Selective activation/coupling of polyhalogenated nucleophiles in ni/cr-mediated reactions: synthesis of c1-c19 building block of halichondrin bs.

The C1-C19 building block 46 of halichondrin Bs was synthesized via a selective activation/coupling of β-bromoenone 34 with aldehyde 35 in a Ni/Cr-mediated reaction. The first phase of study was a method development to effect a coupling of a "naked" vinylogous anion with an aldehyde. The study with the coupling of 9 + 10 → 11 revealed: (1) β-bromoenone 9b is a better nucleophile than the corresponding β-iodo- and β-chloroenones 9a,c; (2) (Me)2Phen(OMe)2·NiCl2 13b is a better Ni-catalyst than (Me)2Phen(H)2·NiCl2 13a; and (3) a low Ni-catalyst loading, for example, 0.05-0.1 mol % Ni-catalyst against 10 mol % Cr-catalyst, is crucial for an effective coupling. The second phase of study was a method development to realize a selective activation/coupling of polyhalogenated nucleophiles such as 34. The competition experiment of 10 + 9b over 10 + 31a-c revealed: (1) (Me)2Phen(OMe)2·NiCl2 13b is more effective than (Me)2Phen(H)2·NiCl2 13a for the required selective activation/coupling; (2) a low Ni-catalyst loading, for example, 0.05-0.1 mol % Ni-catalyst against 10 mol % Cr-catalyst, is crucial for discriminating β-bromoenone 9b from the three types of vinyl iodides 31a-c. The third phase of study was an application of the developed method to execute the proposed coupling of 34 + 35 → 36. For this application, a polyether-type Ni-catalyst 37c, readily soluble in the reaction medium, was introduced to achieve the selective activation/coupling with higher efficiency. With use of ion-exchange resin-based device, the coupling product 36 was transformed to the C1-C19 building block 46 of halichondrin Bs without purification/separation of the intermediates.