Allenyl ketones gold-catalyzed cyclizations

Acetylenic ketones are viable substrates as well for gold-catalyzed cyclizations to furans. Whereas alk-3-yn-l-ones readily cyclize to substituted furans in the presence of gold(I) or gold(III) chloride (possibly via isomerization to a-allenyl ketones see Section 5.2), the corresponding transformation of alk-4-yn-l-ones takes place in the presence of the cationic gold catalyst generated in situ from PhsPAuCl and AgOTf in toluene (Scheme 4-83). The cyclization is accelerated by... 

In a more recent publication Hashmi et al. reported that propargyl ketones such as 20 and allenyl ketones such as 21 can be cyclized to furans such as 22 in reactions catalyzed by gold(III) chloride (Scheme 5) [12]. This type of reaction had already been described as a... 

The cycloisomerization of a-allenyl ketones to the corresponding substituted furans was the first example of a gold-catalyzed addition of an oxygen nucleophile to an allene (Scheme 4-86). Traditionally, silver or palladium catalysts were employed for cyclizations of this type advantages of gold catalysis incluiie shorter reaction times, milder conditions, an or lower catalyst loadings. Variable amounts of... 

In 2000, Hashmi reported the gold(III)-catalyzed cyclization of a-allenyl ketones to form substituted furans [15]. As an example, treatment of allenyl p-methoxybenzyl ketone 70 with a catalytic amount of AuCls formed a 2 1 mixture of the cycloisomerization product 71 and the cycloisomerization/hydroarylation product 72 in 91% combined yield (Eq. (12.40)). Marshall had previously demonstrated the Ag(I)-catalyzed conversion of allenyl ketones to furans [111]. The Ag(I)-catalyzed transformation was four orders of magnitude slower than was the gold(III)-catalyzed transformation, but the silver-catalyzed process forms the 2-substituted furan as the exclusive product without the competing secondary hydroarylation. The selective conversion of a-allenyl ketones to substituted furans is also catalyzed by gold(III)-porphyrin complexes [112]. 

---