Reaction of Thiocarbonyl S-Methylides with Acetylenic Dipolarophiles and an Unexpected Rearrangement of the Cycloadducts

✍ Scribed by Tomasz Gendek; Grzegorz Mlostoń; Anthony Linden; Heinz Heimgartner

• Publisher: John Wiley and Sons
• Year: 2002
• Tongue: German
• Weight: 222 KB
• Volume: 85
• Category: Article
• ISSN: 0018-019X
• DOI: 10.1002/1522-2675(200202)85:2<451::aid-hlca451>3.0.co;2-9

✦ Synopsis

Dedicated to Professor Hans-J¸rgen Hansen on the occasion of his 65th birthday

The 1,3-dipolar cycloaddition of 2,2,4,4-tetramethyl-3-thioxocyclobutanone S-methylide (2a), generated in situ by thermal extrusion of N 2 from the corresponding 2,5-dihydro-1,3,4-thiadiazole 1a, with electron-deficient acetylenic compounds yields spirocyclic 2,5-dihydrothiophene derivatives of type 4 (Scheme 2). Mixtures of diastereoisomers are obtained in the case of propiolates. The strained cyclooctyne also undergoes smooth cycloadditions with thioketone S-methylides (Scheme 3). Under acidic conditions, the spirocyclic products of type 4 and 6a isomerize, via opening of the cyclobutanone ring and aromatization of the five-membered ring, to thiophene derivatives of type 7 (Scheme 4).