THE USE OF THE PHOTOCHEMICAL (2+2) ADDITION IN THE SYNTHESIS OF NATURAL PRODUCTS: THE TOTAL SYNTHESIS OF DELTA-CADINENE AND THE 6-EPI-ARTEANNUIN B SKELETON.

摘要

The photochemical cycloaddition has been used in the synthesis of germacrane, elemane, cadinane, and guaiane sesquiterpenes. The method involves a {2(pi)+2(pi)} photocycloaddition of substituted cyclobutenes and cyclohexenones to form highly functionalized tricyclo{4.4.0.0.('2,5)}decanones. These highly strained compounds can be thermolysed or modified prior to thermolysis to yield a wide range of natural product skeletons. The regio- and stereoselectivity of the reaction will be discussed along with the scope and limitations of the sequence as a general synthetic approach to the sesquiterpenes.;Application of the photochemical olefin metathesis to the synthesis of non-sesquiterpenoid natural products was also investigated. Initial studies towards the quinazoline alkaloids (147) and the AB ring system (128) of rosenonolactone will be discussed, along with attempts at anion induced isomerization of tricyclo{4.4.0.0.('2,5)}decanes via base treatment of dithianes such as 160.;;Specifically, the readily available cyclobutene ester 41 was photoadded to the enone-ester 79 yielding the diester photoadduct 80. This material was subsequently converted into the 6-epi-arteannuin B skeleton which is related to an important class of plasmodicidal agents. In a similar fashion, cyclobutene 41 was photoadded to piperitone 44 to yield the keto-photoadduct 45. Conversion of the carbonyl to an acetate and subsequent thermolysis led to a transannular acetate displacement of the intermediate cyclodecadiene to form a decalin system lacking oxygenation at the bridgehead. This material was then converted to (delta)-cadinene (107).