Iridoid boschnialactone

Koizumi and coworkers have developed a new enantiomerically pure a,p-unsaturated sulfoxide dienophile bearing the isoborneol group as a chiral auxiliary, dimethyl (f )-2-(10-isoborneolsulfinyl)maleate (214), which has been successfully employed as a chiral synthetic equivalent of dimethyl acetylenedicarboxylate [174], The dienophile (214) underwent cycloaddition with cyclopentadiene, in the presence of zinc chloride, with high stereo- and diastereoselectivity (92% single endo diastereoisomer, 6% single exo diastereoisomer) to yield the major cycloadduct (215), which was subsequently transformed into the half-ester (216), an intermediate in the syntheses of (-)-aristeromycin (199) and (-)-neplanocin A (200). Cycloadduct (215) has also recently been used for the synthesis of bicyclo[2.2.1]heptane lactone (217) [175,177], which itself is an intermediate in the synthesis of the iridoid (-)-boschnialactone [176] (218) (Scheme 5.69), and also in the formal synthesis of (-)-aristeromycin (199) and (-)-neplanocin (200) [177]. 

A striking example of the power of A -heterocyclic carbene (NHC)-bearing catalysts with sterically demanding substrates was disclosed by Chavez and Jacobsen, " who presented a route to several iridoid natural products, exemplified by the enantio- and diastereoselective synthesis of boschnialactone 31 outlined in Scheme 5. Chiral aldehyde 27, available from citronellal by Eschenmoser-methylenation in a single step, reacted despite the presence of an isoprenyl moiety and a gi OT-disubstituted double bond, in the presence of catalyst C smoothly to form... 

Among the first synthetic achievements, based on the use of tricyclooctanones, are the following preparations of mono- and sesqui-terpenes. Four members of the monoterpene iridoid family have been obtained by a single approach (Scheme 17). The disadvantage of the stereo/io/iselective reduction of (21b), affording (82) and (83) in a 1 1 ratio, is compensated for by the benefit of simultaneous access to four natural products ( )-boschnialactone and the epimer (84), this being a suitable precursor to attain ( )-al-... 

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