Formal syntheses of hirsutine and rhynchophylline and progress toward the enantioselective total synthesis of citrinadin A
The diastereoselective formal syntheses of the corynanthe alkaloid hirsutine and oxindole alkaloid rhynchophylline are described. The general approach features the use of ring-closing metathesis (RCM) to construct an [alpha],[beta]-unsaturated lactam, which is subjected to 1,4-addition. The lithium enolate of ethyl-1,3-dithiolane-2-carboxylate was identified as the optimal nucleophile in these systems. A key feature of this approach is that the stereochemical outcome of the 1,4-addition can be effectively controlled by appropriately sequencing the indole Boc-protection step to give either the C(3)-H/C(15)-H cis or C(3)-H/C(15)-H trans stereochemical relationship. As a result, we have developed a unified approach to both the "normal" and "pseudo" corynanthe alkaloids. This finding was highlighted through the synthesis of the complete carbon skeleton of the archetypal normal corynanthe alkaloid dihydrocorynantheol. An efficient synthesis of the tricyclic spiroindolinone ABC-fragment of the marine alkaloid citrinadin A has been achieved. The synthesis relies on a novel asymmetric oxidative rearrangement of an indole to an oxindole using a chiral auxiliary on the indole nitrogen to achieve facial selectivity. The transformation proceeds via the epoxidation of the indole C(2),C(3) double bond using DMDO, followed by a silica gelmediated 1,2-epoxide rearrangement. Using this tactic, the spirooxindole of citrinadin A, which contains two adjacent quaternary centers, was formed in high yield and excellent diastereoselectivity. Efforts toward the fragment coupling of the tricyclic spiroindolinone with a 2,4,6-trisubstituted piperidine coupling partner are described.