Pinacols, cyclization dehydrative

Superacid dehydrative cyclization of pinacols such as (93) gives condensed aromatic compounds as shown, presumably via dicationic species like (94).137... 

Compound (80), prepared from 2-methylcyclopentane-l,3-dione and methyl 2-chloroacrylate followed by the sequence (76)-> (77)-> (78)- (79) (resolved)— (80), combined with (71) (prepared from m-methoxyphenylstyrene and diborane) to form the seco-steroid (82)/ Acetic anhydride-toluene-p-sulphonic acid then cyclized this stereoselectively to furnish the triacetate (83) which on saponification gave the triol (84). Interestingly, this compound reacted with toluene-p-sulphonic acid in alcohol to produce, by dehydration and change of configuration at C-14, the compound (85), which served as a source of various 8a-oestrone compounds. Compound (84) on treatment with boron trifluoride etherate underwent pinacol transformation in preference to dehydration to yield the ketone (86) this ketone was correlated with the known compound (87). 

When aromatic pinacols are reacted with an acid, products often arise from dehydration and rearrangement.5 This general conversion is known as the pinacol rearrangement. The pinacol rearrangement may be promoted by both Brdnsted and Lewis acids.6 In the procedure described here, superacidic triflic acid is reacted with an aryl pinacol and a dehydrative cyclization occurs to give the substituted phenanthrene product. Related to this conversion, the chemistry of benzopinacol in sulfuric acid and triflic acid is contrasted in Scheme 1. We have proposed that the superacidic triflic acid causes the formation of diprotonated intermediates which promote the dehydrative cyclization.4... 

Morgan et al. showed that bacteriochlorin-like macrocycles can also be generated by cyclization of either 5,10- or 5,15-bis[(ethoxycarbonyl)vinyl]-porphyrins. However, the resulting products rapidly decomposed upon exposure to air, thus precluding their use as photosensitizers for PDT. For developing a general synthesis of stable bacteriochlorins, the same authors followed the pinacol-pinacolone approach in preparing ketochlorins 22 and 23. In brief, dehydration of 19 produced a mixture of 20 and... 

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