Quinone ketal structures

The cis-2,3-diaryl-2,3-dihydro-l,4-benzoxathiin is a very unique structural motif. Other than scattered reports in the literature on the formation of this scaffold, there was no effective asymmetric synthesis for it [6]. We explored two major synthetic approaches to realize the key chiral as-diaryl dihydrobenzoxathiin scaffold, as shown in Scheme 5.3. One was the quinone ketal route in which the quinone ketal 13 and the chiral mercaptol alcohol 14 were the key intermediates. The other approach was the stereo- and enantioselective reduction of the diaryl benzoxathiin 16. The key mercaptol alcohol 14 and the diaryl benzoxathiin 16 were both envisioned to be prepared from the key, common iodoketone intermediate 15. 

The construction of only the (S)-hexahydroxydiphenyl unit, a structural analog of secondary plant metabolite ellagitannins, by biomimetic cyclization of suitably protected glucose-derived digaUoyl esters has been achieved by use of lead tetraacetate (Scheme 13.32). No quinone ketal-type product was detected, in contrast to the simpler system described below (Scheme 13.37). 

The Lewis acid-catalysed orientation reversal in the reaction between substituted cyclohexa-1,3-dienes and 2,6-dimethyl-l,4-benzoquinone ° has been employed in an interesting synthesis of quassin (218). ° Thus, reaction at room temperature of the diene (215) with the above quinone in the presence of an equivalent quantity of Bp3,OEt2 gave the adduct (216) which was converted by several subsequent steps into (218). In the absence of the catalyst the alternative adduct (217) was obtained. Periodic acid oxidation of substituted o-cresols ° and of 2-methoxyphenols in methanol solution affords intermediate o-quinol methyl ethers or o-quinone dimethyl ketals which dimerize to give dienediones with structures related to those of (216) and (217). Another report concerns the formation of a Diels-Alder dimer upon hypochlorite oxidation of 2,2 -methylenebis(4-methyl-6-t-butyl)phenol. ... 

---