Aliphatic spiroacetals

Aliphatic Spiroacetals Gold, Platinum, Palladium, and Mercury Catalysts... 

In the failure of the phosphoric acid catalysts, we recognized a much more general issue in current Brpnsted acid catalysis, that is, small aliphatic substrates and loosely bound intermediates such as oxocarbenium ions represent essentially unsolved problems in the area. Instead of focusing on the decoration of the spiroacetalization substrates with bulky or aromatic groups, we turned to the development of a catalyst class that could solve these general problems in asymmetric Brpnsted acid catalysis [57]. 

In the Asteraceae, acetylenes are widely distributed and structurally very diverse, including aliphatic acetylenes, acetylenic thiophenes, aromatics, isocoumarins and spiroacetal enol ethers (Table 5.1 and Figures 5.3,5.4 and 5.5). 

Recent examples of the applicatimi of this strategy for the synthesis of aliphatic natural product spiroacetals have been demonstrated by the syntheses of spirastrellolide A [42], spirofungins A and B [43-45], cephalosporolides E and F [46,47], spirangien A [48,49], and pteridic acids A and B [50]. 

Similarly, Rodriguez et al. [81] have used an iterative sulfone lithiation approach to synthesize a range of aliphatic 5,5-spiroacetals (Scheme 36). Acylation and... 

Alternatively, the HDA approach toward mono(benzannulated) spiroacetals can make use of aliphatic dienes, rather than o-QM, as the 4n reaction partner. This strategy has been demonstrated most recently by Belmont et al. [163], where HDA between a variety of furoquinolines 313 and acrolein affords the spiroacetals 314 in good yield and selectivity (Scheme 77). 

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