Tricyclic 1,3-diol substrate

The total synthesis of (+)-furoscrobiculin B, a lactarane sesquiterpene isolated from basidiomycetes of mushrooms, was accomplished in the laboratory of H. Suemune and K. Kanematsu using a furan ring transfer reaction and a semipinacol rearrangement as key steps. The secondary hydroxyl group of the tricyclic c/s-vicinal diol substrate was converted to the corresponding tosylate that in situ underwent a ring-expansion reaction to afford an azulenofuran in good yield. 

As for cyclic alcohols, bi- or tricyclic diols of different structures are excellent substrates and furnish excellent results in terms of stereosel tivity [263-267]. Scheme 62 presents a few selected examples of the successful asymmetrization of bi- and tricyclic substrates. 

Oxidative hydroboration 91) of lactone 11.49 afforded a mixture of four lactarorufins (Scheme 20), in which diols 11.30 and 11.33, arising from a attack of diborane, largely predominated (more than 90%). The same type of stereoselectivity was observed for other addition reactions, i.e. epoxidation, osmylation, hydrogenation (775), (704), 43) to 2,9, 3,4- or 6,7-double bond of lactaranolides and marasmanes. Apparently, the tricyclic structures of these substrates provided enough conformational and steric bias to direct approach of reagents from the same side as the bridgehead protons H-2 and H-9. However, when the double bond was located in a different position, exceptions were observed 98). 

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