Convergent total syntheses of the potent cytotoxins (+)-tedanolide (1) and (+)-13-deoxytedanolide (2) are described. The carbon framework of these compounds was assembled via a stereoselective aldol reaction that unifies the C(1)-C(12) ketone fragment 5 with a C(13)-C(23) aldehyde fragment 6 (for 13-deoxytedanolide) or 52 (for tedanolide). Multiple obstacles were encountered en route to (+)-1 and (+)-2 that required very careful selection and orchestration of the stereochemistry and functionality of key intermediates. Chief among these issues was the remarkable stability and lack of reactivity of hemiketals 33b and 34 that prevented the tedanolide synthesis from being completed from aldol 4. Key to the successful completion of the tedanolide synthesis was the observation that the 13-deoxy hemiketal 36 could be oxidized to C(11,15)-diketone 38 en route to 13-deoxytedanolide. This led to the decision to pursue the tedanolide synthesis via C(15)-(S)-epimers, since this stereochemical change would destabilize the hemiketal that plagued the attempted synthesis of tedanolide via C(15)-(R) intermediates. However, use of C(15)-(S) configured intermediates required that the side chain epoxide be introduced very late in the synthesis, owing to the ease with which the C(15)-(S)-OH cyclized onto the epoxide of intermediate 50.

A total synthesis of 13-deoxytedanolide is described. The synthesis features a highly stereoselective fragment assembly aldol reaction of methyl ketone 4 and aldehyde 5 to establish the complete carbon skeleton of the natural product in the form of aldol 15. The facile formation of the remarkably unreactive hemiketal 16 thwarted attempts to elaborate 15 to tedanolide. However, deoxygenation of the C(13)-hydroxyl of 16 provided the 13-deoxy hemiketal 17 that was smoothly elaborated to 13-deoxytedanolide.

[reaction: see text] Highly diastereoselective syntheses of diketo esters 6a and 6b are described. These intermediates undergo efficient aldol reactions with protected C(13)-C(21) aldehydes 3 and 23, thereby providing advanced C(1)-C(21) tedanolide seco ester precursors 9a and 9b.

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