Hydroxy dithioketal cyclization

Scheme 3. Construction of didehydrooxocane systems by the hydroxy dithioketal cyclization.

It is important to emphasize that the hydroxy dithioketal cyclization can be conducted under mild reaction conditions and can be successfully applied to a variety of substrates.15 However, the utility of this method for the synthesis of didehydrooxocane-contain-ing natural products requires the diastereoselective, reductive removal of the ethylthio group. Gratifyingly, treatment of 13 with triphenyltin hydride and a catalytic amount of the radical initiator, azobisisobutyronitrile (AIBN), accomplishes a homolytic cleavage of the C-S bond and furnishes didehydrooxocane 14 in diastereo-merically pure form (95 % yield), after hydrogen atom transfer. 

Hydroxy dithioketal cyclization Construction of didehydrooxocane systems... 

Cyclization of hydroxy dithioketals. Nicolaou et al.1 have used this reaction for construction of oxocenes. The most satisfactory method involves activation of the sulfur of the starting material (1) with AgC104 to produce an oxocene derivative (2), followed by C-S cleavage. This second step can be effected with (C6H5)3SnH (AIBN) or by oxidation to the corresponding sulfoxide or sulfone, which then is cleaved with... 

In the synthetic studies of the marine natural products brevetoxins A and B, Nicolaou and coworkers [157] developed a highly efficient cyclization reaction of hydroxy dithioketals leading to oxocenes. As shown in Scheme 88, exposure of the hydroxy dithioketal 261 to 1.1 equiv of AT-chlorosuccinimide (NCS) in CH3CN in the presence of 2 equiv of 2,6-lutidine, 1.1 equiv of AgNOj, molecular sieves and silica gel at 25 °C for 5 min led to the oxocene 262 in 95% yield. 

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