46. 1H COSY, HMBC, and NOESY data.
47. Compound 2i was obtained as a single diastereomer.
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49. (a) Unlike the gold(I)-catalyzed cycloisomerization of 3-methoxy 1,6-enynes previously reported (ref 7), no silyl enol ethers were obtained in heterocyclization-Claisen rearrangement pathway. However, the protodesilylation of initially formed silyl enol ethers cannot be completely excluded. (b) In the absence of i-PrOH, the reaction gave the ketone 3i in significantly lower yield (<30%), with no evidence of formation of the silyl enol ethers.
50. Although the formation of cycloheptenones could be explained by the initial alcoholysis of the silyl ethers under Lewis/protonic acid conditions and the subsequent alkoxycyclization-[3,3]-sigmatropic rearrangement of the corresponding alcohols (e.g., ref 10j), this alternative reaction pathway was reasonably excluded on the basis of the following observations: (i) Upon exposure to the reaction conditions described in eq 5 (5-10%, with or without i-PrOH (1.1 equiv)), the alcohol precursor of silyl ether 1i led to the formation of an intractable mixture of compounds with no evidence of formation of compound 3i. (ii) Compound 1i did not form 3i upon treatment with a catalytic amount of various protonic acids (5-10%, with or without i-PrOH (1.1 equiv)).