A practical route to tertiary diarylmethylamides or -carbamates from imines, organozinc reagents and acyl chlorides or chloroformates is described. This route involves the formation of an imine, which is used without isolation, followed by its activation by the carbonyl-containing electrophile and the trapping of the acyliminium by an organozinc reagent. Most steps are conducted concomitantly to render the procedure as practical and straightforward as possible. Therefore, the whole experimental protocol takes less than two hours.
A coupling reaction between cyclic N-acyliminium ions with organozinc reagents is described. The cyclic N-acyliminium ions, generated in situ from N-substituted-5-hydroxy-2-pyrrolidinones by treatment with boron trifluoride–diethyl ether complex or titanium tetrachloride, are trapped by the organozinc reagent, which is formed from an alkyl bromide in the presence of zinc in the same reaction medium. The N-substituted-5-allyl-2-pyrrolidinones generated using this method serve as versatile intermediates for the synthesis of azabicyclic systems with indolizidine and pyrroloazepinolizidine cores.
The total synthesis of endolides A and B has been achieved in a concise and highly stereoselective fashion (12 steps; 16.2 and 16.0% overall yield, respectively). Key features of the route include a modified Negishi coupling between 3-bromofuran and an organozinc reagent derived from an iodoalanine derivative for the synthesis of a 3-(3-furyl)alanine derivative, and a judicious choice of reaction conditions to overcome the conformational constraints placed by converting a linear peptide into the corresponding macrocycle.
A practical route to tertiary diarylmethylamides or -carbamates from imines, organozinc reagents and acyl chlorides or chloroformates