Science of Synthesis

Inoue, M.; Yamashita, S., Science of Synthesis, (2008) 37, 601.

Treatment of the E-vinylogous sulfones ‌110‌ with tris(trimethylsilyl)silane and triethylborane at room temperature in the presence of air furnishes the 2,5-disubstituted cyclic ethers ‌111‌ with five- to seven-membered rings in 34–99% yield and with excellent diastereoselectivity favoring the cis-isomer (‌Scheme 22‌).[‌93‌] The stereochemical outcome can be rationalized by a favored transition state in which the alkyl substituent is pseudo-equatorial with the vinylogous sulfone s-trans to alleviate A1,3-allylic strain. Similarly, E-vinylogous sulfoxides can also be converted into cis-2,5-disubstituted tetrahydrofurans. Reaction of alcohol ‌113‌ with ethynyl tolyl (R)-sulfoxide (‌112‌) in the presence of 4-methylmorpholine produces the corresponding (E)-β-alkoxyvinyl (R)-sulfoxide, which is converted into iodide ‌114‌ via a substitution reaction. Radical cyclization of iodide ‌114‌ proceeds stereoselectively to form sulfoxide ‌115‌. Importantly, Pummerer rearrangement can convert sulfoxide ‌115‌ into aldehyde ‌116‌, which corresponds to the partial structure of Annonaceous acetogenins.[‌94‌–‌96‌] Finally, the utility of this class of radical reactions is further demonstrated by the synthesis of the core skeleton of garsubellin A. Syringe-pump addition of triphenyltin hydride and 2,2'-azobisisobutyronitrile to a refluxing solution of sulfone ‌117‌ provides tetracyclic ether ‌118‌ as a sole product.[‌97‌]

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