Aldol-type ring opening reaction, Retro

Interest in the synthesis of the mitomycins has resulted in a variety of procedures for the preparation of pyrrolo-indoles and related structures. Now Ban and his colleagues have reported a biomimetic route to the mitosane skeleton which involves a novel retro-aldol type of ring-opening reaction followed by a trans-annular cyclization (Scheme 102). A mitosane-like compound is available via... 

Treatment of cycloprop [c]pyrans 520 with acid leads to chromans 521 with a 5,8-substitution pattern, which is difficult to obtain by other synthetic methods (Scheme 114). This novel reaction is believed to proceed via a retro-hDA opening of the dihydropyran ring to afford 522 followed by an acid-catalyzed aldol-type cyclization and dehydration (Scheme 114) <20040L3191>. 

A possible origin of this ring-opened alkaloid is from the methyl chanofruticosinate (238) which on oxidation provides the iminium ion 360. Hydrolysis of this iminium ion gives the presumably unstable carbinol amine 361 which could then undergo a retro-aldol-type reaction... 

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