Thiepin 1,1-dioxide cycloaddition

This strategy has also been extended to [6 + 2]-cycloaddition [61]. Thus, substituted cyclooctatetraenes can be prepared in two steps employing a Cr-(0)-promoted [67t + 27t] thiepin dioxide-alkyne cycloaddition, followed by photoactivated sulfur dioxide extrusion (Scheme 9.37) [62]. 

In contrast, cycloaddition of the thiazuleno derivative 9, prepared from dibenzo[fr,/]thiepin-10,11-dione (2), with diphenylacetylene yields 77% of the same product 8.94 Other cycloadducts from 9, and from its S, 5-dioxide, were also obtained in good yields. 

Thiepin-1,1-dioxide undergoes a number of chromium(0) mediated [6jt + 4jt] cycloaddition reactions with a range of 1,3-dienes. The intermediate adduct undergoes a Ramberg-Backlund rearrangement to form new benzannulated products <96JOC7644>. 

Rigby and coworkers305,309 also performed metal mediated [6 + 4] cycloadditions of heterocyclic trienes and tropones with various dienes. In concurrence with the all-carbon trienes, the electronic nature of the diene partners generally had little influence on the cycloaddition efficiency. The only reported exceptions are the reactions of thiepin-1,1-dioxides. Lower yields were observed in the reactions involving electron-deficient dienes in comparison with the reactions with electron-rich dienes. The reaction of complex 514... 

Rigby et al. developed higher-order cycloadditions of ( -thiepine I,l-dioxide)tricarbonylchromium(0) <1992TL5873, 1993JA1382, 1996CHEC-II(9)67> and applied them to various ring systems. 

Furthermore, a four-component cycloaddition reaction (7]6-thiepine I,l-dioxide)tricarbonylchromium(0) 52 with tethered diynes under photoactivation afforded pentacyclic adducts formally derived from a sequential [671+271]/ [67l+27t]/[2(T+27i] cycloaddition process <1999OL507>. Photocycloaddition of the complex 52 with excess 1,7-octadiyne 66a or 1,8-nonadiyne 66b (C1CH2CH2C1, hv (Pyrex filter)) afforded the pentacyclic triene sulfones 67a and 67b in 45% and 38% yields, respectively (Equation 4). In contrast, 1,6-heptadiyne 66c afforded only the three-component cycloadduct 68 in 56% yield (Equation 5). 

A very different sulfur dioxide extrusion protocol has been applied to the total synthesis of (+)-estradiol (30) by Rigby and coworkers. In the event, the trimethylsilyl-substituted chromium thiepin complex 27 participated in a [6 r+4 r] cycloaddition reaction to provide a 70% yield of the structurally complex dihy-drothepin 28. Subsequent sulfur dioxide extrusion via a Ramberg-Backlund-type ring contraction gave tetracycle 29. The total synthesis of (+)-estradiol (30) was completed using a four-step sequence of standard reactions (eq 12). ... 

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